Search

We developed a multichannel high time resolution data acquisition (DAQ) system for time-over-threshold (TOT) application specific integrated circuit (ASIC). The developed field-programmable gate array-based DAQ is composed of 144 channels per board with an Ethernet readout to measure energy and timing information simultaneously. The measured time precision of the developed DAQ board was reduced to 62.5 ps. The proposed DAQ with TOT-ASIC can potentially be used for several radiation detection applications, such as positron emission tomography (PET) and Compton imaging. The DAQ board is applied to PET and named PET readout board with a high-speed network (PETNET). In the first version developed, the PETNET had a time precision of 1 ns by implementing a double data rate method. This demonstrates that this method is useful for capturing electron–positron pairs of 22Na using two PETNETs. We obtained 2-D cross-sectional images placing two 8 × 8 detectors facing each other. In the second version developed, the PETNET had a time precision of 125 or 62.5 ps by implementing a delay-line method. Cable length measurements proved that PETNET has a position resolution of 4.5 cm at the speed of light. To synchronize the time of data on several PETNETs, a time-of-flight method was adopted, and all the data were compared with each other with accurate time precision. [ABSTRACT FROM AUTHOR]

The aim of the performed technical attenuation length measurement is to compare light attenuation of a few commercially available plastic scintillator strips and to select the best scintillator type for the total-body Jagiellonian Positron Emission Tomograph (J-PET) construction. Few models of plastic scintillators obtained from different manufacturers were tested. All strips had the same rectangular cross section and dimensions 6 mm × 24 mm × 1000 mm with all surfaces polished. The light attenuation length was measured by exciting the scintillator strip at different positions with an ultraviolet lamp emitting at 365 nm and reading light signal collected at one side of the strip by a silicon photodiode. Among measured plastic scintillators, EJ-200 possesses the highest technical light attenuation length and is suitable for construction of total-body J-PET scanner. [ABSTRACT FROM AUTHOR]

In developing a signal separation algorithm, an obvious challenge relates to the ways in which the photon measurements from dual-tracer positron emission tomography (PET) are overlapped. Aside from this issue, the ill-conditioning and the noisy measures are still obstacles to achieve accurate and robust result. In this paper, we develop a novel approach from a data-driven perspective that separates dynamic dual-tracer PET signals into individual-tracer components explicitly with a simultaneous-injection single-scan. Our approach is divided into two phases: training and reconstruction. In the training phase, our proposed framework will learn the potential relationship of different frames for dynamic dual-tracer PET data. In the reconstruction phase, given the sinogram of a dual tracer, the PET activity map of an individual tracer can be obtained directly. Experiments on Monte Carlo simulation data sets are conducted as validation. The experimental results have demonstrated the superior performance of the proposed approach by comparing with the ground truth of the individual-tracer. [ABSTRACT FROM AUTHOR]

In the context of the development of an in-beam time-of-flight positron emission tomography demonstrator dedicated to the in vivo monitoring of delivered dose in hadrontherapy, we evaluate the potential performance of front-end architectures based on sampling and digital pulse processing to reconstruct the energy and time of event. In this paper, we evaluate the requirements for sampling frequency and analog-to-digital converter (ADC) resolution. The timing algorithm is a digital adaptation of the constant fraction discriminator principle, with a step of interpolation using a low-pass filter based on the cubic spline technique. We demonstrate the interest of the interpolation to lower the sampling frequency requirement, by improving the signal reconstruction compared with a simple linear interpolation. Experimental tests were performed on pulse libraries acquired from a set up composed of LYSO and LaBr3 scintillators coupled to H6533 photomultipliers, and a sampling oscilloscope operating at 10 GHz. By offline processing of the signals with variable parameters (initial frequency, interpolator bandwidth, resampling frequency, and ADC resolution), we examined the impact of these parameters on time resolution. Results for the tested detectors suggest a minimal required sampling rate of 1.5 GHz, while the ADC resolution can be as small as 5 b. A logarithmic ADC could be more efficient, with a strict minimum of 4 b. [ABSTRACT FROM AUTHOR]

Recent advances in the development of silicon photomultipliers (SiPM) offer new opportunities for medical imaging applications. Specifically, novel imaging devices for positron emission (PET) and single photo-emission computed tomography (SPECT) are becoming feasible. In this investigation, we tested a monolithic array of new generation SiPMs, an SB-8 array from SensL. 8\times8 array of 6-mm square SiPMs was studied with two different multiplexing readouts, 4- and 16-channels. SB-8 detector was coupled to various scintillation arrays and resulting prototypes were evaluated. Testing demonstrated that the new device is capable of resolving 1 mm LYSO and 1.5 mm NaI crystals with 16-channel readout and 1.57 mm LYSO and 3 mm NaI crystals with 4-channel readout. Energy resolution of approximately 15% at 511 keV and 19% at 122 keV were obtained with LYSO and NaI crystals, respectively. A timing resolution of 1.52 ns was measured with 1.57 mm LYSO array and the 16-channel readout. It is concluded that new B-series SiPMs from SensL are suitable for use in high spatial resolution nuclear medicine particle detectors. [ABSTRACT FROM PUBLISHER]

This paper describes the design and performance of a high spatial resolution positron emission tomography (PET) detector with time-of-flight capabilities. With an emphasis on high spatial resolution and sensitivity, we initially evaluated the performance of several 1.5 \times 1.5 and 2.0 \times 2.0~\mm^2 and 12–15 mm long LYSO crystals read out by several appropriately sized PMTs. Experiments to evaluate the impact of reflector on detector performance were performed and the final detector consisted of a 32 \times 32 array of 1.5 \times 1.5 \times 15~\mm^3 LYSO crystals packed with a diffuse reflector and read out by a single Hamamatsu 64 channel multi-anode PMT. Such a design made it compact, modular and offered a cost-effective solution to obtaining excellent energy and timing resolution. To minimize the number of readout signals, a compact front-end readout electronics that summed anode signals along each of the orthogonal directions was also developed. Experimental evaluation of detector performance demonstrates clear discrimination of the crystals within the detector. An average energy resolution (FWHM) of 12.7 \pm 2.6\% and average coincidence timing resolution (FWHM) of 348 ps was measured, demonstrating suitability for use in the development of a high spatial resolution time-of-flight scanner for dedicated breast PET imaging. [ABSTRACT FROM AUTHOR]

Small animal positron emission tomography (PET) systems are often designed by employing close geometry configurations. Due to the different characteristics caused by geometrical factors, these tomographs require data acquisition protocols that differ from those optimized for conventional large diameter ring systems. In this work we optimized the energy window for data acquisitions with PETbox4, a 50 mm detector separation (box-like geometry) pre-clinical PET scanner, using the Geant4 Application for Tomographic Emission (GATE). The fractions of different types of events were estimated using a voxelized phantom including a mouse as well as its supporting chamber, mimicking a realistic mouse imaging environment. Separate code was developed to extract additional information about the gamma interactions for more accurate event type classification. Three types of detector backscatter events were identified in addition to the trues, phantom scatters and randoms. The energy window was optimized based on the noise equivalent count rate (NECR) and scatter fraction (SF) with lower-level discriminators (LLD) corresponding to energies from 150 keV to 450 keV. The results were validated based on the calculated image uniformity, spillover ratio (SOR) and recovery coefficient (RC) from physical measurements using the National Electrical Manufacturers Association (NEMA) NU-4 image quality phantom. These results indicate that when PETbox4 is operated with a more narrow energy window (350-650 keV), detector backscatter rejection is unnecessary. For the NEMA NU-4 image quality phantom, the SOR for the water chamber decreases by about 45% from 15.1% to 8.3%, and the SOR for the air chamber decreases by 31% from 12.0% to 8.3% at the LLDs of 150 and 350 keV, without obvious change in uniformity, further supporting the simulation based optimization. The optimization described in this work is not limited to PETbox4, but also applicable or helpful to other small inner diameter geometry scanners. [ABSTRACT FROM AUTHOR]

