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Background: In x‐ray computed tomography (CT), the gain inconsistency of detector units leads to ring artifacts in the reconstructed images, seriously destroys the image structure, and is not conducive to image recognition. In addition, to reduce radiation dose and scanning time, especially photon counting CT, low‐dose CT is required, so it is important to reduce the noise and suppress ring artifacts in low‐dose CT images simultaneously. Purpose: Deep learning is an effective method to suppress ring artifacts, but there are still residual artifacts in corrected images. And the feature recognition ability of the network for ring artifacts decreases due to the effect of noise in the low‐dose CT images. In this paper, a method is proposed to achieve noise reduction and ring artifact removal simultaneously. Methods: To solve these problems, we propose a ring artifact correction method for low‐dose CT based on detector shifting and deep learning in this paper. Firstly, at the CT scanning stage, the detector horizontally shifts randomly at each projection to alleviate the ring artifacts as front processing. Thus, the ring artifacts are transformed into dispersed noise in front processed images. Secondly, deep learning is used for dispersed noise and statistical noise reduction. Results: Both simulation and real data experiments are conducted to evaluate the proposed method. Compared to other methods, the results show that the proposed method in this paper has better effect on removing ring artifacts in the low‐dose CT images. Specifically, the RMSEs and SSIMs of the two sets of simulated and experiment data are better compared to the raw images significantly. Conclusions: The method proposed in this paper combines detector shifting and deep learning to remove ring artifacts and statistical noise simultaneously. The results show that the proposed method is able to get better performance. [ABSTRACT FROM AUTHOR]

Background: Cone‐beam computed tomography (CBCT), an important medical modality for disease detection and diagnosis, is currently widely used in clinical practice. However, due to the inconsistent response of CBCT detectors, the lack of proper calibration often leads to the occurrence of ring artifacts in CBCT‐reconstructed images. These artifacts may affect physicians' assessment and diagnosis. Therefore, effective elimination of ring artifacts in CBCT images without degrading image quality is important. Purpose: Given the pros and cons of existing methods for removing ring artifacts in CBCT images, this paper is devoted to designing a specific Transformer for this task, leveraging the global and local modeling ability of Transformer. Methods: We design a loss function with dual‐domain information fusion for the vanilla Transformer to correct ring artifacts in CBCT images. The method operates in image domain to predict artifact‐free outputs and preserve more image details. Meanwhile, we design a tailored loss function incorporating polar domain optimization to remove ring artifacts more effectively. Specifically, an unidirectional gradient loss that constrains vertical gradients in polar domain is imposed, based on the geometric prior that in polar coordinates, ring artifacts predominately affect horizontal gradients while minimally influencing vertical gradients. Results: We conduct extensive ablative and comparative experiments on CBCT/CT image set to validate the performance of the proposed method. First, four ablation experiments demonstrate the feasibility of our approach. Then, we compare our method with several classical methods and the latest state‐of‐the‐arts, and our method achieves the highest quality of corrected images as well as the best evaluation metrics. In these experiments, 5332 CT images were used for training, and 550 CT images, and 500 real CBCT images were used for testing. The source code is available at https://github.com/shasha521/CBCT. Conclusions: Experimental results demonstrate that our method can significantly improve the effectiveness of ring artifact correction. By capitalizing on dual‐domain information fusion and a customized loss function, the improved Transformer can not only effectively remove ring artifacts in CBCT images, but also preserve the details of original images quite well. [ABSTRACT FROM AUTHOR]

Background: Given the large number of readout pixels in clinical positron emission tomography (PET) scanners, signal multiplexing is an indispensable feature to reduce scanner complexity, power consumption, heat output, and cost. Purpose: In this paper, we introduce interleaved multiplexing (iMux) scheme that utilizes the characteristic light‐sharing pattern of depth‐encoding Prism‐PET detector modules with single‐ended readout. Methods: In the iMux readout, four anodes from every other silicon photomultiplier (SiPM) pixels across rows and columns, which overlap with four distinct light guides, are connected to the same application‐specific integrated circuit (ASIC) channel. The 4‐to‐1 coupled Prism‐PET detector module was used which consisted of a 16 × 16 array of 1.5 × 1.5 × 20 mm3 lutetium yttrium oxyorthosilicate (LYSO) scintillator crystals coupled to an 8 × 8 array with 3 × 3 mm2 SiPM pixels. A deep learning‐based demultiplexing model was investigated to recover the encoded energy signals. Two different experiments were performed with non‐multiplexed and multiplexed readouts to evaluate the spatial, depth of interaction (DOI), and timing resolutions of our proposed iMux scheme. Results: The measured flood histograms, using the decoded energy signals from our deep learning‐based demultiplexing architecture, achieved perfect crystal identification of events with negligible decoding error. The average energy, DOI, and timing resolutions were 9.6 ± 1.5%, 2.9 ± 0.9 mm, and 266 ± 19 ps for non‐multiplexed readout and 10.3 ± 1.6%, 2.8 ± 0.8 mm, and 311 ± 28 ps for multiplexed readout, respectively. Conclusions: Our proposed iMux scheme improves on the already cost‐effective and high‐resolution Prism‐PET detector module and provides 16‐to‐1 crystal‐to‐readout multiplexing without appreciable performance degradation. Also, only four SiPM pixels are shorted together in the 8 × 8 array to achieve 4‐to‐1 pixel‐to‐readout multiplexing, resulting in lower capacitance per multiplexed channel. [ABSTRACT FROM AUTHOR]

Background: Simultaneous positron emission tomography/magnetic resonance imaging (PET/MRI) has shown promise in acquiring complementary multiparametric information of disease. However, designing these hybrid imaging systems is challenging due to the propensity for mutual interference between the PET and MRI subsystems. Currently, there are integrated PET/MRI systems for clinical applications. For neurologic imaging, a brain‐dedicated PET insert provides superior spatial resolution and sensitivity compared to body PET scanners. Purpose: Our first‐generation prototype brain PET insert ("PETcoil") demonstrated RF‐penetrability and MR‐compatibility. In the second‐generation PETcoil system, all analog silicon photomultiplier (SiPM) signal digitization is moved inside the detectors, which results in substantially better PET detector performance, but presents a greater technical challenge for achieving MR‐compatibility. In this paper, we report results from MR‐compatibility studies of two fully assembled second‐generation PET insert detector modules. Methods: We studied the effect of the presence of the two second‐generation TOF‐PET insert detectors on parameters that affect MR image quality and evaluated TOF‐PET detector performance under different MRI pulse sequence conditions. Results: With the presence of operating PET detectors, no RF noise peaks were induced in the MR images, but the relative average noise level was increased by 15%, which led to a 3.1 to 4.2‐dB degradation in MR image signal‐to‐noise ratio (SNR). The relative homogeneity of MR images degraded by less than 1.5% with the two operating TOF‐PET detectors present. The reported results also indicated that ghosting artifacts (percent signal ghosting (PSG) ⩽ 1%) and MR susceptibility artifacts (0.044 ppm) were insignificant. The PET detector data showed a relative change of less than 5% in detector module performance between running outside and within the MR bore under different MRI pulse sequences except for energy resolution in EPI sequence (13% relative difference). Conclusions: The PET detector operation did not cause any significant artifacts in MR images and the performance and time‐of‐flight (TOF) capability of the former were preserved under different tested MR conditions. [ABSTRACT FROM AUTHOR]

Purpose: Parallax error is a major issue in small animal positron emission tomography (PET) scanners which are used in preclinical studies or detailed scanning of human organs. Several methods have been proposed and investigated to reduce this radial artifact in PET images by estimating the depth of interaction (DOI) of 511‐keV photons in the crystal. Among all, the dual‐ended readout method seems to be very simple and effective as it does not have any fabrication and readout complications. In the past, some studies suggested that increasing the roughness of crystal lateral surfaces improves DOI resolution. In this paper, this was experimentally examined for four Ce:GAGG crystals with different surface structures. Methods: Four 1.2 × 1.2 × 20 mm3 GAGG crystals with following surface treatment were examined: polished with optical finishing, fine grinding (using a fine surface grinding machine), fine cutting (no treatment), and coarse cutting (no treatment). These crystals were coupled individually to two SiPMs for dual‐ended readout and placed in a coincidence detection circuit for electronic collimation of 511 keV incidents. The crystals were compared in terms of energy response and DOI estimation capability. Results: DOI function for each crystal was extracted and FWHM DOI resolution was calculated. DOI resolution for the polished crystal varied in the range of 0.54–4.14 mm throughout the length of the crystal due to its nonlinear DOI function. The fine grinding crystal showed a linear DOI function within the dynamic range of (−0.75, 0.75), and its DOI resolution varied in the range of 1.24–1.50 mm (1.37 ± 0.13 mm DOI resolution). The fine‐cut crystal had almost a linear DOI function and a wider dynamic range of (−0.85, 0.85) and therefore the best performance with 1.2 ± 0.08 mm DOI resolution. However, for the crystal with the roughest surface (coarse‐cut crystal), even though the dynamic range expanded to (−0.95, 0.95), its DOI function became nonlinear resulting in 1.24 ± 0.28 mm DOI resolution. This means there is an optimum surface roughness to provide the crystal with the best DOI capability. The pulse‐height spectrum measured at each depth was used as a measure to compare the energy performance of the four crystals. The photopeak of 511 keV was observed for all depths, all crystals. The photopeak position for the coarse‐cut crystal had extensive depth dependency which results in poorer energy resolution unless the energy window is calibrated for each depth. This variation of photopeak for the fine‐cut and fine grinding crystals was comparable with that of polished crystal. Conclusion: This paper reports 1.2 ± 0.08 mm FWHM DOI resolution for a fine‐cut unpolished crystal. This resolution is as narrow as the crystal width, resulting in the complete elimination of parallax error in PET images. Results suggest that there is an optimum roughness for the best performance of the dual‐ended method and further increase in the roughness, degrades DOI resolution. Thanks to the high light yield of GAGG, the energy performance of the fine‐cut crystal is acceptable, and the depth dependency of the spectrum is negligible. [ABSTRACT FROM AUTHOR]

The clinical translation of FLASH radiotherapy (RT) requires challenges related to dosimetry and beam monitoring of ultra‐high dose rate (UHDR) beams to be addressed. Detectors currently in use suffer from saturation effects under UHDR regimes, requiring the introduction of correction factors. There is significant interest from the scientific community to identify the most reliable solutions and suitable experimental approaches for UHDR dosimetry. This interest is manifested through the increasing number of national and international projects recently proposed concerning UHDR dosimetry. Attaining the desired solutions and approaches requires further optimization of already established technologies as well as the investigation of novel radiation detection and dosimetry methods. New knowledge will also emerge to fill the gap in terms of validated protocols, assessing new dosimetric procedures and standardized methods. In this paper, we discuss the main challenges coming from the peculiar beam parameters characterizing UHDR beams for FLASH RT. These challenges vary considerably depending on the accelerator type and technique used to produce the relevant UHDR radiation environment. We also introduce some general considerations on how the different time structure in the production of the radiation beams, as well as the dose and dose‐rate per pulse, can affect the detector response. Finally, we discuss the requirements that must characterize any proposed dosimeters for use in UDHR radiation environments. A detailed status of the current technology is provided, with the aim of discussing the detector features and their performance characteristics and/or limitations in UHDR regimes. We report on further developments for established detectors and novel approaches currently under investigation with a view to predict future directions in terms of dosimetry approaches, practical procedures, and protocols. Due to several on‐going detector and dosimetry developments associated with UHDR radiation environment for FLASH RT it is not possible to provide a simple list of recommendations for the most suitable detectors for FLASH RT dosimetry. However, this article does provide the reader with a detailed description of the most up‐to‐date dosimetric approaches, and describes the behavior of the detectors operated under UHDR irradiation conditions and offers expert discussion on the current challenges which we believe are important and still need to be addressed in the clinical translation of FLASH RT. [ABSTRACT FROM AUTHOR]

