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The use of monolithic scintillator-based photon detectors in positron emission tomography (PET) has emerged as an attractive alternative to traditional pixelated array designs. Monolithic-based detector designs employ the scintillation light distribution (LD) shape to provide a single 3-D photon interaction position per event, enabling high spatial resolution throughout the crystal volume. Since there are no intercrystal gaps, monolithic designs provide higher intrinsic detection efficiency compared to pixelated designs. However, in order to make the monolithic detector design practical for clinical PET systems, some major drawbacks need to be addressed, such as the time-consuming and complex calibration procedures to obtain precise spatial and timing information. This article gives a historical review of monolithic-based PET detectors, a description of their main advantages and challenges, describes the state-of-the-art, including their use in current commercial system, and ends with a future prospective.

A new small animal PET based on SiPM and monolithic LYSO crystals has been developed. Eight detector modules form the PET ring, each mounting an array of ${\text{12}} \times {\text{12}}$ SiPMs coupled to a readout providing the summed signals of the pixels on each of the 12 rows and 12 columns of the SiPM array. This design makes it possible to accurately determine the centroid of the scintillation light distribution with about 1.6 mm full width at half maximum (FWHM) resolution without correction for the 1 mm source size, and the photon depth of interaction (DOI) with nearly 2 mm FWHM. This single ring PET system has a homogeneous spatial resolution across the entire 80 mm transaxial field of view (FOV) of about 1 mm FWHM. The noise equivalent count rate (NECR) peak is estimated to occur at around 39.2 MBq with a rate of approximately 82.7 kcps for the mouse-like phantom and 22 kcps at 48.1 MBq for the rat-like phantom. Following the NEMA protocol, the peak absolute sensitivity in the center of the FOV is 2.8% for a 30% peak energy window. A pilot test injecting NaF to a mouse of 20 grams is also presented. Finally, the PET ring has been tested in front of a high field 15.2 T Magnetic Resonance (MR). No significant variation on energy and spatial resolution across the FOV has been observed due to the presence of the magnetic field.

Before microCT can be exploited to its full potential for longitudinal monitoring of transgenic and experimental mouse models of lung diseases, radiotoxic side-effects such as inflammation or fibrosis must be considered. We evaluated dose and potential radiotoxicity to the lungs for long-term respiratory gated high-resolution microCT protocols. Free-breathing C57Bl/6-mice underwent four different retrospectively respiratory gated microCT imaging schedules of repeated scans during five or twelve weeks, followed by ex vivo microCT and detailed histological and biochemical assessment of lung damage. Radiation exposure, dose and absorbed dose were determined by ionization chamber, thermoluminescent dosimeter measurements and Monte Carlo calculations. Despite the relatively large radiation dose delivered per microCT acquisition, mice did not show any signs of radiation-induced lung damage or fibrosis when scanned weekly during 5 and up to 12 weeks. Doubling the scanning frequency and once tripling the radiation dose, e.g. mimicking the instant repetition of a failed scan, also stayed without detectable toxicity after 5 weeks of scanning. Histological analyses confirmed the absence of radiotoxic damage to the lungs, demonstrating that long-term monitoring of mouse lungs using high-resolution microCT is safe. This opens perspectives for longitudinal monitoring of (transgenic) mouse models of lung diseases and therapeutic response on an individual basis with high spatial and temporal resolution, without concerns for radiation toxicity that could potentially influence the readout of microCT-derived lung biomarkers. This work further supports the introduction of microCT for routine use in the preclinical pulmonary research field where post mortem histological approaches are still the gold standard. ispartof: American Journal of Physiology. Lung Cellular and Molecular Physiology vol:309 issue:3 pages:L271-L279 ispartof: location:United States status: published

The main aim of this work is to provide a method to retrieve the intrinsic resolution of detector blocks based on monolithic crystals in a fully assembled scanner. This method suggests a software collimation to the original data. The results are compared with the traditional approach of separating two detector blocks far enough, resulting in geometrical collimation.An empirical equation has been deduced to fit the experimental data in which the detector intrinsic resolution follows a Gaussian distribution and the contribution of the source, given the small size of 0.25 mm in diameter, follows a Lorentzian profile. The experiments resulted in an average detector intrinsic spatial resolution of 0.6 mm FWHM, with a standard deviation error of 0.1 mm. These tests show a method to determine the intrinsic resolution of monolithic-based detector blocks, once assembled in the PET system, with high accuracy.