Scatter Fraction (SF) and Noise Equivalent Count Rate (NECR) of a 2400 mm wide axial field-of-view Positron Emission Tomography (PET) system based on Resistive Plate Chamber (RPC) detectors with 300 ps Time Of Flight (TOF) resolution were studied by simulation using Geant4. The study followed the NEMA NU2-2001 standards, using the standard 700 mm long phantom and an axially extended one with 1800 mm, modeling the foreseeable use of this PET system. Data was processed based on the actual RPC readout, which requires a 0.2~\mu\s non-paralyzable dead time for timing signals and a paralyzable dead time (\taups) for position signals. For NECR, the best coincidence trigger consisted of a multiple time window coincidence sorter retaining single coincidence pairs (involving only two photons) and all possible coincidence pairs obtained from Multiple coincidences, keeping only those for which the direct TOF-reconstructed point falls inside a tight region surrounding the phantom. For the 700 mm phantom, the SF was 51.8% and, with \taups = 3.0~\mu\s, the peak NECR was 167 kcps at 7.6~\kBq/cm^3. Using \tau ps = 1.0~\mu\s the NECR was 349 kcps at 7.6~\kBq/cm^3, and no peak was found. For the 1800 mm phantom, the SF was slightly higher, and the NECR curves were identical to those obtained with the standard phantom, but shifted to lower activity concentrations. Although the higher SF, the values obtained for NECR allow concluding that the proposed scanner is expected to outperform current commercial PET systems. [ABSTRACT FROM AUTHOR]

The GATE package is used to perform Monte Carlo hadrontherapy simulations of a cancer treatment combined with the complete description of an associated positron emission tomography (PET) imaging device for dose monitoring. This study aimed to demonstrate that the GATE platform has the capability to perform realistic simulations in the field of hadrontherapy, combining both dose and imaging system. We defined the simulation configuration as a carbon ion pencil beam scanning of a thorax CT phantom together with a complete PET imaging system. Two main positron emitters resulting from nuclear ^12C reactions are considered in this study: ^11C and ^15O. We studied the produced data to analyse the interest in using a full PET system simulation instead of the usual Gaussian smoothing applied on the positron emitters map. We found differences in the distal position of the signal falloff of 20% between full PET system simulation and the Gaussian model. We also studied the influence of the ^15O isotope in PET images and found the contribution to falloff of this isotope to be negligible (4%), which suggests the inclusion of ^15O isotope in the simulation is not necessary. Finally, we analysed the impact of dose delivery on PET image quality and found a difference of 20% on the PET estimation falloff between doses of 10 Gy and 1 Gy. This study shows that GATE, implemented on a computing system with large number of CPUs (>1000), has the potential to be used for quantitative evaluation of imaging protocols for radiation monitoring. [ABSTRACT FROM PUBLISHER]

Positron range is a limiting factor in spatial resolution of positron emission tomography (PET) imaging. In this work we investigated the tissue-dependent nature of positron range and its effect on imaging with the new Biograph mMR 3 T whole-body PET/MR system. Positron range simulations were performed for Rb-82 and Ga-68 in homogeneous and non-homogeneous configurations of soft and lung tissue using Geant4. In addition, a 3D MLEM reconstruction framework was extended to incorporate correction kernels that compensate for the effect of positron range. We developed methods for selecting and adapting such correction kernels based on the MR-derived attenuation map and pre-simulated positron range data. The methods were evaluated using Monte Carlo (MC) simulation data on a model of the mMR system including the 3 T magnetic field (MF) and the MR body coil. The results prove that the use of tissue-specific positron range kernels results in better spatial resolution. Especially, the application of generalized kernels at the borders of different tissue types has the potential to provide better contrast and lesion localization. [ABSTRACT FROM PUBLISHER]

We describe techniques that extend the usefulness of real-time data-handling architectures designed for clinical positron emission tomography (PET)—especially for instances of extremely high (>10 M events/s) count rate. As is widely known, Rubidium-82 (82Rb) with a 1.3-min half-life is often used in clinical PET. When used, 82Rb is more often applied for dynamic and/or gated studies—typically with little or no delay between tracer injection and start of acquisition. The use of 82Rb for short-duration-framed studies in clinical PET has its own set of challenges. For example, a “too large” dose may temporarily exceed the acquisition throughput. When such saturation occurs, coincidence events are lost. In the case of 82Rb, such loss is typically short-lived and limited to the count-rate peak. Such saturation also leads to a truncation of the count-rate profile—a truncation that often limits curve fitting essential for an accurate compartmental model across the entire study. The techniques offered here help to resolve this issue. By electronically keeping track of those events that are subject to saturation loss in the acquisition channel, the complete count-rate profile itself is preserved. Such tracking is done in real time with an accurate log of loss quantities added to the list-mode stream. This approach enables the clinician to raise the 82Rb dose without a specific concern over count-rate profile truncation. We describe a special 64-bit nonevent (i.e., “tag”) packet that is automatically inserted into the list-mode stream. The “Lost-Event Tally” tag packet stream enables a full recovery of the rate profile even when event packets are lost. During acquisition, this tag packet is repeatedly inserted into the stream to report the counted event-packet loss (up to 1\,048\,575) since the previous such tag packet was inserted. [ABSTRACT FROM PUBLISHER]

Positron Emission Tomography (PET) has emerged as a valuable molecular imaging modality for quantitative measurement of biochemical processes in vivo in the clinical and preclinical imaging domains. However, PET imaging suffers from various physical degrading factors including photon attenuation, which can be corrected using CT-based attenuation correction (CTAC) on combined PET/CT scanners. The attenuation map is calculated by converting CT numbers derived from low-energy polyenergetic x-ray spectra to linear attenuation coefficients at 511 keV. Generation of accurate attenuation maps is crucial for reliable attenuation correction of PET data and hence is a prerequisite for accurate quantification of biological processes. In this study, we implemented the CTAC procedure on the FLEX Triumph™ preclinical PET/CT scanner and evaluated tube voltage dependence for different kVps (40, 50, 60, 70, and 80). The quantitative impact of both bilinear and quadratic based energy-mapping methods on linear attenuation coefficients, attenuation maps and corrected PET images was assessed at different CT tube voltages. Attenuation maps were calculated from CT images of a cylindrical polyethylene phantom containing different concentrations of K2HPO4 in water. Correlation coefficients and best regression fit equations were calculated for both methods. Phantom and rodent PET/CT images were used to assess improvements in image quality and quantitative accuracy. It was observed that the slopes of the bilinear calibration curves for CT numbers greater than 0 HU increase with increasing tube voltage. In addition, higher correlation coefficients were obtained for the quadratic compared to the bilinear energy-mapping method. Tube voltage of 70 kVp produced the smallest relative error and higher correlation coefficient compared to other tube voltages. For low concentrations of K2HPO4, the mean relative difference (in \%) between theoretical and calculated attenuation coefficients when using bilinear and quadratic energy-mapping methods are 1.39\pm 1.9 and 1.33\pm 1.8, respectively. They are 2.78\pm 1.3 and 2.5\pm 1.3, respectively, for high concentrations of K2HPO4. As expected, higher activity concentrations were obtained for PET after attenuation correction. The increased PET signal for mouse tissues ranged between 21 and 31% for bilinear energy-mapping and between 21.8 and 35% for quadratic energy-mapping, whereas these varied from 40 to 51% and from 41 to 56%, respectively, for rat tissues. For biological tissues having a high atomic number such as bone, the quadratic energy-mapping method produced slightly improved results compared to the bilinear energy-mapping method. Phantom and rodent PET studies were successfully corrected for photon attenuation using the developed CTAC procedure. [ABSTRACT FROM AUTHOR]