Purpose: The fixed pattern noise in radiography image detectors is caused by various sources. Multiple readout circuits with gate drivers and charge amplifiers are used to efficiently acquire the pixel voltage signals. However, the multiple circuits are not identical and thus yield nonuniform system gains. Nonuniform sensitivities are also produced from local variations in the charge collection elements. Furthermore, in phosphor-based detectors, the optical scattering at the top surface of the columnar CsI growth, the grain boundaries, and the disorder structure causes spatial sensitivity variations. These nonuniform gains or sensitivities cause fixed pattern noise and degrade the detector performance, even though the noise problem can be partially alleviated by using gain correction techniques. Hence, in order to develop good detectors, comparative analysis of the energy spectrum of the fixed pattern noise is important. Methods: In order to observe the energy spectrum of the fixed pattern noise, a normalized noise power spectrum (NNPS) of the fixed pattern noise is considered in this paper. Since the fixed pattern noise is mainly caused by the nonuniform gains, we call the spectrum the gain NNPS. We first asymptotically observe the gain NNPS and then formulate two relationships to calculate the gain NNPS based on a nonuniform-gain model. Since the gain NNPS values are quite low compared to the usual NNPS, measuring such a low NNPS value is difficult. By using the average of the uniform exposure images, a robust measuring method for the gain NNPS is proposed in this paper. Results: By using the proposed measuring method, the gain NNPS curves of several prototypes of general radiography and mammography detectors were measured to analyze their fixed pattern noise properties.We notice that a direct detector, which is based on the a-Se photoconductor, showed lower gain NNPS than the indirect-detector case, which is based on the CsI scintillator. By comparing the gain NNPS curves of the indirect detectors, we could analyze the scintillator properties depending on the techniques for the scintillator surface processing. Conclusions: A robust measuring method for the NNPS of the fixed pattern noise of a radiography detector is proposed in this paper. The method can measure a stable gain NNPS curve, even though the fixed pattern noise level is quite low. From the measured gain NNPS curves, we can compare and analyze the detector properties in terms of producing the fixed pattern noise. [ABSTRACT FROM AUTHOR]

Purpose: High‐dose rate (HDR) and pulsed‐dose rate (PDR) brachytherapy would benefit from an independent treatment verification system to monitor treatment delivery and to detect errors in real time. This paper characterizes and provides an uncertainty budget for a detector based on a fiber‐coupled high‐Z inorganic scintillator capable of performing time‐resolved in vivo dosimetry during HDR and PDR brachytherapy. Method: The detector was composed of a detector probe and an optical reader. The detector probe consisted of either a 0.5 × 0.4 × 0.4 mm3 (HDR) or a 1.0 × 0.4 × 0.4 mm3 (PDR) cuboid ZnSe:O crystal glued onto an optical‐fiber cable. The outer diameter of the detector probes was 1 mm, and fit inside standard brachytherapy catheters. The signal from the detector probe was read out at 20 Hz by a photodiode and a data acquisition device inside the optical reader. In order to construct an uncertainty budget for the detector, six characteristics were determined: (1) temperature dependence of the detector probe, (2) energy dependence as a function of the probe‐to‐source position in 2D (determined with 2 mm resolution using a robotic arm), (3) the signal‐to‐noise ratio (SNR), (4) short‐term stability over 8 h, and (5) long‐term stability of three optical readers and four probes used for in vivo monitoring in HDR and PDR treatments over 21 months (196 treatments and 189 detector calibrations, and (6) dose‐rate dependence. Results: The total uncertainty of the detector at a 20 mm probe‐to‐source distance was < 5.1% and < 5.8% for the HDR and PDR versions, respectively. Regarding the above characteristics, (1) the sensitivity of the detector decreased by an average of 1.4%/°C for detector probe temperatures varying from 22 to 37°C; (2) the energy dependence of the detector was nonlinear and depended on both probe‐to‐source distance and the angle between the probe and the brachytherapy source; (3) the median SNRs were 187 and 34 at a 20 mm probe‐to‐source distance for the HDR and PDR versions, respectively (corresponding median source activities of 4.8 and 0.56 Ci, respectively); (4) the detector response varied by 0.6% in 11 identical irradiations over 8 h; (5) the sensitivity of the four detector probes decreased systematically by 0–1.2%/100 Gy of dose delivered to the probes, and random fluctuations of 4.8% in the sensitivity were observed for the three probes used in PDR and 1.9% for the probe used in HDR; and (6) the detector response was linear with dose rate. Conclusion: ZnSe:O detectors can be used effectively for in vivo dosimetry and with high accuracy for HDR and PDR brachytherapy applications. [ABSTRACT FROM AUTHOR]

A cone-angle-based window function is defined in this manuscript for image reconstruction using helical cone beam filtered backprojection (CB-FBP) algorithms. Rather than defining the window boundaries in a two-dimensional detector acquiring projection data for computed tomographic imaging, the cone-angle-based window function deals with data redundancy by selecting rays with the smallest cone angle relative to the reconstruction plane. To be computationally efficient, an asymptotic approximation of the cone-angle-based window function is also given and analyzed in this paper. The benefit of using such an asymptotic approximation also includes the avoidance of functional discontinuities that cause artifacts in reconstructed tomographic images. The cone-angle-based window function and its asymptotic approximation provide a way, equivalent to the Tam-Danielsson-window, for helical CB-FBP reconstruction algorithms to deal with data redundancy, regardless of where the helical pitch is constant or dynamically variable during a scan. By taking the cone-parallel geometry as an example, a computer simulation study is conducted to evaluate the proposed window function and its asymptotic approximation for helical CB-FBP reconstruction algorithm to handle data redundancy. The computer simulated Forbild head and thorax phantoms are utilized in the performance evaluation, showing that the proposed cone-angle-based window function and its asymptotic approximation can deal with data redundancy very well in cone beam image reconstruction from projection data acquired along helical source trajectories. Moreover, a numerical study carried out in this paper reveals that the proposed cone-angle-based window function is actually equivalent to the Tam-Danielsson-window, and rigorous mathematical proofs are being investigated. [ABSTRACT FROM AUTHOR]

Background: Photon-counting x-ray detectors may enable single-exposure dual-energy (DE) x-ray angiography. Purpose: The purpose of this paper is to experimentally optimize the energy thresholds and tube voltage for single-exposure DE x-ray angiography. Methods: We optimized single-exposure DE x-ray angiography using the iodine signal-difference-to-noise ratio (SDNR) per root patient air kerma (1) as a figure of merit. We measured the iodine SDNR by imaging an iodine stepwedge immersed in a water tank with a depth of 30 cmin the direction of x-ray propagation. The stepwedge was imaged using tube voltages ranging from 90 to 150 kV and a cadmium telluride (CdTe) x-ray detector with two energy bins and analog charge summing for charge sharing suppression. The energy threshold that separates the two energy bins was varied from approximately 35 keV to approximately 75% of the maximum energy of the x-ray beam. Curve fitting was used to determine the threshold that maximized SDNR/v 1. The effect of scatter was determined from measurements of the scatter-to-primary ratios (SPRs) of the low-energy and high-energy images and a semi-empirical model of the relationship between SDNR and SPR. Using the optimal parameters, we imaged a phantom with vessel-simulating structures and background clutter. Results: The optimal energy thresholds increased monotonically from ~50 to ~85 keV over the range of tube voltages considered. For tube voltages greater than 90 kV, the optimal energy thresholds consistently allocated approximately two thirds of all detected primary photons to the low energy bin; this ratio was preserved without scatter. Consistent with prior modeling studies, SDNR/v increased monotonically with tube voltage from 90 to 150 kV; SDNR/v at 150 kV was approximately 38% higher than that at 90 kV for an iodine area density of ~50 mg/cm2.Scatter reduced SDNR by approximately 25% for SPRs of ~1 and 0.4 in low-energy and high-energy images, respectively. Conclusions: Achieving optimal image quality in single-exposure DE angiography with photon-counting x-ray detectors will require high tube voltages (i.e., >130 kV) and, for thick patients, energy thresholds that allocate approximately two thirds of all primary photons to the low-energy image. Future work will compare the image quality of singe-exposure photon-counting and kV-switching approaches to DE x-ray angiography. [ABSTRACT FROM AUTHOR]

Purpose: Data on the output correction factor for small photon beam dosimetry of the microDiamond detector manufactured by the company PTW can be found in a variety of papers. Referring either to measurements or to Monte Carlo (MC) calculations, they show substantial disagreements particularly at very small fields. This work reports results of a further MC study aiming at a better understanding of how specific properties of the microDiamond detector are influencing its output correction factor and whether this can explain at least some of the disagreements. Methods: In this study the method of a fluence‐based decomposition of the dose conversion factor was used which is considered as a useful tool to understand the response of a detector in nonreference conditions. This decomposition method yields the following three factors: (a) the stopping power ratio water to diamond, (b) a perturbation factor pint taking into account all fluence changes in the transition from a small water voxel at the point of dose determination to the bare diamond detector, and (c) a perturbation factor pext taking into account all additional fluence changes in the fully simulated diamond detector caused by the material and design details outside the sensitive volume. Results: Monte Carlo calculated output correction factors were obtained for Co‐60, 6 MV and 10 MV photon beams showing that the maximum variation with field size remained in the order of 2% for quadratic field sizes larger than about 0.3 cm. For field sizes smaller than about 0.5 cm a clear under‐response is obtained at all three radiation qualities in agreement with all known MC calculations, however, in contrast to some measured result. The shape of the output correction factor can be well explained by an opposite mode of action between under‐response expressed by the perturbation factor pint and over‐response expressed by the perturbation factor pext where the first one is mainly influenced by volume averaging, and the second one by a back scatter effect of electrons from the diamond substrate into the sensitive volume. Conclusion: The response of microDiamond detector can be well described under various measuring conditions by the dose conversion factor and the dependency of its fluence‐based subfactors on detector characteristics. Monte Carlo simulations offer an improvement in the understanding particularly of small‐field effects by relating the output correction factor to spectral fluence changes in the sensitive volume of the detector. The most significant influence factors are the finite size of the active volume and the presence of the high‐density diamond substrate causing a field size‐dependent backscattering. These perturbations are opposite in their effects. The diamond in the sensitive volume itself and in particular its density has almost no influence. Scattering of results at very small field sizes can be explained by different gradients of dose profiles around the beam axis at identical full width half maximum (FWHM) field size parameters and by possible deviations of the radius of the sensitive volume from the nominal radius. The backscattering effect also has an influence on the determination of profiles and for very small field sizes on the response at different rotation angles. [ABSTRACT FROM AUTHOR]

Purpose: The dose conversion factor plays an important role in the dosimetry by enabling the absorbed dose in the sensitive volume of a detector to be converted into the absorbed dose in the surrounding medium (in most cases water). The purpose of this paper is to demonstrate that a specific fluence‐based approach for the decomposition of the dose conversion factor is in particular useful for the interpretation of the influences of detector properties on measurements under nonreference conditions. Methods: Data for the dose conversion factor and secondary fluence spectra were obtained by the Monte Carlo method. The calculation of the secondary charged particle fluence (electrons and positrons) in the sensitive detector volume was imbedded into the code for the calculation of absorbed dose in the detector. The decomposition method into subfactors is based on the use of these fluence data applied to a stepwise transition from the dose at the point of measurement next to a pure water detector and finally to the fully simulated detector geometry. Each subfactor is obtained as a ratio, at which the stopping power only is different in the numerator and the denominator or at which the fluence only is different in the numerator and the denominator. This method was applied at photon dose profiles obtained in water at different radiation qualities and with various detectors of cylindrical type. Results: The resulting subfactors can be well identified as a stopping power ratio and as perturbation factors each reflecting particular detector properties. Two of them ( f 1 and f4) are equivalent with perturbation factors which have already been introduced by other authors previously. These are the volume perturbation factor and the extracameral perturbation factor. Subfactor f 2 denoted as medium perturbation factor was found to resemble the density perturbation factor. Results obtained for the volume perturbation factor applied to dose profiles measured with cylindrical detectors confirm that the volume effect can be well described by a convolution of the true profile in water with a Gaussian kernel. It was found that the sigma parameter depends on the cylinder radius only and amounts almost exactly to half of its value. The medium perturbation factor strongly depends on the density of the detector medium. For an air‐filled detector, the influence of the air again can be described by a Gauss convolution, however, with a less good agreement. For detectors with a density of the cavity medium larger than that of water, for instance, for a diamond detector, it was found that there is a tendency of compensation between the volume averaging effect and the medium effect. Conclusion: The fluence‐based decomposition of the dose conversion factor leads to a fluence‐based formulation of perturbation factors, referred to as volume, medium, and extracameral perturbation factor. These factors offer useful explanations for the behavior of detectors in nonreference conditions. An example was given for cylindrical detectors at dose profile measurements. [ABSTRACT FROM AUTHOR]