Noise equivalent counts (NEC) have been used as a measure or proxy of PET image quality for many years. It has been shown to be a useful metric, for example to determine clinical patient dosage. However, NEC should be used cautiously in evaluating image quality since it is a global data quality measure that does not take into account localized effects due to spatial resolution and image reconstruction, as well as the effect of time-of-flight (TOF) imaging on resultant images. In this work, we study the use of a numerical observer that uses a generalized scan-statistic model to estimate lesion detectability with localization in a uniform background phantom, for varying activity levels and scan times. Data were acquired on a clinical whole-body TOF PET scanner. Data show that ALROC increases as a function of NEC but at high activity levels it approaches a peak value earlier than the NEC peak. Also, the ALROC for images acquired with the same NEC, but at two different activity levels and scan times, is similar. Our results show that with TOF information we can either achieve improved clinical performance for heavy patients, or reduce the scan time or injected activity while maintaining similar ALROC value as in a Non-TOF image. [ABSTRACT FROM AUTHOR]

In order to improve image quality in Positron Emission Tomography (PET) different routes are being pursued such as fast timing resolution for time-of-flight PET, higher spatial resolution by the use of smaller scintillator pixels and the use of depth-of-interaction information. The detection of the depth-of-interaction (DOI) of a gamma ray within a detector, deploying pulse shape discrimination (PSD), has been used to increase sensitivity and spatial resolution, especially at the edge of the field of view (FOV). The DOI information is used to reduce the parallax error; thus improving spatial resolution. Commonly, different scintillator materials with different decay times and light output and other differentiating factors, such as density, emission spectra, etc., are used for DOI detectors. We present a multilayer phoswich detector comprised of LSO with different decay times in the range from 30 ns to 47 ns. The difference in decay times is achieved by co-doping LSO:Ce with Ca, resulting in short decay times of ~ 30 ns. The use of a cut light guide allows the use of regular Photomultiplier tubes, giving the opportunity of a potential DOI detector replacement for current detectors. We were able to identify each pixel in the different detector layers and thus, to determine the depth of the photon interaction and achieve a timing resolution of 345 ps. [ABSTRACT FROM AUTHOR]

Some Positron Emission Tomography (PET) scanners achieve improved resolution or Depth-Of-Interaction (DOI) measurement using avalanche photodiode-based phoswich detectors, with different challenges (noise limitation) than traditional photomultiplier tubes (space limitation). DOI measurement is necessary in small-animal PET for parallax mitigation, while side-by-side phoswich detectors improve resolution without an equal increase in electronics complexity. Future improvements in scanner performance now require the improvement of the current Parameter Estimation (PE) digital Crystal Identification (CI) algorithms. Indeed PE CI becomes mandatory to the APD signal analysis of crystals with very similar scintillation properties (decay time difference less than ~ 15 ns), or when the Signal-to-Noise Ratio (SNR) is degraded, for Compton events with an energy below 150 keV, for instance. PE CI currently relies on one-parameter discrimination of crystal species after a Wiener parametric estimation of a pole-zero model. Neither that nor traditional Pulse-Shape Discrimination (PSD) can correctly accommodate the need for CI of low-energy scattered photons in very similar scintillation materials. This paper studies a Recursive Least-Squares (RLS) PE method based on Auto-Regressive Moving Average with eXogenous variable (ARMAX) modeling of the acquisition chain conjugated with simultaneous 3-parameter CI adapted from Vector Quantization (VQ). The RLS algorithm presents a significant improvement for the discrimination of similar materials (~ 80% less primary CI error versus Wiener CI for 15 ns decay time difference) and excellent performance in heavy noise (negligible CI error for 0 dB SNR 30-keV BGO photons versus LSO). Issues remaining include the handling of noise through the exogenous variable and the computational burden, still too high for existing hardware. [ABSTRACT FROM AUTHOR]

In order to detect the breast cancer at the early stage with high efficiency, we intend to develop positron emission mammography (PEM), using Pr:LuAG single crystal as a scintillator of this scanner. PEM has the same mechanism of PET, where two 511 keV annihilation gamma-rays are detected by two gamma cameras at opposite side, and reconstruction of the data shows the position of the cancer with malignancy grades. After cutting and polishing processes for each crystal to a physical dimensions of 2.1 x 2.1 x 15 mm³, Pr:LuAG array covered with BaSO4 reflector was used as a sensor head. The one camera unit consisted of 20 x 64 pixels optically coupled with three H8500-03 multi anode PMTs. Our PEM required four cameras at each side. Finally, eight cameras were installed in both sides of the prototype instrument and the spatial resolution was evaluated using the 18F in the breast phantom. [ABSTRACT FROM AUTHOR]

PETbox is a low-cost benchtop PET scanner dedicated to high throughput preclinical imaging that is currently under development at our institute. This paper presents the design and characterization of the detectors that are used in the PETbox system. In this work, bismuth germanate scintillator was used for the detector, taking advantage of its high stopping power, high photoelectric event fraction, lack of intrinsic background radiation and low cost. The detector block was segmented into a pixelated array consisting of 20 x 44 elements, with a crystal pitch of 2.2 mm and a crystal cross section of 2 mm x 2 mm. The effective area of the array was 44 mm x 96.8 mm. The array was coupled to two Hamamatsu H8500 position sensitive photomultiplier tubes, forming a flat-panel type detector head with a sensitive area large enough to cover the whole body of a typical laboratory mouse. Two such detector heads were constructed and their performance was characterized. For one detector head, the energy resolution ranged from 16.1% to 38.5% full width at half maximum (FWHM), with a mean of 20.1%; for the other detector head, the energy resolution ranged from 15.5% to 42.7% FWHM, with a mean of 19.6%. The intrinsic spatial resolution was measured to range from 1.55 mm to 2.39 mm FWHM along the detector short axis and from 1.48 mm to 2.33 mm FWHM along the detector long axis, with an average of 1.78 mm. Coincidence timing resolution for the detector pair was measured to be 4.1 ns FWHM. These measurement results show that the detectors are suitable for our specific application. [ABSTRACT FROM AUTHOR]

For animal PET systems to achieve high sensitivity without adversely affecting spatial resolution, they must have the ability to measure depth-of-interaction (DOI). In this paper, we propose a novel four-layer PET system, and present the performances of modules built to verify the concept of the system. Each layer in the four-layer PET system has a relative offset of half a crystal pitch from other layers. Performances of the four-layer detector were estimated using a GATE Monte Carlo simulation code. The proposed system consists of six H9500 PMTs, each of which contains 3193 crystals. A sensitivity of 11.8% was obtained at the FOV center position of the proposed system. To verify the concept, we tested a PET module constructed using a H9500 flat panel PMT and LYSO crystals of cross-sectional area 1.5 x 1.5 mm². The PET module was irradiated with a 1.8 MBq 22Na radiation source from the front or side of the crystals to obtain flood images of each crystal. Collimation for side irradiation was achieved using a pair of lead blocks of dimension 50 x 100 x 200 mm³. All crystals in the four layers were clearly identified in flood images, thus verifying the DOI capability of the proposed four-layer PET system. We also investigated the optimal combination of crystal lengths in the four-layer PET system using the GATE Monte Carlo simulation code to generate events from simulated radiation sources, and using the ML-EM algorithm to reconstruct simulated radiation sources. The combination of short crystal lengths near radiation sources and long crystal lengths near the PMT provides better spatial resolution than combinations of same crystal lengths in the four-layer PET system. [ABSTRACT FROM AUTHOR]