Purpose: The recent introduction of a specific electronic readout chip, designed for the processing of the same signal pulses from an x-ray sensor in one integrating and one counting channel in each pixel E. Kraft et al., [IEEE Trans. Nucl. Sci. 54, 383-390 (2007)]; E. Kraft, 'Counting and integrating microelectronics development for direct conversion X-ray imaging,' Ph.D. thesis, 2007; J. Fink, 'Characterization of the imaging performance of the simultaneously counting and integrating X-ray detector CIX,' Ph.D. thesis, 2010, enables an alternative to conventional dual-energy imaging methods. As shown in a previous paper, the proposed alternative benefits from positively correlated noise in the raw data, inherent to the counting-integrating readout (CIX), which results in a reduction of basis image noise after dual-energy decomposition. In the present paper, the authors compare the new approach to dual-energy imaging to two conventional dual-energy techniques, the dual-kV technique and the dual-crystal technique and to a photon-counting technique based on two energy windows. Methods: The study is based on x-ray computed tomography (CT) simulations of an anthropomorphic head phantom, where the signal-to-noise ratios and the contrast-to-noise ratios in basis material images and quasi-monochromatic images at 60 keV are compared. Moreover, a simple pictorial illustration of the relevance of input noise correlations in the noise-propagation process during the dual-energy basis material decomposition are presented, as well as measurement data for the correlation obtained as a function of the x-ray flux rate. Results: Under the idealized assumptions of the absence of scatter and detector imperfections like K-escape, crosstalk and detector noise, our comparison shows that among the dual-energy techniques investigated, the CIX concept together with the dual-kV technique performs best, confirming the mitigating effect of correlated measurement data on the dual-energy basis material decomposition. Conclusions: The novel concept of simultaneous counting of photons and integrating the x-ray energy flux has a large potential for dual-energy applications in both projection and tomographic x-ray imaging. Future work will have to focus on the sensitivity of the CIX dual-energy concept to scattered radiation, detector imperfections and high x-ray fluxes. [ABSTRACT FROM AUTHOR]

This paper deals with the application of the adjoint transport theory in order to optimize Monte Carlo based radiotherapy treatment planning. The technique is applied to Boron Neutron Capture Therapy where most often mixed beams of neutrons and gammas are involved. In normal forward Monte Carlo simulations the particles start at a source and lose energy as they travel towards the region of interest, i.e., the designated point of detection. Conversely, with adjoint Monte Carlo simulations, the so-called adjoint particles start at the region of interest and gain energy as they travel towards the source where they are detected. In this respect, the particles travel backwards and the real source and real detector become the adjoint detector and adjoint source, respectively. At the adjoint detector, an adjoint function is obtained with which numerically the same result, e.g., dose or flux in the tumor, can be derived as with forward Monte Carlo. In many cases, the adjoint method is more efficient and by that is much quicker when, for example, the response in the tumor or organ at risk for many locations and orientations of the treatment beam around the patient is required. However, a problem occurs when the treatment beam is mono-directional as the probability of detecting adjoint Monte Carlo particles traversing the beam exit (detector plane in adjoint mode) in the negative direction of the incident beam is zero. This problem is addressed here and solved first with the use of next event estimators and second with the application of a Legendre expansion technique of the angular adjoint function. In the first approach, adjoint particles are tracked deterministically through a tube to a (adjoint) point detector far away from the geometric model. The adjoint particles will traverse the disk shaped entrance of this tube (the beam exit in the actual geometry) perpendicularly. This method is slow whenever many events are involved that are not contributing to the point detector, e.g., neutrons in a scattering medium. In the second approach, adjoint particles that traverse an adjoint shaped detector plane are used to estimate the Legendre coefficients for expansion of the angular adjoint function. This provides an estimate of the adjoint function for the direction normal to the detector plane. In a realistic head model, as described in this paper, which is surrounded by 1020 mono-directional neutron/gamma beams and from which the best ones are to be selected, the example calculates the neutron and gamma fluxes in ten tumors and ten organs at risk. For small diameter beams (5 cm), and with comparable relative errors, forward Monte Carlo is seen to be 1.5 times faster than the adjoint Monte Carlo techniques. For larger diameter neutron beams (10 and 15 cm), the Legendre technique is found to be 6 and 20 times faster, respectively. In the case of gammas alone, for the 10 and 15 cm diam beams, both adjoint Monte Carlo Legendre and point detector techniques are respectively 2 and 3 times faster than forward Monte Carlo. [ABSTRACT FROM AUTHOR]

Purpose: Photon counting detectors (PCDs) are an emerging technology with applications in spectral and low-dose radiographic and tomographic imaging. This paper develops an analytical model of PCD imaging performance, including the system gain, modulation transfer function (MTF), noise-power spectrum (NPS), and detective quantum efficiency (DQE). Methods: A cascaded systems analysis model describing the propagation of quanta through the imaging chain was developed. The model was validated in comparison to the physical performance of a silicon-strip PCD implemented on an experimental imaging bench. The signal response, MTF, and NPS were measured and compared to theory as a function of exposure conditions (70 kVp, 1-7 mA), detector threshold, and readout mode (i.e., the option for coincidence detection). The model sheds new light on the dependence of spatial resolution, charge sharing, and additive noise effects on threshold selection and was used to investigate the factors governing PCD performance, including the fundamental advantages and limitations of PCDs in comparison to energy-integrating detectors (EIDs) in the linear regime for which pulse pileup can be ignored. Results: The detector exhibited highly linear mean signal response across the system operating range and agreed well with theoretical prediction, as did the system MTF and NPS. The DQE analyzed as a function of kilovolt (peak), exposure, detector threshold, and readout mode revealed important considerations for system optimization. The model also demonstrated the important implications of false counts from both additive electronic noise and charge sharing and highlighted the system design and operational parameters that most affect detector performance in the presence of such factors: for example, increasing the detector threshold from 0 to 100 (arbitrary units of pulse height threshold roughly equivalent to 0.5 and 6 keV energy threshold, respectively), increased the f50 (spatial-frequency at which the MTF falls to a value of 0.50) by ~30% with corresponding improvement in DQE. The range in exposure and additive noise for which PCDs yield intrinsically higher DQE was quantified, showing performance advantages under conditions of very low-dose, high additive noise, and high fidelity rejection of coincident photons. Conclusions: The model for PCD signal and noise performance agreed with measurements of detector signal, MTF, and NPS and provided a useful basis for understanding complex dependencies in PCD imaging performance and the potential advantages (and disadvantages) in comparison to EIDs as well as an important guide to task-based optimization in developing new PCD imaging systems. [ABSTRACT FROM AUTHOR]

Purpose: This paper describes an extension to a wide field calibration method implemented on a commercial detector array in order to improve the reproducibility of the calibration procedure. Methods: Following the standard array calibration procedure, two additional 10 x 10 cm exposures were acquired for each array axis with the detector array shifted by ±10 cm in the transverse or axial axes, or by ± 10√2 cm in the positive or negative diagonal axes. These exposures were compared with a final baseline 10 x 10 cm exposure captured with the detector repositioned at the isocenter. The measurements were used to calculate a linear off-axis correction gradient which was then applied to the stored calibration factors. Results: The mean coefficient of variation between five repeat calibrations was reduced from 4.17% to 0.48% and the maximum percentage error in individual calibration factors was reduced from 6.46% to 0.77%. Conclusions: The reproducibility of the calibration factors of an ionization chamber array was increased by capturing a baseline exposure and two further off-axis readings per calibration axis. [ABSTRACT FROM AUTHOR]

Purpose: This paper introduces a novel autocalibration method for cone-beam-CTs (CBCT) or flat-panel CTs, assuming a perfect rotation. The method is based on ellipse-fitting. Autocalibration refers to accurate recovery of the geometric alignment of a CBCT device from projection images alone, without any manual measurements. Methods: The authors use test objects containing small arbitrarily positioned radio-opaque markers. No information regarding the relative positions of the markers is used. In practice, the authors use three to eight metal ball bearings (diameter of 1 mm), e.g., positioned roughly in a vertical line such that their projection image curves on the detector preferably form large ellipses over the circular orbit. From this ellipse-to-curve mapping and also from its inversion the authors derive an explicit formula. Nonlinear optimization based on this mapping enables them to determine the six relevant parameters of the system up to the device rotation angle, which is sufficient to define the geometry of a CBCT-machine assuming a perfect rotational movement. These parameters also include out-of-plane rotations. The authors evaluate their method by simulation based on data used in two similar approaches [L. Smekal, M. Kachelriess, S. E, and K. Wa, 'Geometric misalignment and calibration in cone-beam tomography,' Med. Phys. 31(12), 3242-3266 (2004); K. Yang, A. L. C. Kwan, D. F. Miller, and J. M. Boone, 'A geometric calibration method for cone beam CT systems,' Med. Phys. 33(6), 1695-1706 (2006)]. This allows a direct comparison of accuracy. Furthermore, the authors present real-world 3D reconstructions of a dry human spine segment and an electronic device. The reconstructions were computed from projections taken with a commercial dental CBCT device having two different focus-to-detector distances that were both calibrated with their method. The authors compare their reconstruction with a reconstruction computed by the manufacturer of the CBCT device to demonstrate the achievable spatial resolution of their calibration procedure. Results: Compared to the results published in the most closely related work [K. Yang, A. L. C. Kwan, D. F. Miller, and J. M. Boone, 'A geometric calibration method for cone beam CT systems,' Med. Phys. 33(6), 1695-1706 (2006)], the simulation proved the greater accuracy of their method, as well as a lower standard deviation of roughly 1 order of magnitude. When compared to another similar approach [L. Smekal, M. Kachelriess, S. E, and K. Wa, 'Geometric misalignment and calibration in cone-beam tomography,' Med. Phys. 31(12), 3242-3266 (2004)], their results were roughly of the same order of accuracy. Their analysis revealed that the method is capable of sufficiently calibrating out-of-plane angles in cases of larger cone angles when neglecting these angles negatively affects the reconstruction. Fine details in the 3D reconstruction of the spine segment and an electronic device indicate a high geometric calibration accuracy and the capability to produce state-of-the-art reconstructions. Conclusions: The method introduced here makes no requirements on the accuracy of the test object. In contrast to many previous autocalibration methods their approach also includes out-of-plane rotations of the detector. Although assuming a perfect rotation, the method seems to be sufficiently accurate for a commercial CBCT scanner. For devices which require higher dimensional geometry models, the method could be used as a initial calibration procedure. [ABSTRACT FROM AUTHOR]

Purpose: During radiological interventional procedures (RIP) the skin of a patient under examination may undergo a prolonged x-ray exposure, receiving a dose as high as 5 Gy in a single session. This paper describes the use of the OneDoseTM cable-free system based on p-type MOSFET detectors to determine the entrance skin dose (ESD) at selected points during RIP. Methods: At first, some dosimetric characteristics of the detector, such as reproducibility, linearity, and fading, have been investigated using a C-arc as a source of radiation. The reference setting (RS) was: 80 kV energy, 40 cm × 40 cm field of view (FOV), current-time product of 50 mAs and source to skin distance (SSD) of 50 cm. A calibrated PMX III solid state detector was used as the reference detector and Gafchromic® films have been used as an independent dosimetric system to test the entire procedure. A calibration factor for the RS and correction factors as functions of tube voltage and FOV size have been determined. Results: Reproducibility ranged from 4% at low doses (around 10 cGy as measured by the reference detector) to about 1% for high doses (around 2 Gy). The system response was found to be linear with respect to both dose measured with the PMX III and tube voltage. The fading test has shown that the maximum deviation from the optimal reading conditions (3 min after a single irradiation) was 9.1% corresponding to four irradiations in one hour read 3 min after the last exposure. The calibration factor in the RS has shown that the system response at the kV energy range is about four times larger than in the MV energy range. A fifth order and fourth order polynomial functions were found to provide correction factors for tube voltage and FOV size, respectively, in measurement settings different than the RS. ESDs measured with the system after applying the proper correction factors agreed within one standard deviation (SD) with the corresponding ESDs measured with the reference detector. The ESDs measured with Gafchromic® films were in agreement within one SD compared to the ESDs measured using the OneDoseTM system, as well. The global uncertainty associated to the OneDoseTM system established in our experiments, ranged from 7% to 10%, depending on the duration of the RIP due to fading. These values are much lower than the uncertainty commonly accepted for general diagnostic practices (20%) and of about the same size of the uncertainty recommended for practices with high risk of deterministic side effects (7%). Conclusions: The OneDoseTM system has shown a high sensitivity in the kV energy range and has been found capable of measuring the entrance skin dose in RIP. [ABSTRACT FROM AUTHOR]