The performance of multi pixel photon counters (MPPC) of 3 mm x 3 mm size, with 14400 and 3600 pixels, were studied by means of the signal from a laser light pulser and using the 3 mm x 3 mm x 20 mm LSO pixel scintillator. Special attention was paid to measure number of fired pixels, generated by the light of pulser and that of the LSO crystal, using a direct method of a comparison of the light peak position in the pulse height spectrum with that of the single photoelectron. The tests of the LSO crystal showed 1550 ± 80 fired pixels per MeV in the MPPC with 14400 pixels assuring a good linearity of the response up to about 1 MeV energy of gamma rays absorbed in the LSO crystal. Energy resolution of 14.8% for 662 keV gamma rays from 137Cs source and a time resolution of about 850 ps for 511 keV annihilation quanta were limited by a rather low number of the fired pixels compared to the number of photoelectrons in photomultipliers. [ABSTRACT FROM AUTHOR]

We have designed and constructed an MR-compatible PET system for fully simultaneous PET/MR studies of small animals. The scanner uses long optical fibers to distance the magnetic field sensitive PET PMTs from the high magnetic field at the center of an MR scanner. It is a single slice system with an inner diameter of 7 cm. A full evaluation of the performance of the PET system and the results of an MR compatibility assessment in a Philips Achieva whole body 3 T MRI scanner are presented. The reconstructed resolution of the PET scanner is 1.5 mm at the center falling to 2.5 mm at the edge of the field of view; the system sensitivity is 0.95%; the count rate is linear up to an activity of 6 MBq (~4 kcps) and the scatter fraction is 42% which can be reduced to 26% using MR-compatible gamma shields. Simultaneous PET/MR images of phantoms and a mouse have been acquired. The system is highly MR compatible, as demonstrated here, showing no degradation in performance of either the MR or PET system in the presence of the other modality. The system will be used to demonstrate novel pre-clinical applications of simultaneous PET/MR. [ABSTRACT FROM AUTHOR]

The integration of a high resolution PET insert into a conventional PET system can significantly improve the resolution and the contrast of its images within a reduced imaging field of view. For the rest of the scanner imaging field of view, the insert is a highly attenuating and scattering media. In order to use all available coincidence events (including coincidences between 2 detectors in the original scanner, namely the scanner-scanner coincidences), appropriate scatter and attenuation corrections have to be implemented. In this work, we conducted a series of Monte Carlo simulations to estimate the composition of the scattering background and the importance of the scatter correction. We implemented and tested the Single Scatter Simulation (SSS) algorithm for a hypothetical system and show good agreement between the estimated scatter using SSS and Monte Carlo simulated scatter contribution. We further applied the SSS to estimate scatter contribution from an existing prototype PET insert for a clinical PET/CT scanner. The results demonstrated the applicability of SSS to estimate the scatter contribution within a clinical PET/CT system even when there is a high resolution half ring PET insert device in its imaging field of view. [ABSTRACT FROM AUTHOR]

Head movements during brain imaging using high resolution positron emission tomography (PET) impair the image quality which, along with the improvement of the spatial resolution of PET scanners, in general, raises the importance of motion correction. Here, we present a new design for an automatic, movable, mechanical PET phantom to simulate patients' head movements while being scanned. This can be used for evaluating motion correction methods. A low-cost phantom controlled by a rotary stage motor was built and tested for axial rotations of 1° - 10° with the multiple acquisition frame method. The phantom is able to perform stepwise and continuous axial rotations with submillimeter accuracy, and the movements are repeatable. The scans were acquired on the high resolution research tomograph dedicated brain scanner. The scans were reconstructed with the new 3-D ordered subset expectation maximization algorithm with modeling of the point spread function (3DOSEM-PSF), and they were corrected for motions based on external tracking information using the Polaris Vicra real-time stereo motion-tracking system. The new automatic, movable phantom has a robust design and is a potential quality assessment tool for the development and evaluation of future motion correction methods. [ABSTRACT FROM AUTHOR]

We report on a high resolution, monolithic crystal PET detector design concept that provides depth of interaction (DOI) positioning within the crystal. Our design utilizes a novel sensor on the entrance surface (SES) approach combined with a maximum likelihood positioning algorithm. We compare the intrinsic spatial resolution characteristics (i.e., X, Y and Z) using our SES design versus conventional placement of the photo-sensors on the rear surface of the crystal. The sensors can be any two-dimensional array of solid state readout devices (e.g., silicon photomultipliers (SiPM) or avalanche photodiodes (APD)). SiPMs are a new type of solid-state photodetector with Geiger mode operation that can provide signal gain similar to a photomltipiler tube (PMT). Utilizing a multi-step simulation process, we determined the intrinsic spatial resolution characteristics for a variety of detector configurations. The SES design was evaluated via simulation for three different two-dimensional array sizes: 8 x 8 with 5.8 x 5.8 mm² pads; 12 x 12 with 3.8 x 3.8 mm² pads; and 16 x 16 with 2.8 x 2.8 mm² pads. To reduce the number of signal channels row-column summing readout was used for the 12 x 12 and 16 x 16 channel array devices. The crystal was modeled as a 15 mm monolithic slab of a lutetium-based scintillator with the large area surface varying from 48.8 x 48.8 mm² up to 49.6 x 49.6 mm² depending upon the dimensions of the two-dimensional photo-sensor array. The intrinsic spatial resolution for the 8 x 8 array is 0.88 mm FWHM in X and Y, and 1.83 mm FWHM in Z (i.e., DOI). Comparing the results versus using a conventional design with the photo-sensors on the backside of the crystal, an average improvement of ~24% in X and Y and 20% in Z is achieved. The X, Y intrinsic spatial resolution improved to 0.67 mm and 0.64 mm FWHM for the 12 x 12 and 16 x 16 arrays using row-column readout. Using the 12 x 12 and 16 x 16 arrays also led to a slight improvement in the DOI positioning accuracy. [ABSTRACT FROM AUTHOR]

This paper presents design techniques of a multiphase clock generator using a low-jitter delay-locked loop (DLL) or its array for the developments of high-resolution multi-channel time-to-digital converters (TDCs). The low-jitter technologies for both a single DLL and an array of DLL are discussed. Based on the previous work on the design of a single DLL with 32 delay cells, an array of mixed-mode low-jitter DLLs is proposed for achieving smaller time taps. The array of DLL is successfully designed and embedded into a prototype chip of a three-channel high-resolution TDC in 0.35 µm CMOS process. The operational range of the DLL in the array is from 50 MHz to 120 MHz. The RMS value of measured cycle-to-cycle jitter in the DLL is about 7 ps while the peak-to-peak value is about 20 ps. A bin size of 71 ps can be achieved by using a reference clock of 100 MHz. The DNL and INL of the evaluated chip are 0.58 LSB and 0.63 LSB, respectively. The static power dissipation of the DLL array is about 23 mW. [ABSTRACT FROM AUTHOR]

This paper represents the design of a low-noise, wide band multi-channel readout integrated circuit (IC) used as front end readout electronics of avalanche photo diodes (APD) dedicated to a small animal positron emission tomography (PET) system. The first ten-channel prototype chip (APD_Chip) of the analog parts has been designed and fabricated in a 0.35 µm CMOS process. Every channel of the APD_Chip includes a charge-sensitive preamplifier (CSA), a CR-(RC)² shaper, and an analog buffer. In a channel, the CSA reads charge signals (10 bits dynamic range) from an APD array having 10 pF of capacitance per pixel. A linearized degenerated differential pair which ensures high linearity in all dynamical range is used as the high feedback resistor for preventing pile up of signals. The designed CSA has the capability of compensating automatically up to 200 nA leakage current from the detector. The CR-(RC)² shaper filters and shapes the output signal of the CSA. An equivalent input noise charge obtained from test is 275 e- + 10 e-/pF. In this paper the prototype is presented for both its theoretical analysis and its test results. [ABSTRACT FROM AUTHOR]

PEM-guided biopsy is successful in localizing sub-centimeter lesions that cannot be seen by other modalities. To help the physician quickly visualize the position of an interventional device in relation to a lesion, we have developed a method to view PEM images in near real time, allowing visualization of lesion displacement within 30 seconds. Phantom and patient data were collected on a commercially available high-resolution, organ-specific PET scanner. The real-time display is realized by showing only one imaging plane and calculating the intersection point of each line of response through a plane where the lesion was targeted based on a prior diagnostic-quality scan. A variety of refresh and persistence times from 2 to 30 seconds were evaluated. In a phantom study both point source and line source movements were successfully visualized in real-time with a refresh time of two seconds and a persistence time of 10 to 15 seconds. Simulation based on real clinical data shows that the localization of lesions can be obtained almost real-time and even relatively faint lesions with a lesion to background ratio of 2:1 can be localized in 10 seconds. [ABSTRACT FROM AUTHOR]

The recently published NEMA NU 4-2008 Standards has been specially designed for evaluating the performance of small animal PET scanners used in preclinical applications. In this paper, we report on the NU 4 performance of a clinical positron emission mammography (PEM) system. Since there are no PEM specific performance test protocols available, and the NU 2 protocol (intended for whole-body PET scanners) cannot be applied without modification due to the compact design of the PEM scanner, we decided to evaluate the NU 4 Standards as an alternative. We obtained the following results: Trans-axial spatial resolution 1.8mmFWHMfor high resolution reconstruction mode and 2.4 mm FWHM for standard resolution reconstruction mode with no significant variation within the field of view. The total system sensitivity was 0.16 cps/Bq. In image quality testing, the uniformity was found to be 3.9% STD at the standard resolution mode and 5.6% at the high resolution mode when measured with a 34 mm paddle separation. The NEMA NU 4-2008 Standards were found to be a practicable tool to evaluate the performance of the PEM scanner after some modifications to address the specifics of its detector configuration. Furthermore, the PEM scanner's in-plane spatial resolution was comparable to other small animal PET scanners with good image quality. [ABSTRACT FROM AUTHOR]

A lower-cost high-sensitivity high-resolution positron emission mammography (PEM) camera is developed. It consists of two detector modules with the planar detector bank of 20 × 12 cm². Each bank has 60 low-cost PMT-Quadrant-Sharing (PQS) LYSO blocks arranged in a 10 × 6 array with two types of geometries. One is the symmetric 19.36 × 19.36 mm² block made of 1.5 × 1.5 × mm³ crystals in a 12 × 12 array. The other is the 19.36 × 26.05 mm² asymmetric block made of 1.5 × 1.9 × 10 mm³ crystals in 12 × 13 array. One row (10) of the elongated blocks are used along one side of the bank to reclaim the half empty PMT photocathode in the regular PQS design to reduce the dead area at the edge of the module. The bank has a high overall crystal packing fraction of 88%, which results in a very high sensitivity. Mechanical design and electronics have been developed for low-cost, compactness, and stability purposes. Each module has four Anger-HYPER decoding electronics that can handle a count-rate of 3 Mcps for single events. A simple two-module coincidence board with a hardware delay window for random coincidences has been developed with an adjustable window of 6 to 15 ns. Some of the performance parameters have been studied by preliminary tests and Monte Carlo simulations,including the crystal decoding map and the 17% energy resolution of the detectors, the point source sensitivity of 11.5% with 50 mm bank-to-bank distance, the 1.2 mm-spatial resolutions, 42 kcps peak Noise Equivalent Count Rate at 7.0-mCi total activity in human body, and the resolution phantom images. Those results show that the design goal of building a lower-cost, high-sensitivity, high-resolution PEM detector is achieved. [ABSTRACT FROM AUTHOR]

Cerium-doped lutetium oxyorthosilicate (LSO:Ce) is a well-known scintillator whose high density (7.4 g/cc), high light yield (∼6 times that of BGO), and fast decay time (∼43 ns) make it especially well suited for use in Positron Emission Tomography. Recent work suggests that divalent calcium co-doping eliminates many of the shallow electron traps, thus facilitating fast energy transfer to the Ce3+ luminescence centers and shortening the scintillation decay time to ∼30 ns. Here we report preliminary observations of the effect of Ca2+ co-doping on the crystal growth, photoluminescence, and scintillation properties of LSO:Pr. Single crystals with Pr concentrations ranging from 0.05% to 0.2% and Ca2+ concentrations up to 0.2% were grown via the Czochralski. technique. Absorption, emission, and excitation spectra revealed the numerous 5d-4f and 4f-4f transitions characteristic of trivalent Pr. The main optical absorption due to Ca2+ occurs in the far UV, similar to previous observations of LSO:Ce, but with a more in- tense tail extending through the visible region. Consequently, 4f to 5d optical excitation (∼260 nm) is greatly reduced in Ca co-doped samples, and the related Sd to 4f emission (∼400 nm) is largely absent. Optical excitation (∼450-500 nm) of the red (∼610 nm) emission appears to be unaffected, for the most part, by the presence of Ca2+. The scintillation decay of LSO:Pr is the sum of two exponential components with time constants of 7-8 and 26-29 ns, similar to previous reports. The addition of Ca2+ appears to relatively enhance the shorter component while reducing the longer one, although one or more additional slow components appear. [ABSTRACT FROM AUTHOR]

The aim of this study was to improve image quality of statistical reconstruction by using the 2-D system matrix (SM) trained with an artificial neural network (ANN). For training the ANN SM (SMANN), the inputs of ANN, the digital images, were generated by scanning the mini-deluxe cold spot phantom at ten different orientations using an optical scanner (resolution: 0.01 mm/pixel). The desired outputs were generated by acquiring the projection data with the corresponding angles using the micro positron emission tomography (microPET) R4. In the ANN method, the ADALINE network with a bias vector and a momentum term were used. Moreover, a multiline-source phantom and a four-segment phantom were scanned to obtain the spatial resolutions and the quantitative accuracy for comparison, respectively. A rat FDG microPET image was acquired to compare the difference between the results reconstructed by the microPET's built-in 2-D-ordered subsets expectation maximization algorithm (OSEM), OSEMmicroPET, OSEM by Monte Carlo simulated SM, OSEMSMd, and OSEM by the SMANN, OSEMANN. In the multiline-source experiment, the resolutions of OSEMmicroPET measured at center, 10, and 20 mm from the center were 1.61, 1.78, and 2.30 mm, respectively. The resolutions of OSEMSMd and OSEMANN were 1.24, 1.68, and 1.87, and 1.28, 1.62, and 1.72 mm, respectively. In the results of the four-segment phantom, the sum of absolute error of the truth versus the values reconstructed by OSEMmicroPET, OSEMSMd, and OSEMANN were 1085.53, 913.48, and 435.02, respectively. By interpreting the results of the evaluation, the image quality reconstructed by the SMANN is better than that reconstructed by the original SM The results indicated that SM can be updated toward ideal SM using ANN for statistical reconstruction. [ABSTRACT FROM AUTHOR]