Purpose: Compressed sensing theory has enabled an accurate, low-dose cone-beam computed tomography (CBCT) reconstruction using a minimal number of noisy projections. However, the reconstruction time remains a significant challenge for practical implementation in the clinic. In this work, we propose a novel gradient projection algorithm, based on the Gradient-Projection-Barzilai-Borwein formulation (GP-BB), that handles the total variation (TV)-norm regularization-based least squares problem for the CBCT reconstruction in a highly efficient manner, with speed acceptable for routine use in the clinic. Methods: CBCT is reconstructed by minimizing an energy function consisting of a data fidelity term and a TV-norm regularization term. Both terms are simultaneously minimized by calculating the gradient projection of the energy function with the step size determined using an approximate Hessian calculation at each iteration, based on the Barzilai-Borwein formulation. To speed up the process, a multiresolution optimization is used. In addition, the entire algorithm was designed to run with a single graphics processing unit (GPU) card. To evaluate the performance, the Shepp-Logan numerical phantom, the CatPhan 600 physical phantom, and a clinically-treated head-and-neck patient were acquired from the TrueBeam™ system (Varian Medical Systems, Palo Alto, CA). For each scan, in total, 364 projections were acquired in a 200° rotation. The imager has 1024 × 768 pixels with 0.388 × 0.388-mm resolution. This was down-sampled to 512 × 384 pixels with 0.776 × 0.776-mm resolution for reconstruction. Evenly spaced angles were subsampled and used for varying the number of projections for the image reconstruction. To assess the performance of our GP-BB algorithm, we have implemented and compared with three compressed sensing-type algorithms, the two of which are popular and published (forward-backward splitting techniques), and the other one with a basic line-search technique. In addition, the conventional Feldkamp-Davis-Kress (FDK) reconstruction of the clinical patient data is compared as well. Results: In comparison with the other compressed sensing-type algorithms, our algorithm showed convergence in ≤30 iterations whereas other published algorithms need at least 50 iterations in order to reconstruct the Shepp-Logan phantom image. With the CatPhan phantom, the GP-BB algorithm achieved a clinically-reasonable image with 40 projections in 12 iterations, in less than 12.6 s. This is at least an order of magnitude faster in reconstruction time compared with the most recent reports utilizing GPU technology given the same input projections. For the head-and-neck clinical scan, clinically-reasonable images were obtained from 120 projections in 34-78 s converging in 12-30 iterations. In this reconstruction range (i.e., 120 projections) the image quality is visually similar to or better than the conventional FDK reconstructed images using 364 projections. This represents a dose reduction of nearly 67% (120/364 projections) while maintaining a reasonable speed in clinical implementation. Conclusions: In this paper, we proposed a novel, fast, low-dose CBCT reconstruction algorithm using the Barzilai-Borwein step-size calculation. A clinically viable head-and-neck image can be obtained within ∼34-78 s while simultaneously cutting the dose by approximately 67%. This makes our GP-BB algorithm potentially useful in an on-line image-guided radiation therapy (IGRT). [ABSTRACT FROM AUTHOR]

Purpose: Theoretical reasoning suggests that direct conversion digital x-ray detectors based upon photoconductive amorphous-selenium (a-Se) could attain very high values of the MTF (modulation transfer function) at spatial frequencies well beyond 20 cycles mm-1. One of the fundamental factors affecting resolution loss, particularly at x-ray energies just above the K-edge of selenium (12.66 keV), is the K-fluorescence reabsorption mechanism, wherein energy can be deposited in the detector at locations laterally displaced from the initial x-ray interaction site. This paper compares measured MTF changes above and below the Se K-edge of a CCD based a-Se x-ray detector with theoretical expectations. Methods: A prototype 25 μm sampling pitch (Nyquist frequency = 20 cycles mm-1, 200 μm thick a-Se layer based x-ray detector, utilizing a specialized CCD readout device (200 × 400 area array), was used to make edge images with monochromatic x-rays above and below the K-edge of Se. A vacuum double crystal monochromator, exposed to polychromatic x-rays from a synchrotron, formed the monochromatic x-ray source. The monochromaticity of the x-rays was 99% or better. The presampling MTF was determined using the slanted edge method. The theory modeling the MTF performance of the detector includes the basic x-ray interaction physics in the a-Se layer as well as effects related to the operation of the CCD and charge trapping at a blocking layer present at the CCD/a-Se interface. Results: The MTF performance of the prototype a-Se CCD was reduced from the theoretical value prescribed by the basic Se x-ray interaction physics, principally by the presence of a blocking layer. Nevertheless, the K-fluorescence reduction in the MTF was observed, approximately as predicted by theory. For the CCD prototype detector, at five cycles mm-1, there was a 14% reduction of the MTF, from a value of 0.7 below the K-edge of Se, to 0.6 just above the K-edge. Conclusions: The MTF of an a-Se x-ray detector has been measured using monochromatic x-rays above and below the K-edge of selenium. The MTF is poorer above the K-edge by an amount consistent with theoretical expectations. [ABSTRACT FROM AUTHOR]

Purpose: This paper proposes the further development of a resolution modification routine which is used to simulate the presampling modulation transfer function (pMTF) of digital x-ray detectors. Methods: It suggests a method to reconstruct anisotropic two dimensional (2D) pMTF matrices from the experimentally measured horizontal and vertical 1D pMTFs. In this study, the horizontal dimension of the detector is 17.3 cm, while the vertical one is 24 cm. This matrix is multiplied with the 2D Fourier transform of the super-sampled ideal input image to simulate blurring. Then, the restored image is sampled to form the pixels of the digital image. The authors suggest convolution with the comb function instead of the rectangular function to avoid the correction with the sinc function required by the latter. It is demonstrated that this correction is avoided when the comb function is used. Moreover, this study suggests a way to effectively sample the images in the case when the ratio between the 'analog' pitch of the super-sampled input image and the pixel pitch of the digital x-ray detector is a semi-integer. Results: The validation of the simulation algorithm demonstrated that when the comb function was used the average absolute difference between the pMTF measured from the output images and the input ones was less than 1%, while this was of 13% when the rectangular function was used. When a sinc correction was applied in the latter case the difference decreased again to less than 1%. Conclusions: The developed modification routine provides the means to simulate the spatial resolution of digital x-ray detectors under a wider range of conditions. [ABSTRACT FROM AUTHOR]

Purpose: In digital breast tomosynthesis (DBT), a volumetric reconstruction of the breast is generated from a limited range of x-ray projections. One trade-off of DBT is resolution loss in the projections due to non-normal (i.e., oblique) x-ray incidence. Although degradation in image quality due to oblique incidence has been studied using empirical data and Monte Carlo simulations, a theoretical treatment has been lacking. The purpose of this work is to extend Swank's calculations of the transfer functions of turbid granular phosphors to oblique incidence. The model is ultimately used as a tool for optimizing the design of DBT detectors. Methods: A quantum-limited system and 20 keV x-rays are considered. Under these assumptions, the modulation transfer function (MTF) and noise power spectra (NPS) are derived using the diffusion approximation to the Boltzmann equation to model optical scatter within the phosphor. This approach is applicable to a nonstructured scintillator such as gadolinium oxysulfide doped with terbium (Gd2O2S:Tb), which is commonly used in breast imaging and which can reasonably approximate other detector materials. The detective quantum efficiency (DQE) is then determined from the Nishikawa formulation, where it is written as the product of the x-ray quantum detection efficiency, the Swank factor, and the Lubberts fraction. Transfer functions are calculated for both front- and back-screen configurations, which differ by positioning the photocathode at the exit or entrance point of the x-ray beam, respectively. Results: In the front-screen configuration, MTF and DQE are found to have considerable angular dependence, while NPS is shown to vary minimally with projection angle. As expected, the high frequency MTF and DQE are degraded substantially at large angles. By contrast, all transfer functions for the back-screen configuration have the advantage of significantly less angular dependence. Using these models, we investigated the possibility for optimizing the design of DBT detectors. As an example optimization strategy, the phosphor thickness which maximizes the DQE at a fixed frequency is analyzed. This work demonstrates that the optimal phosphor thickness for the front-screen is angularly dependent, shifting to lower thickness at higher angles. Conversely, the back-screen is not optimized by a single thickness but instead attains reasonably high DQE values over a large range of thicknesses. Although the back-screen configuration is not suited for current detectors using a glass substrate, it may prove to be preferred in future detectors using newly proposed plastic thin-film transistor (TFT) substrates. Conclusions: Using the diffusion approximation to the Boltzmann equation to model the spread of light in a scintillator, this paper develops an analytical model of MTF, NPS, and DQE for a phosphor irradiated obliquely. The model is set apart from other studies on oblique incidence in being derived from first principles. This work has applications in the optimization of DBT detector design. [ABSTRACT FROM AUTHOR]

Purpose: This paper describes a noniterative estimator for the energy dependent information from photon counting detectors with multibin pulse height analysis (PHA). The estimator uses the two function decomposition of the attenuation coefficient [R. E. Alvarez and A. Macovski, Phys. Med. Biol. 21, 733-744 (1976)] and its output is the line integrals of the basis set coefficients. The output noise variance and bias is compared to other noniterative estimators and to the Cramèr-Rao lower bound (CRLB). Methods: The estimator first computes an initial estimate from a linearized maximum likelihood estimator. The errors in the initial estimates are determined at a set of points from measurements on a calibration phantom. The errors at these known points are interpolated to create two-dimensional look up tables of corrections to the initial estimates. During image acquisition, the linearized maximum likelihood estimate for each data point is used as an input to the correction look up tables, and the final output is the sum of the estimate and the correction. The performance of the estimator is compared to generalizations of the polynomial and rational polynomial estimators for multibin data. The estimators are compared by the mean square error (MSE) and its components, the bias, and the variance of the output. The variance is also compared to the CRLB. The performance is simulated with two to five bins PHA data. The CRLB at a fixed object thickness is also computed as a function of the number of bins. Results: For two bin data, all the estimators' variances are equal to the CRLB. With three or more bins, only the proposed estimator achieves the CRLB while the others, which were not optimized for noise performance, have much larger output variance. The bias of the proposed estimator is equal to the polynomial estimator for calibration phantoms with 40 or more steps, that is, 1600 combinations of basis materials, but is larger than the rational polynomial bias. In all cases at the photon counts tested, the MSE is essentially equal to the variance, indicating that the bias errors are negligible compared to the variance. Conclusions: The estimator provides a noniterative method to compute the energy dependent information from multibin PHA data that achieves the CRLB over a wide range of operating conditions and has low output bias. The estimator can be calibrated based on the measurements of a calibration phantom; so, it does not require measurements of the x-ray energy spectrum or the detector response functions. [ABSTRACT FROM AUTHOR]

Purpose: This paper uses film dosimetry to investigate the relationship between multiple scan parameters of multidetector CT with automatic exposure control (AEC) and the surface dose distribution produced on a chest phantom.Methods: The characteristics of the film used in the film dosimeter were evaluated with regard to linearity, relative film response, and directional dependence. Measurements with an ionization chamber dosimeter and a water phantom were used to evaluate the accuracy of the film dosimeter measurements and to validate the dose profile measurements while changing the tube current, detector dimensions and pitch. When using AEC, the surface dose distribution on the chest phantom was analyzed while changing the detector dimensions and pitch.Results: The linearity, relative film response, and directional dependence of the film were established. The measurement difference between the film dosimeter and ionization chamber dosimeter was within ±5% and the dose profile measurement results were validated. It was found that the surface dose distribution changed helically in the direction of the body axis depending on the scan parameters and the phantom.Conclusions: Using a film dosimeter, the relationship between various multidetector CT scan parameters and the surface dose distribution on a chest phantom was investigated and clarified. [ABSTRACT FROM AUTHOR]

This paper investigates a method of reconstructing images from energy-resolved CT data with negligible beam-hardening artifacts and improved contrast-to-nosie ratio (CNR) compared to conventional energy-weighting methods. Conceptually, the investigated method first reconstructs separate images from each energy bin. The final image is a linear combination of the energy-bin images, with the weights chosen to maximize the CNR in the final image. The optimal weight of a particular energy-bin image is derived to be proportional to the contrast-to-noise-variance ratio in that image. The investigated weighting method is referred to as “image-based” weighting, although, as will be described, the weights can be calculated and the energy-bin data combined prior to reconstruction. The performance of optimal image-based energy weighting with respect to CNR and beam-hardening artifacts was investigated through simulations and compared to that of energy integrating, photon counting, and previously studied optimal “projection-based” energy weighting. Two acquisitions were simulated: dedicated breast CT and a conventional thorax scan. The energy-resolving detector was simulated with five energy bins. Four methods of estimating the optimal weights were investigated, including task-specific and task-independent methods and methods that require a single reconstruction versus multiple reconstructions. Results demonstrated that optimal image-based weighting improved the CNR compared to energy-integrating weighting by factors of 1.15–1.6 depending on the task. Compared to photon-counting weighting, the CNR improvement ranged from 1.0 to 1.3. The CNR improvement factors were comparable to those of projection-based optimal energy weighting. The beam-hardening cupping artifact increased from 5.2% for energy-integrating weighting to 12.8% for optimal projection-based weighting, while optimal image-based weighting reduced the cupping to 0.6%. Overall, optimal image-based energy weighting provides images with negligible beam-hardening artifacts and improved CNR compared to energy-integrating and photon-counting methods. [ABSTRACT FROM AUTHOR]