Respiratory motion in PET degrades image quality and limits detectability of small or low-contrast lesions. Although image quality can be improved using respiratory-gating, this adds to the complexity and expense of acquiring PET data. We aimed to develop a data-driven method, based on individual voxel signal fluctuations, for accomplishing electronic respiratory gating of clinical, PET data, requiring no additional hardware or end-user input. We tested our methods using both simulated PET scans and actual human PET acquisitions. For the simulations, our methods correctly identified the start frame of each respiratory cycle defined for the phantom. Resultant gated images demonstrated improved effective resolution and increased measured uptake for lesions located in the thorax. For human PET data, we were able to recover respiratory phase information with a high signal-to-noise ratio. We report here a method to achieve fully automated voxel-based respiratory gating of PET images, without the need for gating hardware or additional user input, capable of improving effective resolution and increasing lesion detectability. [ABSTRACT FROM AUTHOR]

It has been shown that I-124 PET imaging can be used for accurate dose estimation in radio-immunotherapy techniques. However, I-124 is not a pure positron emitter, leading to two types of coincidence events not typically encountered: increased random coincidences due to non-annihilation cascade photons, and true coincidences between an annihilation photon and primarily a coincident 602 keV cascade gamma (true coincidence γ-ray background). The increased random coincidences are accurately estimated by the delayed window technique. Here we evaluate the radial and time distributions of the true coincidence γ-ray background in order to correct and accurately estimate lesion uptake for I-124 imaging in a time-of-flight (TOF) PET scanner. We performed measurements using a line source of activity placed in air and a water-filled cylinder, using F-18 and I-124 radio-isotopes. Our results show that the true coincidence γ-ray backgrounds in I-124 have a uniform radial distribution, while the time distribution is similar to the scattered annihilation coincidences. As a result, we implemented a TOF-extended single scatter simulation algorithm with a uniform radial offset in the tail-fitting procedure for accurate correction of TOF data in I-124 imaging. Imaging results show that the contrast recovery for large spheres in a uniform activity background is similar in F-18 and I-124 imaging. There is some degradation in contrast recovery for small spheres in I-124, which is explained by the increased positron range, and reduced spatial resolution, of I-124 compared to F-18. Our results show that it is possible to perform accurate TOF based corrections for I-124 imaging. [ABSTRACT FROM AUTHOR]

Silicon double-sided strip detectors offer outstanding instrinsic spatial resolution with reasonable detection efficiency for iodine-125 emissions. This spatial resolution allows for multiple-pinhole imaging at low magnification, minimizing the problem of multiplexing. We have conducted imaging studies using a prototype system that utilizes a detector of 300-micrometer thickness and 50-micrometer strip pitch together with a 23-pinhole collimator. These studies include an investigation of the synthetic-collimator imaging approach, which combines multiple-pinhole projections acquired at multiple magnifications to obtain tomographic reconstructions from limited-angle data using the ML-EM algorithm. Sub-millimeter spatial resolution was obtained, demonstrating the basic validity of this approach. [ABSTRACT FROM AUTHOR]

State-of-the-art data-acquisition systems for positron emission tomography (PET) block detectors use anger logic to determine preliminary 2-D coordinates of the detection location within the block. Typically, a crystal lookup table (CLT) is then used to map from. this position to the index of the crystal in which the particle was actually absorbed. Depending on the complexity of the detector design, it is difficult to calculate these CLTs automatically without need for visual inspection and manual readjustment. We propose a novel technique to construct the CLT. The assignment problem is formulated as an optimization problem, where the number of free parameters is initially twice the number of crystals in the block. The core idea is to reduce the dimension of the search space by principal component analysis. The technique was tested successfully for three different block detector designs. (Siemens ECAT Exact HR, Siemens microPET Focus 220, and a prototype block detector built of pixelated lutetium oxyorthosilicate crystals with avalanche photodiodes attached). Important virtues of the new algorithm are very fast processing and a high tolerance toward missing peaks, fused peaks, and nonlinear distortions. Furthermore, the algorithm calculates a performance control value that can be used to quickly identify problematic blocks. [ABSTRACT FROM AUTHOR]

A way to improve the spatial resolution in positron emission tomography (PET) is to determine the depth-of-interaction (DO!) in the detector. A way to achieve this is to use the phoswich approach, a detector with two or more layers of different scintillators. The layer identification is done by using differences in scintillation decay time and pulse shape discrimination techniques. The advantages of the concept have been demonstrated in the HRRT high resolution PET system using a LSO/LYSO combination giving a high spatial resolution uniformity of around 2.5 mm within a larger part of the imaged volume. A phoswich combination that lately has received attention is LuAP/LSO or LuYAP/LSO. The suggestions come from the Crystal Clear Collaboration and there is a patent application for its use in PET. This particular combination of phoswich may, however, have a complication since both LuAP and LuYAP emit in the excitation band of LSO, thus making the functionality more complex. In the present paper we have looked into this and suggested different ways to overcome potential drawbacks. [ABSTRACT FROM AUTHOR]

The development of a prototype dual-modality optical and PET (OPET) small animal imaging tomograph is underway in the Crump Institute for Molecular Imaging at the University of California Los Angeles. OPET consists of a single ring of six detector modules with a diameter of 3.5 cm. Each detector has an 8 × 8 array of optically isolated BGO scintillators which are coupled to multichannel photomultiplier tubes and open on the front end. The system operates in either PET or optical mode and reconstructs the data sets as 3D tomograms. The detectors are capable of detecting both annihilation events (511 keV) from PET tracers as well as Single Photon Events (SPEs) (2-3 eV) from bioluminescence. Detector channels are readout using a custom multiplex readout scheme and then filtered in analog circuitry using either a γ-ray or SPE specific filter. Shaped pulses are sent to a Digital Signal Processing (DSP) unit for event processing. The DSP unit has 100 MHz Analog-to-Digital Converters on the front-end which send digitized samples to Field Programmable Gate Arrays which are programmed via user configurable algorithms to process PET coincidence events or bioluminescence SPEs. Information determined using DSP includes: event timing, energy determination-discrimination, position determination-lookup, and coincidence processing. Coincidence or SPE events are recorded to an external disk and minimal post processing is required prior to image reconstruction. Initial imaging results from a phantom filled with 18FDG solution and an optical pattern placed on the front end of a detector module in the vicinity of a SPE source are shown. [ABSTRACT FROM AUTHOR]

This paper summarizes CERN R&D work done in the framework of the European Commission's FP6 BioCare Project. The objective was to develop a novel "time-based" signal processing technique to read out LSO-APD photodetectors for medical imaging. An important aspect was to employ the technique in a combined scenario for both Computer Tomography (CT) and Positron Emission Tomography (PET) with effectively no tradeoffs in efficiency and resolution compared to traditional single mode machines. This made the use of low noise and yet very high-speed monolithic front-end electronics essential so as to assure the required timing characteristics together with a high signal-to-noise ratio. Using APDs for photon detection, two chips, traditionally employed for particle physics, could be identified to meet the above criteria. Although both were not optimized for their intended new medical application, excellent performance in conjunction with LSO-APD sensors could be derived. Whereas a measured energy resolution of 16% (FWHM) at the 511 keV photo peak competes favorably with that of 'classical' PMTs, the coincidence time resolution of 1.6 ns FWHM with dual APD readout is typically lower. This is attributed to the stochastic photon production mechanism in LSO and the photon conversion characteristic of the photo diode, as well as to the fluctuations in photon conversion, albeit the APD's superior quantum efficiency. Also in terms of CT counting speed, the chosen readout principle is limited by the intrinsic light decay in LSO (49 as) for each impinging X-ray. [ABSTRACT FROM AUTHOR]

Patient motion is inevitable in SPECT and PET due to the lengthy period of time patients are imaged and patient motion can degrade diagnostic accuracy. The goal of our studies is to perfect a methodology for tracking and correcting patient motion when it occurs. In this paper we report on enhancements to the calibration, camera stability, accuracy of motion tracking, and temporal synchronization of a low-cost visual tracking system (VTS) we are developing. The purpose of the VTS is to track the motion of retro-reflective markers on stretchy bands wrapped about the chest and abdomen of patients. We have improved the accuracy of 3D spatial calibration by using a MATLAB optical camera calibration package with a planar calibration pattern. This allowed us to determine the intrinsic and extrinsic parameters for stereo-imaging with our CCD cameras. Locations in the VTS coordinate system are transformed to the SPECT coordinate system by a VTS/SPECT mapping using a phantom of 7 retro-reflective spheres each filled with a drop of Tc99m. We switched from pan, tilt and zoom (PTZ) network cameras to fixed network cameras to reduce the amount of camera drift. The improved stability was verified by tracking the positions of fixed retro-reflective markers on a wall. The ability of our VTS to track movement, on average, with sub-millimeter and sub-degree accuracy was established with the 7-sphere phantom for 1 cm vertical and axial steps as well as for an arbitrary rotation and translation. The difference in the time of optical image acquisition as decoded from the image headers relative to synchronization signals sent to the SPECT system was used to establish temporal synchrony between optical and list-mode SPECT acquisition. Two experiments showed better than 100 ms agreement between VTS and SPECT observed motion for three axial translations. We were able to track 3 reflective markers on an anthropomorphic phantom with a precision that allowed us to correct motion such- - that no loss in visual quality was noted in motion corrected slices relative to motion free slices. [ABSTRACT FROM PUBLISHER]

Several developments were made in the GATE simulation platform to allow accurate modeling of the count rate performances of PET scanners over a wide range of activity concentrations. A background noise module, a dead time and limited bandwidth modeling for the coincidences, and a delayed coincidence builder were added in the code. The results obtained for the modeling of the ECAT HRRT and Focus 220 scanners with the newly developed modules are discussed. They show that GATE can be used to accurately simulate the single event, prompt coincidence and delayed coincidence rates, from very low activity levels in the field of view up to levels that saturate the acquisition system. The new developments were committed into the public release of GATE, making them available for the whole community, thanks to the open source license under which GATE is published (LGPL). [ABSTRACT FROM AUTHOR]

Many PET scanners nowadays have the possibility to record event-by-event information, known as list mode data. This has the advantage of keeping the data in the highest possible resolution (both temporal and spatial). In most cases, list mode data are then binned into sinogram format before reconstruction. In this paper, we discuss at which stage normalisation factors should be introduced. It is shown that noise is greatly reduced by per- forming the normalisation after the binning. We illustrate this with acquired and simulated data for the quad HiDAC camera. [ABSTRACT FROM AUTHOR]

A small-animal positron emission tomography (PET) scanner using PS-APD (position-sensitive avalanche photodiode) detectors has been developed for simultaneous PET/MRI imaging. In this scanner, up to 16 detector modules (one PS-APD per module) are used, and each detector module produces 4 signals to be digitized with their peak values. This leads to as many as 64 analog outputs to the data acquisition (DAQ) system, requiring 64 DAQ channels. In the future, the system will be extended to 32 modules, resulting in 128 channels. It is possible to sample all channels simultaneously, but most of them do not contain useful data, since only one coincidence event (producing data on 8 channels) is identified each time. The purpose of this work was to develop a general-purpose method for reducing the number of analog inputs to the data acquisition for the sparse fast analog signals produced in a PET scanner. To achieve this, a multiplexer board was designed to sample coincidence events from the PET scanner. The effect of the multiplexer on signal quality was evaluated and the average peak-to-valley ratios in detector flood histograms with and without multiplexer were 2.97 and 3.02 respectively. On-board coincidence, pile-up rejection and multiple coincidence rejection functions were implemented and worked as expected. The dead time performance was also characterized. [ABSTRACT FROM AUTHOR]

MADPET-II is a small animal PET tomograph that features individual Lutetium Oxyorthosilicate (LSO) crystal readout from Avalanche PhotoDiodes (APDs). The detector signals are preamplified by 16-channel fully integrated ASICs which are placed as close as possible to the detector in order to avoid attenuation of the signal or unwanted stray capacitance. However, the power consumption of the preamplifier (30 mW per channel) can cause heat transfer and, consequently, gain drift to temperature sensitive detectors. Temperature measurements on the front-end electronics of MADPET-II have shown a maximum increase of approximately 30 °C in the area around the preamplifier and 10 °C in the area around the APD-LSO detector with respect to room temperature. In the presence of this temperature gradient, energy spectra have been acquired from which a significant drift of the photopeak (3.4% per °C) and a small increase of the mean energy resolution (3% over the whole temperature range studied) with increasing temperature has been observed. The effect of temperature on the time resolution is small in comparison to the effect of walk and jitter introduced by the analog processing electronics. The behavior of two 4 × 8 LSO-APD front-end detector arrays in coincidence at temperatures below ambient and at various values of the APD bias voltage in terms of energy and time resolution has also been studied. The total current drawn by the APDs (leakage current and photocurrent) has been monitored at various temperatures T and APD bias and was modelled and fitted by a theoretical function demonstrating a T and e-1/T dependence. No significant improvement on time resolution with decreasing temperature has been observed. For temperature stabilization and monitoring, thermoelectric cooling is considered appropriate for mounting in the limited free space of a PET scanner, especially when this is inside an MR scanner for simultaneous PET/MR imaging. [ABSTRACT FROM AUTHOR]

We describe a simple theoretical framework for linear time-of-flight (TOF) PET reconstruction and evaluate several alternative TOF filters. We implement a capability for direct computation of TOF noise variance images and evaluate the gain in image variance for TOF vs. non-TOF for 1.26 nsec TOF resolution on the Siemens biograph HiRez PET/CT. The variance calculation is validated by comparing to pixel variances computed from replicated synthetic data. TOF reconstruction does not give a uniform improvement in image noise variance, but reduces variance more toward the periphery of an object than at its center. The TOF performance gain depends in part on how random coincidences are processed, and is greater when the randoms fraction is higher, or when randoms smoothing is not employed. We confirm that the TOF gain is greater for larger objects than for smaller ones, but find discrepancies with the D/Δx rule of thumb. With 1.26 nsec resolution on the HiRez, TOF image noise gain factors (non-TOF/TOF variance) range from 1.5 to 2.2 on the measured data at hand. We predict that an improvement in TOF resolution by a factor of 2 (to 600 psec) could improve image variance on this machine by an additional factor of 1.8. We find that confidence weighted TOF reconstruction under-performs non-TOF reconstruction for objects whose diameters are small compared to the TOF resolution. [ABSTRACT FROM AUTHOR]