When choosing an electromagnetic tracking system (EMTS) for image-guided procedures several factors must be taken into consideration. Among others these include the system’s refresh rate, the number of sensors that need to be tracked, the size of the navigated region, the system interaction with the environment, whether the sensors can be embedded into the tools and provide the desired transformation data, and tracking accuracy and robustness. To date, the only factors that have been studied extensively are the accuracy and the susceptibility of EMTSs to distortions caused by ferromagnetic materials. In this paper the authors shift the focus from analysis of system accuracy and stability to the broader set of factors influencing the utility of EMTS in the clinical environment. The authors provide an analysis based on all of the factors specified above, as assessed in three clinical environments. They evaluate two commercial tracking systems, the Aurora system from Northern Digital Inc., and the 3D Guidance system with three different field generators from Ascension Technology Corp. The authors show that these systems are applicable to specific procedures and specific environments, but that currently, no single system configuration provides a comprehensive solution across procedures and environments. [ABSTRACT FROM AUTHOR]

The measured radiation beam profiles need to be corrected for the detector size effect to derive the real profiles. This paper describes two new semi-empirical procedures to determine the real profiles of high-energy x-ray beams by removing the detector size effect from the measured profiles. Measured profiles are corrected by shifting the position of each measurement point by a specific amount determined from available theoretical and experimental knowledge in the literature. The authors developed two procedures to determine the amount of shift. In the first procedure, which employs the published analytical deconvolution procedure of other investigators, the shift is determined from the comparison of the analytical fit of the measured profile and the corresponding analytical real profile derived from the deconvolution of the fitted measured profile and the Gaussian detector response function. In the second procedure, the amount of shift at any measurement point is considered to be proportional to the value of an analytical function related to the second derivative of the real profile at that point. The constant of proportionality and a parameter in the function are obtained from the values of the shifts at the 90%, 80%, 20%, and 10% dose levels, which are experimentally known from the published results of other investigators to be approximately equal to half of the radius of the detector. These procedures were tested by correcting the profiles of 6 and 18 MV x-ray beams measured by three different ionization chambers and a stereotactic field diode detector with 2.75, 2, 1, and 0.3 mm radii of their respective active cylindrical volumes. The corrected profiles measured by different detectors are found to be in close agreement. The detector size corrected penumbra widths also agree with the expected values based on the results of an earlier investigation. Thus, the authors concluded that the proposed procedures are accurate and can be used to derive the real profiles of clinical high-energy x-ray beams. [ABSTRACT FROM AUTHOR]

The H operator represents the deterministic performance of any imaging system. For a linear, digital imaging system, this system operator can be written in terms of a matrix, H, that describes the deterministic response of the system to a set of point objects. A singular value decomposition of this matrix results in a set of orthogonal functions (singular vectors) that form the system basis. A linear combination of these vectors completely describes the transfer of objects through the linear system, where the respective singular values associated with each singular vector describe the magnitude with which that contribution to the object is transferred through the system. This paper is focused on the measurement, analysis, and interpretation of the H matrix for digital x-ray detectors. A key ingredient in the measurement of the H matrix is the detector response to a single x ray (or infinitestimal x-ray beam). The authors have developed a method to estimate the 2D detector shift-variant, asymmetric ray response function (RRF) from multiple measured line response functions (LRFs) using a modified edge technique. The RRF measurements cover a range of x-ray incident angles from 0° (equivalent location at the detector center) to 30° (equivalent location at the detector edge) for a standard radiographic or cone-beam CT geometric setup. To demonstrate the method, three beam qualities were tested using the inherent, Lu/Er, and Yb beam filtration. The authors show that measures using the LRF, derived from an edge measurement, underestimate the system’s performance when compared with the H matrix derived using the RRF. Furthermore, the authors show that edge measurements must be performed at multiple directions in order to capture rotational asymmetries of the RRF. The authors interpret the results of the H matrix SVD and provide correlations with the familiar MTF methodology. Discussion is made about the benefits of the H matrix technique with regards to signal detection theory, and the characterization of shift-variant imaging systems. [ABSTRACT FROM AUTHOR]

Helical CTs are well known to suffer from aliasing artifacts because of their finite longitudinal sampling pitch. The artifact pattern is typically strong streaks from bone edges in clinical images. Especially in the case of multidetector row CT, the artifact resulting from longitudinal aliasing is often called a windmill artifact because the visible streaks form a windmill pattern when the object is of a particular shape. The scan must be performed using a very thin slice thickness, i.e., fine sampling in the longitudinal direction, with a longer scan time to mitigate this aliasing artifact. Some elaborate longitudinal interpolation methods to remediate longitudinal aliasing have been proposed, but they have not been successful in practice despite their theoretical importance. A periodic swing of the focal spot in the longitudinal direction, a so-called z-flying focal spot, was introduced recently to achieve finer sampling. Although it is a useful technique, some important deficiencies exist: It is sufficiently effective only near the isocenter and is difficult to apply to a scan using a thick slice thickness, even though longitudinal aliasing is more serious at the thicker scan. In this paper, the author addresses the nature of interlaced (or unequally spaced) sampling and derives a new principle of data treatment that can suppress the aliased spectra selectively. According to this principle, the common practice of image reconstruction, which backprojects data along the original sampling ray path, is never the best choice. The author proposes a new scheme of backprojection, which involves the longitudinal shift of projection data. A proper choice of longitudinal shift for backprojection provides effective and selective suppression of aliased spectra, with retention of the original frequency spectrum depending on the level of focus swing. With this shifted backprojection, the swing of focus can be made much smaller than for a conventional z-flying focal spot. The required amount of shift for backprojection is position dependent. Nevertheless, its implementation in the reconstruction process can be achieved simply by relocating the x-ray source and detector assembly from positions of actual scanning. Through simulation, the combination of shifted backprojection and the small swing of focus is evaluated. Results confirm that the artifact attributable to longitudinal aliasing is well suppressed in the entire field of view, whereas the penalty on the slice sensitivity profile (or longitudinal resolution) can be kept minimal. Moreover, this method solves other deficiencies of z-flying focus, such as inapplicability to scans with a thicker slice thickness. [ABSTRACT FROM AUTHOR]

In coronary calcium scoring, motion artifacts affecting calcified plaques are commonly characterized using descriptive terms, which incorporate an element of subjectivity in their interpretations. Quantitative indices may improve the objective characterization of these motion artifacts. In this paper, an automated method for generating 12 quantitative indices, i.e., features that characterize the motion artifacts affecting calcified plaques, is presented. This method consists of using the rapid phase-correlated region-of-interest (ROI) tracking algorithm for reconstructing ROI images of calcified plaques automatically from the projection data obtained during a cardiac scan, and applying methods for extracting features from these images. The 12 features include two dynamic, six morphological, and four intensity-based features. The two dynamic features are three-dimensional (3D) velocity and 3D acceleration. The six morphological features include edge-based volume, threshold-based volume, sphericity, irregularity, average margin gradient, and variance of margin gradient. The four intensity-based features are maximum intensity, mean intensity, minimum intensity, and standard deviation of intensity. The 12 features were extracted from 54 reconstructed sets of simulated four-dimensional images from the dynamic NCAT phantom involving six calcified plaques under nine heart rate/multi-sector gating combinations. In order to determine how well the 12 features correlated with a plaque motion index, which was derived from the trajectory of the plaque, partial correlation coefficients adjusted for heart rate, number of gated sectors, and mean feature values of the six plaques were calculated for all 12 features. Features exhibiting stronger correlations (|r|∈[0.60,1.00]) with the motion index were 3D velocity, maximum intensity, and standard deviation of intensity. Features demonstrating stronger correlations (|r|∈[0.60,1.00]) with other features mostly involved intensity-based features. Edge-based volume/irregularity and average margin gradient/variance of margin gradient were the only two feature pairs out of 12 with stronger correlations that did not involve intensity-based features. Automatically extracted features of the motion artifacts affecting calcified plaques in cardiac computed tomography images potentially can be used to develop models for predicting image assessability with respect to motion artifacts. [ABSTRACT FROM AUTHOR]

Background: Flat panel detector (FPD) based cone‐beam computed tomography (CT) has made tremendous progress in the last two decades, with many new and advanced medical and industrial applications keeping emerging from diagnostic imaging and image guidance for radiotherapy and interventional surgery. The current cone‐beam CT (CBCT), however, is still suboptimal for head CT scan which requires a high standard of image quality. While the dual‐layer FPD technology is under extensive development and is promising to further advance CBCT from qualitative anatomic imaging to quantitative dual‐energy CT, its potential of enabling head CBCT applications has not yet been fully investigated. Purpose: The relatively moderate energy separation from the dual‐layer FPD and the overall low signal level especially at the bottom‐layer detector, could raise significant challenges in performing high‐quality dual‐energy material decomposition (MD). In this work, we propose a hybrid, physics and model guided, MD algorithm that attempts to fully use the detected x‐ray signals and prior‐knowledge behind head CBCT using dual‐layer FPD. Methods: Firstly, a regular projection‐domain MD is performed as initial results of our approach and for comparison as conventional method. Secondly, based on the combined projection, a dual‐layer multi‐material spectral correction (dMMSC) is applied to generate beam hardening free images. Thirdly, the dMMSC corrected projections are adopted as a physics‐model based guidance to generate a hybrid MD. A set of physics experiments including fan‐beam scan and cone‐beam scan using a head phantom and a Gammex Multi‐Energy CT phantom are conducted to validate our proposed approach. Results: The combined reconstruction could reduce noise by about 10% with no visible resolution degradation. The fan‐beam studies on the Gammex phantom demonstrated an improved MD performance, with the averaged iodine quantification error for the 5–15 mg/ml iodine inserts reduced from about 5.6% to 3.0% by the hybrid method. On fan‐beam scan of the head phantom, our proposed hybrid MD could significantly reduce the streak artifacts, with CT number nonuniformity (NU) in the selected regions of interest (ROIs) reduced from 23 Hounsfield Units (HU) to 4.2 HU, and the corresponding noise suppressed from 31 to 6.5 HU. For cone‐beam scan, after scatter correction (SC) and cone‐beam artifact reduction (CBAR), our approach can also significantly improve image quality, with CT number NU in the selected ROI reduced from 24.2 to 6.6 HU and the noise level suppressed from 22.1 to 8.2 HU. Conclusions: Our proposed physics and model guided hybrid MD for dual‐layer FPD based head CBCT can significantly improve the robustness of MD and suppress the low‐signal artifact. This preliminary feasibility study also demonstrated that the dual‐layer FPD is promising to enable head CBCT spectral imaging. [ABSTRACT FROM AUTHOR]

Background: The incorporation of multi‐energy capabilities into radiotherapy flat‐panel detectors offers advantages including enhanced soft tissue visualization by reduction of signal from overlapping anatomy such as bone in 2D image projections; creation of virtual monoenergetic images for 3D contrast enhancement, metal artefact reduction and direct acquisition of relative electron density. A novel dual‐layer on‐board imager offering dual energy processing capabilities is being designed. As opposed to other dual‐energy implementation techniques which require separate acquisition with two different x‐ray spectra, the dual‐layer detector design enables simultaneous acquisition of high and low energy images with a single exposure. A computational framework is required to optimize the design parameters and evaluate detector performance for specific clinical applications. Purpose: In this study, we report on the development of a Monte Carlo (MC) model of the imager including model validation. Methods: The stack‐up of the dual‐layer imager (DLI) was implemented in GEANT4 Application for Tomographic Emission (GATE). The DLI model has an active area of 43×43 cm2, with top and bottom Cesium Iodide (CsI) scintillators of 600 and 800 μm thickness, respectively. Measurement of spatial resolution and imaging of dedicated multi‐material dual‐energy (DE) phantoms were used to validate the model. The modulation transfer function (MTF) of the detector was calculated for a 120 kVp x‐ray spectrum using a 0.5 mm thick tantalum edge rotated by 2.5o. For imaging validation, the DE phantom was imaged using a 140 kVp x‐ray spectrum. For both validation simulations, corresponding measurements were done using an initial prototype of the imager. Agreement between simulations and measurement was assessed using normalized root mean square error (NRMSE) and 1D profile difference for the MTF and phantom images respectively. Further comparison between measurement and simulation was made using virtual monoenergetic images (VMIs) generated from basis material images derived using precomputed look‐up tables. Results: The MTF of the bottom layer of the dual‐layer model shows values decreasing more quickly with spatial frequency, compared to the top layer, due to the thicker bottom scintillator thickness and scatter from the top layer. A comparison with measurement shows NRMSE of 0.013 and 0.015 as well as identical MTF50 of 0.8 mm1 and 1.0 mm1 for the top and bottom layer respectively. For the DE imaging of the DE‐phantom, although a maximum deviation of 3.3% is observed for the 10 mm aluminum and Teflon inserts at the top layer, the agreement for all other inserts is less than 2.2% of the measured value at both layers. Material decomposition of simulated scatter‐free DE images gives an average accuracy in PMMA and aluminum composition of 4.9% and 10.3% for 11–30 mm PMMA and 1–10 mm aluminum objects respectively. A comparison of decomposed values using scatter containing measured and simulated DE images shows good agreement within statistical uncertainty. Conclusion: Validation using both MTF and phantom imaging shows good agreement between simulation and measurements. With the present configuration of the digital prototype, the model can generate material decomposed images and virtual monoenergetic images. [ABSTRACT FROM AUTHOR]

Background: The beam energy is one of the most significant parameters in particle therapy since it is directly correlated to the particles' penetration depth inside the patient. Nowadays, the range accuracy is guaranteed by offline routine quality control checks mainly performed with water phantoms, 2D detectors with PMMA wedges, or multi‐layer ionization chambers. The latter feature low sensitivity, slow collection time, and response dependent on external parameters, which represent limiting factors for the quality controls of beams delivered with fast energy switching modalities, as foreseen in future treatments. In this context, a device based on solid‐state detectors technology, able to perform a direct and absolute beam energy measurement, is proposed as a viable alternative for quality assurance measurements and beam commissioning, paving the way for online range monitoring and treatment verification. Purpose: This work follows the proof of concept of an energy monitoring system for clinical proton beams, based on Ultra Fast Silicon Detectors (featuring tenths of ps time resolution in 50 μm active thickness, and single particle detection capability) and time‐of‐flight techniques. An upgrade of such a system is presented here, together with the description of a dedicated self‐calibration method, proving that this second prototype is able to assess the mean particles energy of a monoenergetic beam without any constraint on the beam temporal structure, neither any a priori knowledge of the beam energy for the calibration of the system. Methods: A new detector geometry, consisting of sensors segmented in strips, has been designed and implemented in order to enhance the statistics of coincident protons, thus improving the accuracy of the measured time differences. The prototype was tested on the cyclotron proton beam of the Trento Protontherapy Center (TPC). In addition, a dedicated self‐calibration method, exploiting the measurement of monoenergetic beams crossing the two telescope sensors for different flight distances, was introduced to remove the systematic uncertainties independently from any external reference. Results: The novel calibration strategy was applied to the experimental data collected at TPC (Trento) and CNAO (Pavia). Deviations between measured and reference beam energies in the order of a few hundreds of keV with a maximum uncertainty of 0.5 MeV were found, in compliance with the clinically required water range accuracy of 1 mm. Conclusions: The presented version of the telescope system, minimally perturbative of the beam, relies on a few seconds of acquisition time to achieve the required clinical accuracy and therefore represents a feasible solution for beam commission, quality assurance checks, and online beam energy monitoring. [ABSTRACT FROM AUTHOR]

Background: Small‐animal positron emission tomography (PET) systems are widely used in molecular imaging research and drug development. There is also growing interest in organ‐dedicated clinical PET systems. In these small‐diameter PET systems, the measurement of the depth‐of‐interaction (DOI) of annihilation photons in scintillation crystals allows for the correction of parallax error in PET system, leading to an improvement on the spatial resolution uniformity. The DOI information is also useful for improving the timing resolution of PET system as it enables the correction of DOI‐dependent time walk in the arrival time difference measurement of annihilation photon pairs. The dual‐ended readout scheme is one of the most widely investigated DOI measurement methods, which collects visible photons using a pair of photosensors located at both ends of the scintillation crystal. Although the dual‐ended readout allows for simple and accurate DOI estimation, it requires twice the number of photosensors compared to the single‐ended readout scheme. Purpose: To effectively reduce the number of photosensors in a dual‐ended readout scheme, we propose a novel PET detector configuration that employs 45° tilted and sparsely arranged silicon photomultipliers (SiPMs). In this configuration, the angle between the scintillation crystal and SiPM is 45°. Therefore, and thus, the diagonal of the scintillation crystal matches one of the lateral sides of the SiPM. Accordingly, it allows for the use of SiPM device larger than the scintillation crystal, thereby improving light collection efficiency with a higher fill factor and reducing SiPM quantity. In addition, all scintillation crystals can achieve more uniform performance than other dual‐ended readout methods with a sparse SiPM arrangement because 50% of the scintillation crystal cross section is commonly in contact with the SiPM. Methods: To demonstrate the feasibility of our proposed concept, we implemented a PET detector that employs a 4×${\rm{\;}} \times \;$4 LSO block with a single crystal dimension of 3.03 × 3.03 × 20 mm3 and a 45° tilted SiPM array. The 45° tilted SiPM array consists of 2 × 3 SiPM elements at the top ("Top SiPM") and 3 × 2 SiPM elements at the bottom ("Bottom SiPM"). Each crystal element of the 4 × 4 LSO block is optically coupled with each quarter section of the Top SiPM and Bottom SiPM pair. To characterize the performance of the PET detector, the energy, DOI, and timing resolution were measured for all 16 crystals. The energy data was obtained by summing all the charges from the Top SiPMs and Bottom SiPMs, and the DOI resolution was measured by irradiating the side of the crystal block at five different depths (2, 6, 10, 14, and 18 mm). The timing was estimated by averaging the arrival time of the annihilation photons measured at the Top SiPMs and Bottom SiPMs (Method 1). The DOI‐dependent time‐walk effect was further corrected by using DOI information and statistical variations in the trigger times at the Top SiPMs and Bottom SiPMs (Method 2). Results: The average DOI resolution of the proposed PET detector was 2.5 mm, thereby resolving the DOI at five different depths, and the average energy resolution was 16% full width at half maximum (FWHM). When Methods 1 and 2 were applied, the coincidence timing resolutions were 448 and 411 ps FWHM, respectively. Conclusions: We expect that our novel low‐cost PET detector design with 45° tilted SiPMs and a dual‐ended readout scheme would be a suitable solution for constructing a high‐resolution PET system with DOI encoding capability. [ABSTRACT FROM AUTHOR]

Background: In Magnetic Resonance-Linac (MR-Linac) dosimetry formalisms, a new correction factor, kB, Q, has been introduced to account for corresponding changes to detector readings under the beam quality, Q, and the presence of magnetic field, B. Purpose: This study aims to develop and implement a Monte Carlo (MC)-based framework for the determination of kB, Q correction factors for a series of ionization chambers utilized for dosimetry protocols and dosimetric quality assurance checks in clinical 1.5 T MR-Linacs. Their dependencies on irradiation setup conditions are also investigated. Moreover, to evaluate the suitability of solid phantoms for dosimetry checks and end-to-end tests, changes to the detector readings due to the presence of small asymmetrical air gaps around the detector's tip are quantified. Methods: Phase space files for three irradiation fields of the ELEKTA Unity 1.5 T/7 MV flattening-filter-free MR-Linac were provided by the manufacturer and used as source models throughout this study. Twelve ionization chambers (three farmer-type and nine small-cavity detectors, from three manufacturers) were modeled (including their dead volume) using the EGSnrc MC code package. kB, Q values were calculated for the 10 × 10 cm2 irradiation field and for four cardinal orientations of the detectors' axes with respect to the 1.5 T magnetic field. Potential dependencies of kB,Q values with respect to field size, depth, and phantom material were investigated by performing additional simulations. Changes to the detectors' readings due to the presence of small asymmetrical air gaps (0.1 up to 1 mm) around the chambers'sensitive volume in an RW3 solid phantom were quantified for three small-cavity chambers and two orientations. Results: For both parallel (to the magnetic field) orientations, kB,Q values were found close to unity. The maximum correction needed was 1.1%.For each detector studied, the kB,Q values calculated for the two parallel orientations agreed within uncertainties. Larger corrections (up to 5%) were calculated when the detectors were oriented perpendicularly to themagnetic field. Results were compared with corresponding ones found in the literature, wherever available. No considerable dependence of kB,Q with respect to field size (down to 3 × 3 cm2), depth, or phantom material was noticed, for the detectors investigated. As compared to the perpendicular one, in the parallel to the magnetic field orientation, the air gap effect is minimized but is still considerable even for the smallest air gap considered (0.1 mm). Conclusion: For the 10 × 10 cm2 field, magnetic field correction factors for 12 ionization chambers and four orientations were determined. For each detector, the kB,Q value may be also applied for dosimetry procedures under different irradiation parameters provided that the orientation is taken into account. Moreover, if solid phantoms are used, even the smallest asymmetrical air gap may still bias small-cavity chamber response. This work substantially expands the availability and applicability of kB,Q correction factors that are detector- and orientationspecific, enabling more options in MR-Linac dosimetry checks, end-to-end tests, and quality assurance protocols. [ABSTRACT FROM AUTHOR]

Purpose: Accurate measurement of out‐of‐field dose in radiotherapy directly impacts beam data modeling in treatment planning systems, verification of implanted electronic devices/lens/fetus dose, secondary cancer risk estimation, and organ‐at‐risk dose reporting. When performing out‐of‐field dosimetry, it is therefore imperative that the response of the detector has been well characterized. Due to the softening of the radiation beam out‐of‐field, many detectors will exhibit energy dependence. This study investigated the energy dependence of a range of clinical available detectors over typical energies experienced out‐of‐field. Methods: The response of detectors to photon beams from 70 kV to 6 MV was measured. The relative change in response from 6 MV down to 70 kV highlighted the expected deviation in the response of detectors that would typically be calibrated in‐field for use out‐of‐field. Results: The Pinpoint detector displayed the most energy‐independent response over the energy range investigated. The Micro‐Lion detector was the only detector to show an under‐response to all low‐energy beams relative to 6 MV. The diode‐type detectors showed the largest energy dependence. Conclusions: When considering detectors for use in out‐of‐field dose measurements, it is important that the energy dependence is investigated over a low‐energy range as out‐of‐field the energy spectra comprise a larger component of photons in the 50–100‐keV range. This study highlights the variation in response of a range of clinically available detectors to low‐energy radiation beams relative to 6 MV for out‐of‐field dosimetry. The Pinpoint detector was the most energy‐independent detector with a response close to unity over the entire energy range investigated. [ABSTRACT FROM AUTHOR]

Background: Current clinical positron emission tomography (PET) systems utilize detectors where the scintillator typically contains single elements of 3–6‐mm width and about 20‐mm height. While providing good time‐of‐flight performance, this design limits the spatial resolution and causes radial astigmatism as the depth‐of‐interaction (DOI) remains unknown. Purpose: We propose an alternative, aiming to combine the advantages of current detectors with the DOI capabilities shown for monolithic concepts, based on semi‐monolithic scintillators (slabs). Here, the optical photons spread along one dimension enabling DOI‐encoding with a still small readout area beneficial for timing performance. Methods: An array of eight monolithic LYSO slabs of dimensions 3.9 × 32 × 19 mm3 was read out by a 64‐channel photosensor containing digital SiPMs (DPC3200‐22‐44, Philips Digital Photon Counting). The position estimation in the detector's monolithic and DOI direction was based on a calibration with a fan beam collimator and the machine learning technique gradient tree boosting (GTB). Results: We achieved a positioning performance in terms of mean absolute error (MAE) of 1.44 mm for the monolithic direction and 2.12 mm for DOI considering a wide energy window of 300–700 keV. The energy resolution was determined to be 11.3%, applying a positional‐dependent energy calibration. We established both an analytical and machine‐learning‐based timing calibration approach and applied them for a first‐photon trigger. The analytical timing calibration corrects for electronic and optical time skews leading to 240 ps coincidence resolving time (CRT) for a pair of slab‐detectors. The CRT was significantly improved by utilizing GTB to predict the time difference based on specific training data and applied on top of the analytical calibration. We achieved 209 ps for the wide energy window and 198 ps for a narrow selection around the photopeak (411–561 keV). To maintain the detector's sensitivity, no filters were applied to the data during processing. Conclusion: Overall, the semi‐monolithic detector provides attractive performance characteristics. Especially, a good CRT can be achieved while introducing DOI capabilities to the detector, making the concept suitable for clinical PET scanners. [ABSTRACT FROM AUTHOR]