Growing interest in the development of dual modality positron emission/X-rays tomography (PET/CT) systems prompts researchers to face a new challenge: to acquire both the anatomical and functional information in the same measurement, simultane- ously using the same detection system and electronics. The aim of this work was to study a detector consisting of LaBr3, LSO or LYSO pixel crystals coupled to an avalanche photodiode (APD). The measurements covered tests of the detectors in PET and CT modes, respectively. The measurements included the determination of light output, energy resolution, the non-proportionality of the light yield and the time resolution for 511 keV annihilation quanta; analysis also included characterizing the PET detector, and determining the dependence of counting rate versus mean cur- rent of the APD in the X-ray detection. In the present experiment, the use of counting and current modes in the CT detection increases the dynamic range of the measured dose of X-rays by a factor of 20, compared to the counting mode alone. [ABSTRACT FROM AUTHOR]

In this paper, we describe a Monte Carlo simulation of the performance of a high-sensitivity and high-resolution small animal positron emission tomography (PET) scanner with a large axial field-of-view (AFOV). The simulated camera is based on the photomultiplier-quadrant-sharing (PQS) concept and composed of 180 blocks of 14 × 14 lutetium oxyorthosilicate (LSO) crystals each measuring 1.16 mm transaxially, 1.27 mm transaxially, and 9.4 mm radially. The camera has 84 detector rings with an 11.6 cm AFOV and a ring diameter of 16.6 cm. For the simulation, we used the Geant4 Application for Tomographic Emission (GATE) simulation package. We validated GATE by comparing its predictions for spatial resolution, absolute sensitivity, and count rate with measured data obtained using an existing bismuth germanate (BGO) based dedicated animal PET scanner that had a similar AFOV and ring diameter and was based on the PQS technique. Simulated and experimental images of the Data Spectrum Micro Deluxe phantom were also compared. The simulation data suggested that new LSO-based scanner could have reconstructed radial (tangential) spatial resolutions of 1.14 mm (1.14 mm), 1.31 mm (1.32 mm), 1.54 mm (1.52 mm), 2.01 mm (1.8 mm), and 2.4 mm (2.1 mm) at the center and 1 cm, 2 cm, 3 cm, and 4 cm off center, respectively. The simulation data also suggested that 1.2-mm hot rods in the Micro Deluxe phantom will be distinguishable. Simulation predicted an absolute sensitivity of about 7.3% for a point source at the center of the camera assuming an energy window of 300 keV to 750 keV, a coincidence time window of 8 ns, and a system dead time of 60 ns. [ABSTRACT FROM AUTHOR]

Component-based normalization is an important technique for PET scanners with a high number of lines of response (LOR), e.g., 4.5 × 109 for the HRRT. It reduces the problem of measuring the sensitivity of each LOR to that of estimating the individual crystal efficiencies (ϵ), e.g., 119808 for the HRRT. We propose a component-based method to compute ϵ for the HRRT. In addition, the block design of the HRRT produces pulse pile-up which causes apparent changes in ϵ with count rate. These effects occur within the block and between the front (LSO) and back (LYSO) crystal layers. We use a rotating source to measure the ϵ values and a decaying uniform phantom to account for ϵ variations with count rate. The computation of efficiencies is achieved with ∼1% statistical noise with an acquisition of ∼1 h. Count rate dependency of ϵ is implemented as a linear model in terms of block singles rate. Four approaches to modify ϵ with count rate were compared. Among them, an independent parameter for each crystal produced the best results, both visually and quantitatively. Failure to account for the count rate dependency in ϵ leads to high resolution artifacts in the reconstructed images, most visible in the transverse plane, in the center of the field-of-view. [ABSTRACT FROM AUTHOR]

We are developing dedicated breast and small animal positron emission tomography (PET) systems using scintillation detectors comprising 1 × 1 × 3 mm³ LSO crystals coupled to novel, extremely thin position-sensitive avalanche photodiodes (PSAPD). The detectors are placed in a novel configuration such that PSAPDs read the relatively large side faces of the crystals, with normally incident 511 keV photons entering parallel to the PSAPD surface. This configuration facilitates 1 mm³ spatial resolution, directly measured photon interaction depth in 2 cm thick LSO, and >90% scintillation light collection efficiency. For this design, extremely thin (<300 μm) PSAPDs are required to achieve high crystal packing fraction for high intrinsic detection efficiency. The standard PSAPD (8 × 8 mm² active area) is pack- aged on a ceramic substrate, which is not compact enough for the desired detector configuration. The new PSAPD is packaged on a polyimid (Kapton) "flex" circuit with ∼250 μm total thickness. This paper investigates whether the required modification to the PSAPD chip, packaging, and manufacturing processes lead to degraded performance for the thin device compared to the standard PSAPD. Electronically collimated coincidence measurements yielded an average intrinsic spatial resolution of 1.1 ± 0.1 mm FWHM, energy resolution of 10.8 ± 0.6% at 511 keV, and coincidence time resolution of 2.1 ± -0.2 ns FWHM for the standard PSAPD. The corresponding values measured for the thin PSAPD were very similar, 1.1 ± 0.1 mm FWHM, 10.9 ± 0.7%, and 2.0 ± -0.3 ns, respectively. In summary, the new thin PSAPD allows us to attain the design specifications for the proposed ultra-high resolution, high sensitivity PET systems and performs comparable to the standard device. [ABSTRACT FROM AUTHOR]

We are using a novel position sensitive avalanche photodiode (PSAPD) for the construction of a high resolution positron emission tomography (PET) camera. Up to now most researchers working with PSAPDs have been using an Anger-like positioning algorithm involving the four corner readout signals of the PSAPD. This algorithm yields a significant non-linear spatial "pin-cushion" distortion in raw crystal positioning histograms. In this paper, we report an improved positioning algorithm, which combines two diagonal corner signals of the PSAPD followed by a 45° rotation to determine the X or Y position of the interaction. We present flood positioning histogram data generated with the old and new positioning algorithms using a 3 × 4 array of 2 × 2 × 3 mm³ and a 3 × 8 array of 1 × 1 × 3 mm³ of LSO crystals coupled to 8 × 8 mm² PSAPDs. This new algorithm significantly reduces the pin-cushion distortion in raw flood histogram image. [ABSTRACT FROM AUTHOR]

This paper describes a novel x-ray/SPECT dual modality breast imaging system that provides 3D structural and functional information. While only a limited number of views on one side of the breast can be acquired due to mechanical and time constraints, we developed a technique to compensate for the limited angle artifact in reconstruction images and accurately estimate both the lesion size and radioactivity concentration. Various angular sampling strategies were evaluated using both simulated and experimental data. It was demonstrated that quantification of lesion size to an accuracy of 10% and quantification of radioactivity to an accuracy of 20% are feasible from limited-angle data acquired with clinically practical dosage and acquisition time. [ABSTRACT FROM AUTHOR]

Emission computed tomography reconstruction requires compensation for photon attenuation. The usual way to do this is by performing a transmission scan to reconstruct the attenuation map. An important improvement could be achieved if it is possible to retrieve the attenuation map directly from the activity data. Several approaches have been suggested before to do this by using iterative methods for solving Maximum Likelihood (ML) problems (or Penalized Maximum Likelihood, MAP) that take into account the Poisson nature of the noise. One of the main drawbacks has been that these methods tend to retrieve solutions that generate an undesired ‘crosstalk’ between the attenuation and the activity maps. In this paper, we present a new approach that consists of a combination of a minorizing function algorithm applied to the likelihood function plus the application of an appropriate decreasing multiplicative factor and iterative data refinement. We compare this new approach with previous ones and our simulations show very encouraging results as far as solving the ‘crosstalk’ problem is concerned. [ABSTRACT FROM AUTHOR]