Purpose: FLASH radiotherapy (RT) is an emerging technique in which beams with ultra‐high dose rates (UH‐DR) and dose per pulse (UH‐DPP) are used. Commercially available active real‐time dosimeters have been shown to be unsuitable in such conditions, due to severe response nonlinearities. In the present study, a novel diamond‐based Schottky diode detector was specifically designed and realized to match the stringent requirements of FLASH‐RT. Methods: A systematic investigation of the main features affecting the diamond response in UH‐DPP conditions was carried out. Several diamond Schottky diode detector prototypes with different layouts were produced at Rome Tor Vergata University in cooperation with PTW‐Freiburg. Such devices were tested under electron UH‐DPP beams. The linearity of the prototypes was investigated up to DPPs of about 26 Gy/pulse and dose rates of approximately 1 kGy/s. In addition, percentage depth dose (PDD) measurements were performed in different irradiation conditions. Radiochromic films were used for reference dosimetry. Results: The response linearity of the diamond prototypes was shown to be strongly affected by the size of their active volume as well as by their series resistance. By properly tuning the design layout, the detector response was found to be linear up to at least 20 Gy/pulse, well into the UH‐DPP range conditions. PDD measurements were performed by three different linac applicators, characterized by DPP values at the point of maximum dose of 3.5, 17.2, and 20.6 Gy/pulse, respectively. The very good superimposition of three curves confirmed the diamond response linearity. It is worth mentioning that UH‐DPP irradiation conditions may lead to instantaneous detector currents as high as several mA, thus possibly exceeding the electrometer specifications. This issue was properly addressed in the case of the PTW UNIDOS electrometers. Conclusions: The results of the present study clearly demonstrate the feasibility of a diamond detector for FLASH‐RT applications. [ABSTRACT FROM AUTHOR]

Purpose: This paper introduces a new strategy for simulating low-dose computed tomography (CT) scans using real scans of a higher dose as an input. The tool is verified against simulations and real scans and compared to other approaches found in the literature. Methods: The conditional variance identity is used to properly account for the variance of the input high-dose data, and a formula is derived for generating a new Poisson noise realization which has the same mean and variance as the true low-dose data. The authors also derive a formula for the inclusion of real samples of detector noise, properly scaled according to the level of the simulated x-ray signals. Results: The proposed method is shown to match real scans in number of experiments. Noise standard deviation measurements in simulated low-dose reconstructions of a 35 cm water phantom match real scans in a range from 500 to 10 mA with less than 5% error. Mean and variance of individual detector channels are shown to match closely across the detector array. Finally, the visual appearance of noise and streak artifacts is shown to match in real scans even under conditions of photon-starvation (with tube currents as low as 10 and 80 mA). Additionally, the proposed method is shown to be more accurate than previous approaches (1) in achieving the correct mean and variance in reconstructed images from pure-Poisson noise simulations (with no detector noise) under photon-starvation conditions, and (2) in simulating the correct noise level and detector noise artifacts in real low-dose scans. Conclusions: The proposed method can accurately simulate low-dose CT data starting from high-dose data, including effects from photon starvation and detector noise. This is potentially a very useful tool in helping to determine minimum dose requirements for a wide range of clinical protocols and advanced reconstruction algorithms. [ABSTRACT FROM AUTHOR]

Purpose: Detectors for positron emission tomography (PET) typically use two types of scintillation crystals, pixelated or monolithic. A variant of these types of scintillators are the so‐called semi‐monolithic crystals. They consist of a monolithic crystal segmented in one direction in pieces called slabs. These scintillators have the potential to successfully combine the benefits of pixelated and monolithic configurations, providing good timing and spatial resolutions as well as the capacity to decode the depth of interaction (DOI) information. In this work, the timing performance of a detector based on semi‐monolithic crystals was studied in depth. The energy response was also evaluated. Methods: The semi‐monolithic detector consists of 1 × 24 LYSO slabs of 25.4 × 12 × 0.95 mm3 each. The bottom surface of the slabs is coupled to an array of 8 × 8 silicon photomultipliers (SiPMs) of 3 × 3 mm2 active area, 50 μm cell size and 3.2 mm pitch. The 64 output signals were independently readout by the TOFPET2 ASIC. In order to achieve the best coincidence time resolution (CTR), four different time walk corrections were tested. Additional work investigated the best method of combining the timestamps belonging to the same event. Results: The resolvability of the slabs in the measured flood maps improves with the thickness of a light guide placed in between the scintillators and photosensors. The energy resolution does not change significantly with values as good as 13.7%. Regarding the CTR, values of 335.8, 363, 369.8, and 402.5 ps have been obtained for the whole detector for no light guide, 0.5, 1.0, and 1.5 mm thickness light guide cases, respectively. These values further improve to 276.1, 302.6, 305.6 and 336.2 ps, respectively, when energy‐weighted averaging of timestamps is applied. Conclusions: We have shown both an excellent timing resolution and good energy resolution for a PET detector based on semi‐monolithic crystals. The use of light guides of different thicknesses does not significantly affect the energy resolution of the whole detector, but the timing capabilities slightly worsen with the increasing thickness of the light guide. [ABSTRACT FROM AUTHOR]

Purpose: This study reports a single‐institution experience with beam data acquisition and film‐based validation for a novel self‐shielded sterotactic radiosurgery unit and investigates detector dependency on field output factors (OFs), off‐axis ratios (OARs), and percent depth dose (PDD) measurements within the context of small‐field dosimetry. Methods: The delivery platform for this unit consists of a 2.7‐MV S‐band linear accelerator mounted on coupled gimbals that rotate around a common isocenter (source‐to‐axis distance [SAD] = 450 mm), allowing for more than 260 noncoplanar beam angles. Beam collimation is achieved via a tungsten collimator wheel with eight circular apertures ranging from 4 mm to 25 mm in diameter. Three diodes (PTW 60012 Diode E, PTW 60018 SRS Diode, and Sun Nuclear EDGE) and a synthetic diamond detector (PTW 60019 micro Diamond [µD] detector) were used for OAR, PDD, and OF measurements. OFs were also acquired with a PTW 31022 PinPoint ionization chamber. Beam scanning was performed using a 3D water tank at depths of 7, 50, 100, 200, and 250 mm with a source‐to‐surface distance of 450 mm. OFs were measured at the depth of maximum dose (dmax = 7 mm) with the SAD at 450 mm. Gafchromic EBT3 film was used to validate OF and profile measurements and as a reference detector for estimating correction factors for active detector OFs. Deviations in field size, penumbra, and PDDs across the different detectors were quantified. Results: Relative OFs (ROFs) for the diodes were within 1.4% for all collimators except for 5 and 7.5 mm, for which SRS Diode measurements were higher by 1.6% and 2.6% versus Diode E. The µD ROFs were within 1.4% of the diode measurements. PinPoint ROFs were lower by >10% for the 4‐mm and 5‐mm collimators versus the Diode E and µD. Corrections to OFs using EBT3 film as a reference were within 1.2% for all diodes and the µD detector for collimators 10 mm and greater and within 2.0%, 2.8%, and 1.1% for the 7.5‐, 5‐, and 4‐mm collimators, respectively. The maximum difference in full width at half maximum (FWHM) between the Diode E and the other active detectors was for the 25‐mm collimator and was 0.09 mm (µD), 0.16 mm (SRS Diode), and 0.65 mm (EDGE). Differences seen in PDDs beyond the depth of dmax were <1% across the three diodes and the µD. FWHM and penumbra measurements made using EBT3 film were within 1.34% and 3.26%, respectively, of the processed profile data entered into the treatment planning system. Conclusions: Minimal differences were seen in OAR and PDD measurements acquired with the diodes and the µD. ROFs measured with the three diodes were within 2.6% and within 1.4% versus the µD. Gafchromic Film measurements provided independent verification of the OAR and OF measurements. Estimated corrections to OFs using film as a reference were <1.6% for the Diode E, EDGE, and µD detector. [ABSTRACT FROM AUTHOR]

Purpose: This study aims to develop a calibration‐based estimator for the photon‐counting detector (PCD)‐based x‐ray computed tomography. Methods: We propose the nearest neighborhood (NN)‐based estimator, which searches for the nearest calibration data for a given PCD output and sets the associated basis line‐integrals as the estimate. Searching for the nearest neighbors can be accelerated using the pre‐calculated k‐d tree for the data. Results: The proposed method is compared to the model‐based maximum likelihood (ML) estimator. For slab phantom study, both ML and NN‐based methods achieve the Cramér‐Rao lower bound and are unbiased for various combinations of three basis materials (water, bone, and gold). The proposed method is also validated for K‐edge imaging and presents almost unbiased Au concentrations in the region of interest. Conclusions: The proposed NN‐based method is demonstrated to be as accurate as the model‐based ML estimator, but it is computationally efficient and requires only calibration measurements. [ABSTRACT FROM AUTHOR]

Purpose: Spectral CT uses energy‐dependent measurements that enable material discrimination in addition to reconstruction of structural information. Flat‐panel detectors (FPDs) have been widely used in dedicated and interventional systems to deliver high spatial resolution, volumetric cone‐beam CT (CBCT) in compact and OR‐friendly designs. In this work, we derive a model‐based method that facilitates high‐resolution material decomposition in a spectral CBCT system equipped with a prototype dual‐layer FPD. Through high‐fidelity modeling of multilayer detector, we seek to avoid resolution loss that is present in more traditional processing and decomposition approaches. Method: A physical model for spectral measurements in dual‐layer flat‐panel CBCT is developed including layer‐dependent differences in system geometry, spectral sensitivities, and detector blur (e.g., due to varied scintillator thicknesses). This forward model is integrated into a model‐based material decomposition (MBMD) method based on minimization of a penalized weighted least‐squared (PWLS) objective function. The noise and resolution performance of this approach was compared with traditional projection‐domain decomposition (PDD) and image‐domain decomposition (IDD) approaches as well as one‐step MBMD with lower‐fidelity models that use approximated geometry, projection interpolation, or an idealized system geometry without system blur model. Physical studies using high‐resolution three‐dimensional (3D)‐printed water‐iodine phantoms were conducted to demonstrate the high‐resolution imaging performance of the compared decomposition methods in iodine basis images and synthetic monoenergetic images. Results: Physical experiments demonstrate that the MBMD methods incorporating an accurate geometry model can yield higher spatial resolution iodine basis images and synthetic monoenergetic images than PDD and IDD results at the same noise level. MBMD with blur modeling can further improve the spatial‐resolution compared with the decomposition results obtained with IDD, PDD, and MBMD methods with lower‐fidelity models. Using the MBMD without or with blur model can increase the absolute modulation at 1.75 lp/mm by 10% and 22% compared with IDD at the same noise level. Conclusion: The proposed model‐based material decomposition method for a dual‐layer flat‐panel CBCT system has demonstrated an ability to extend high‐resolution performance through sophisticated detector modeling including the layer‐dependent blur. The proposed work has the potential to not only facilitate high‐resolution spectral CT in interventional and dedicated CBCT systems, but may also provide the opportunity to evaluate different flat‐panel design trade‐offs including multilayer FPDs with mismatched geometries, scintillator thicknesses, and spectral sensitivities. [ABSTRACT FROM AUTHOR]

Purpose: Spectral distortion due to charge sharing (CS) and pulse pileup (PP) in photon‐counting detectors (PCDs) degrades the quality of PCD data. We recently proposed multi‐energy inter‐pixel coincidence counters (MEICC) that provided spectral cross‐talk information related to CS. When PP was absent, the normalized Cramér–Rao lower bounds (nCRLBs) of 225‐µm pixel PCDs with MEICC was comparable to those of 450‐µm pixel PCD without MEICC. The aim of this study was to assess the performance of PCDs with MEICC in the presence of both CS and PP using computer simulations. Methods: An in‐house Monte Carlo program was modified to incorporate the following four temporal elements: (1) A pulse shape with a pulse duration of 20 ns, (2) delays of up to 10 ns in anode arrival times when photons were incident on pixel boundaries, (3) offsets proportional to a vertical separation between the primary and secondary charge clouds at the rate of ±4 ns per ±100 µm, and (4) a stochastic fluctuation of anode arrival times for all of the charge clouds with a standard deviation of 2 ns. We assessed the performance of five PCDs, (a)–(f), for three spectral tasks, (A)–(C): (a) The conventional PCD, (b) a PCD with MEICC, (c) a PCD with one coincidence counter (1CC), (d) a PCD with a 3 × 3 analog charge summing scheme (ACS), and (e) a PCD with a 3 × 3 digital count summing scheme (DCS); (A) conventional CT imaging with water (i.e., linear attenuation coefficient maps), (B) water–bone material decomposition, and (C) K‐edge imaging with tungsten. The tube current was changed from 1 mA to 1000 mA and the nCRLB was assessed. Results: The recorded count rate curves were fitted by the non‐paralyzable detection model with the effective deadtime parameter. The best fit was achieved by 25.8 ns for the conventional PCD, 18.6 ns for MEICC and 1CC, 140.5 ns for ACS, and 209.0 ns for DCS. The nCRLBs were strongly dependent on count rates. MEICC provided the best nCRLBs for all of the imaging tasks over the count rate range investigated except for a few conditions such as K‐edge imaging at 1 mA. PP decreased the merit of MEICC over the conventional PCD in addressing CS. Nonetheless, MEICC consistently provided better nCRLBs than the conventional PCD did. The nCRLBs of MEICC were in the range of 49–58% of those of the conventional PCD for K‐edge imaging, 45–76% for water–bone material decomposition, and 81–88% for the conventional CT imaging (i.e., linear attenuation coefficient maps). ACS provided better nCRLBs than the conventional PCD did only when the effect of PP was minor (e.g., when the counting efficiency of the conventional PCD was higher than 0.95 with the tube current of up to 100 mA). Conclusion: Besides a few cases, MEICC provides the best nCRLBs for all of the tasks at all of the count rates. ACS and DCS provide better nCRLBs than the conventional PCD does only when count rates are very low. [ABSTRACT FROM AUTHOR]

Purpose: Medical procedures can be difficult to perform on anatomy that is constantly moving. Respiration displaces internal organs by up to several centimeters with respect to the surface of the body, and patients often have limited ability to hold their breath. Strategies to compensate for motion during diagnostic and therapeutic procedures require reliable information to be available. However, current devices often monitor respiration indirectly, through changes on the outline of the body, and they may be fixed to floors or ceilings, and thus unable to follow a given patient through different locations. Here we show that small ultrasound‐based sensors referred to as "organ configuration motion" (OCM) sensors can be fixed to the abdomen and/or chest and provide information‐rich, breathing‐related signals. Methods: By design, the proposed sensors are relatively inexpensive. Breathing waveforms were obtained from tissues at varying depths and/or using different sensor placements. Validation was performed against breathing waveforms derived from magnetic resonance imaging (MRI) and optical tracking signals in five and eight volunteers, respectively. Results: Breathing waveforms from different modalities were scaled so they could be directly compared. Differences between waveforms were expressed in the form of a percentage, as compared to the amplitude of a typical breath. Expressed in this manner, for shallow tissues, OCM‐derived waveforms on average differed from MRI and optical tracking results by 13.1% and 15.5%, respectively. Conclusion: The present results suggest that the proposed sensors provide measurements that properly characterize breathing states. While OCM‐based waveforms from shallow tissues proved similar in terms of information content to those derived from MRI or optical tracking, OCM further captured depth‐dependent and position‐dependent (i.e., chest and abdomen) information. In time, the richer information content of OCM‐based waveforms may enable better respiratory gating to be performed, to allow diagnostic and therapeutic equipment to perform at their best. [ABSTRACT FROM AUTHOR]

Purpose: The aim of this study is to perform three‐dimensional (3D) source position reconstruction by combining in vivo dosimetry measurements from two independent detector systems. Methods: Time‐resolved dosimetry was performed in a water phantom during HDR brachytherapy irradiation with 192Ir source using two detector systems. The first was based on three plastic scintillator detectors and the second on a single inorganic crystal (CsI:Tl). Brachytherapy treatments were simulated in water under TG‐43U1 conditions, including a HDR prostate plan. Treatment needles were placed in distances covering a range of source movement of 120 mm around the detectors. The distance from each dwell position to each scintillator was determined based on the measured dose rates. The three distances given by the mPSD were recalculated to a position along the catheter (z) and a distance radially away from the mPSD (xy) for each dwell position (a circumference around the mPSD). The source x, y, and z coordinates were derived from the intersection of the mPSD's circumference with the sphere around the ISD based on the distance to this detector. We evaluated the accuracy of the source position reconstruction as a function of the distance to the source, the most likely location for detector positioning within a prostate volume, as well as the capacity to detect positioning errors. Results: Approximately 4000 source dwell positions were tracked for eight different HDR plans. An intersection of the mPSD torus and the ISD sphere was observed in 77.2% of the dwell positions, assuming no uncertainty in the dose rate determined distance. This increased to 100% if 1σ search regions were added. However, only 73(96)% of the expected dwell positions were found within the intersection band for 1(2) σ uncertainties. The agreement between the source's reconstructed and expected positions was within 3 mm for a range of distances to the source up to 50 mm. The experiments on a HDR prostate plan, showed that by having at least one of the detectors located in the middle of the prostate volume, reduces the measurement deviations considerably compared to scenarios where the detectors were located outside of the prostate volume. The analysis showed a detection probability that, in most cases, is far from the random detection threshold. Errors of 1(2) mm can be detected in ranges of 5–25 (25–50) mm from the source, with a true detection probability rate higher than 80%, while the false probability rate is kept below 20%. Conclusions: By combining two detector responses, we enabled the determination of the absolute source coordinates. The combination of the mPSD and the ISD in vivo dosimetry constitutes a promising alternative for real‐time 3D source tracking in HDR brachytherapy. [ABSTRACT FROM AUTHOR]

Purpose: Electronic portal imaging devices (EPIDs) are commonly installed on modern linear accelerators (LINACs) and are convenient for imaging and, potentially, dosimetry. However, owing to their construction with metal and scintillating layers of high atomic number, they exhibit nonwater‐equivalent response and oversensitivity to low‐energy photons. Therefore, EPIDs are not ideal for dosimetry purposes. Additionally, nonlinearities due to the combined use of scintillators and photodiodes have been reported. Here, an EPID which employs a variable gain Gas Electron Multiplier (GEM) and direct detection of electrons is introduced. To investigate its dosimetric performance, measurements characterizing the novel EPID are performed and compared with measurements from ionization chambers and conventional EPIDs. Methods: Linearity, dose rate dependence, field size dependence, off‐axis response, and transmission response were measured for all available energy settings (6, 10, 6 MV Flattening Filter Free (FFF) and 10 MVFFF) using three different detector gain settings. Additionally, an evaluation of the ghosting and image lag of the panel was completed. Reference ionization chamber measurements were performed for the off‐axis and transmission response and existing data for conventional EPIDs and ionization chambers from equivalent measurements were used for comparison of the field size dependence. Elsewhere, values from the linac monitoring chambers were used. Results: In the range from 10 to 1000 Monitor Units (MU), the detector was linear within 1% for all combinations of gain settings and energies. The dose rate dependence was also within 1% for all energies and for two out of three gain settings. Regarding field size dependency, the ratio of ionization chamber and panel values was 0.94 and 0.98 for the conventional EPID and GEMini respectively, at 20 × 20 cm2 and 10 MV. For 6 MV, 6 MVFFF, and 10MVFFF these ratios were 0.97, 0.98, and 0.99 for the GEMini, and 0.95, 0.97, and 0.97 for the conventional EPID. Similar performance between the GEMini and conventional EPID is observed for field sizes smaller than 10 × 10 cm2. The transmission response was within 5% for all energies for thicknesses up to 30 cm, compared to 10–20% for a conventional EPID. The off‐axis response for shifts up to 16 cm was within 1% and 3% for 6 MV and 10 MV, with and without phantom. The rise and fall of the signal from the detector correspond well to monitor chamber measurements indicating little ghosting and image lag, regardless of gain setting. Conclusion: The GEM EPID exhibits dose rate dependence and linearity within 1%, and negligible ghosting and image lag. In this regard, it performs particularly well using 50 and 250 V of gain, and either could be chosen. For higher sensitivity, 250 V is the recommended base gain setting, although other applications may warrant different gains. For most tests performed in this study, the GEM EPID demonstrates a more water‐equivalent response than conventional EPIDs making GEMs a viable technology for dosimetry in radiation therapy. [ABSTRACT FROM AUTHOR]

Purpose: This is a computational study to develop a rugged self‐powered Radioisotope Identification Device (RIID). The principle of operation relies on the High Energy Current (HEC) concept (Zygmanski and Sajo, Med Phys. 43 4–15, 2016) with measurement of fast electron currents between low‐Z and high‐Z thin‐film electrodes separated by nanoporous aerogel films in a multilayer detector structure whose prototypes were previously investigated (Brivio, Albert, Freund, Gagne, Sajo and Zygmanski, Med Phys, 46 4233–4240, 2019), (Brivio, Albert, Gagne, Freund, Sajo and Zygmanski, J Phys D Appl Phys, 53 265303, 2020). Here, we present an optimal detector design that accounts for a wide energy range (keV‐MeV) of x‐ray‐emitting radioisotopes that are of interest to national security and radiation therapy. Materials: We studied numerous multilayer detector geometries with N = 1..24 basic detector elements composed of 3 electrodes: N x (Al‐aerogel‐Ta‐aerogel‐Al). The thicknesses of electrodes and their total number were varied depending on the incident x‐ray spectra and its ability to penetrate and interact with the different layers, producing fast electrons. We used radiation transport simulations to find a balanced geometry that accounts for all energies from 10 keV to 6 MeV in a single design with relatively few detector elements (N = 24). In the balanced design, the electrodes have increasing thickness as a function of depth in the detector, ranging from 0.5 μm‐Ta and 10 μm‐Al at the entrance to 10 mm‐Ta and 2.5 mm‐Al at the exit. Aerogel thickness was fixed at 50 μm. Electron currents forming RIID signals were acquired from all Ta electrodes. A model function M(x, Ei) representing the detector yield as a function of the cumulative Ta thickness (x) for 70 monoenergetic incident beams (E) was derived. We also investigated the detector response to selected radioactive isotopes (Pd‐103, I‐125, Pu‐239, U‐235, Ir‐192, Cs‐137, Co‐60). Additional studies were performed with Bremsstrahlung spectra produced by electron beams in kVp tubes and in MV Linacs used in radiology and radiation therapy departments. We investigated different algorithms for radioisotope identification that would work for unknown unshielded as well as shielded sources. Results: Characteristic features of response functions for monoenergetic beams and radioisotopes were determined and used to develop two inverse algorithms of radioisotope identification. Using these algorithms, we were able to identify the unshielded and shielded sources, quantify the minimum, mean and maximum effective energies of the shielded spectra, and estimate the amount of Compton background in the spectrum. Conclusions: A multilayer sensor based on fast electron current was optimized and studied in its abilities as RIID. A balanced design permits the identification of radioisotopes with of a wide range of keV‐MeV energies. The device is low cost, rugged, self‐powered and can withstand very high dose rates, allowing deployment in difficult conditions, including radiation incidents. The algorithm we developed for radioisotope identification and spectral unfolding is robust and it is an important component in practical applications. [ABSTRACT FROM AUTHOR]

Purpose: A clinical‐prototype, dedicated, cone‐beam breast computed tomography (CBBCT) system with offset detector is undergoing clinical evaluation at our institution. This study is to estimate the normalized glandular dose coefficients (DgNCT) that provide air kerma‐to‐mean glandular dose conversion factors using Monte Carlo simulations. Materials and methods: The clinical prototype CBBCT system uses 49 kV x‐ray spectrum with 1.39 mm 1st half‐value layer thickness. Monte Carlo simulations (GATE, version 8) were performed with semi‐ellipsoidal, homogeneous breasts of various fibroglandular weight fractions (fg=0.01,0.15,0.5,1), chest wall diameters (d=8,10,14,18,20 cm), and chest wall to nipple length (l=0.75d), aligned with the axis of rotation (AOR) located at 65 cm from the focal spot to determine the DgNCT. Three geometries were considered – 40×30‐cm detector with no offset that served as reference and corresponds to a clinical CBBCT system, 30×30‐cm detector with 5 cm offset, and a 30×30‐cm detector with 10 cm offset. Results: For 5 cm lateral offset, the DgNCT ranged 0.177‐0.574 mGy/mGy and reduction in DgNCT with respect to reference geometry was observed only for 18 cm (6.4%±0.23%) and 20 cm (9.6%±0.22%) diameter breasts. For the 10 cm lateral offset, the DgNCT ranged 0.221‐0.581 mGy/mGy and reduction in DgNCT was observed for all breast diameters. The reduction in DgNCT was 1.4%±0.48%, 7.1%±0.13%, 17.5%±0.19%, 25.1%±0.15%, and 27.7%±0.08% for 8, 10, 14, 18, and 20 cm diameter breasts, respectively. For a given breast diameter, the reduction in DgNCT with offset‐detector geometries was not dependent on fg. Numerical fits of DgNCTd,l,fg were generated for each geometry. Conclusion: The DgNCT and the numerical fit, DgNCTd,l,fg would be of benefit for current CBBCT systems using the reference geometry and for future generations using offset‐detector geometry. There exists a potential for radiation dose reduction with offset‐detector geometry, provided the same technique factors as the reference geometry are used, and the image quality is clinically acceptable. [ABSTRACT FROM AUTHOR]