Purpose: To explore the potential of grating-based x-ray phase-contrast imaging for clinical applications, a first compact gantry system was developed. It is designed such that it can be implemented into anin-vivo small-animal phase-contrast computed tomography (PC-CT) scanner. The purpose of the present study is to assess the accuracy and quantitativeness of the described gantry in both absorption and phase-contrast. Methods: A phantom, containing six chemically well-defined liquids, was constructed. A tomography scan with cone-beam reconstruction of this phantom was performed yielding the spatial distribution of the linear attenuation coefficient μ and decrement δ of the complex refractive index. Theoretical values of μ and δ were calculated for each liquid from tabulated data and compared with the experimentally measured values. Additionally, a color-fused image representation is proposed to display the complementary absorption and phase-contrast information in a single image. Results: Experimental and calculated data of the phantom agree well confirming the quantitativeness and accuracy of the reconstructed spatial distributions of μ and δ. The proposed color-fused image representation, which combines the complementary absorption and phase information, considerably helps in distinguishing the individual substances. Conclusions: The concept of grating-based phase-contrast computed tomography(CT) can be implemented into a compact, cone-beam geometry gantry setup. The authors believe that this work represents an important milestone in translating phase-contrast x-ray imaging from previous proof-of-principle experiments to first preclinical biomedical imaging applications on small-animal models.

We have demonstrated that structures down to 150 nm can be visualized in X-ray projection images using nanofocus X-ray sources. Due to their unlimited depth of focus, they do not possess a limit on the specimen size. This is essential for three-dimensional tomographic imaging of samples with a diameter larger than a few microns. Further simulation studies have shown that optimization of the detector response curve and switching from a reflective X-ray target to a transmission target should allow us to reach sub-100-nm resolutions.

PURPOSE: Previously, we demonstrated the potential of positron emission tomography detectors consisting of monolithic scintillation crystals read out by arrays of solid-state light sensors. We reported detector spatial resolutions of 1.1-1.3 mm full width at half maximum (FWHM) with no degradation for angles of incidence up to 30 degrees, energy resolutions of approximately 11% FWHM, and timing resolutions of approximately 2 ns FWHM, using monolithic LYSO:Ce3+ crystals coupled to avalanche photodiode (APD) arrays. Here we develop, validate, and demonstrate a simple model of the detector point spread function (PSF) of such monolithic scintillator detectors. METHODS: A PSF model was developed that essentially consists of two convolved components, one accounting for the spatial distribution of the energy deposited by annihilation photons within the crystal, and the other for the influences of statistical signal fluctuations and electronic noise. The model was validated through comparison with spatial resolution measurements on a detector consisting of an LYSO:Ce3+ crystal read out by two APD arrays. RESULTS: The model is shown to describe the measured detector spatial response well at the noise levels found in the experiments. In addition, it is demonstrated how the model can be used to correct the measured spatial response for the influence of the finite diameter of the annihilation photon beam used in the experiments, thus obtaining an estimate of the intrinsic detector PSF. CONCLUSIONS: Despite its simplicity, the proposed model is an accurate tool for analyzing the detector PSF of monolithic scintillator detectors and can be used to estimate the intrinsic detector PSF from the measured one.

Item does not contain fulltext Much research is being conducted on position-sensitive scintillation detectors for medical imaging, particularly for emission tomography. Monte Carlo simulations play an essential role in many of these research activities. As the scintillation process, the transport of scintillation photons through the crystal(s), and the conversion of these photons into electronic signals each have a major influence on the detector performance; all of these processes may need to be incorporated in the model to obtain accurate results. In this work the optical and scintillation models of the GEANT4 simulation toolkit are validated by comparing simulations and measurements on monolithic scintillator detectors for high-resolution positron emission tomography (PET). We have furthermore made the GEANT4 optical models available within the user-friendly GATE simulation platform (as of version 3.0). It is shown how the necessary optical input parameters can be determined with sufficient accuracy. The results show that the optical physics models of GATE/GEANT4 enable accurate prediction of the spatial and energy resolution of monolithic scintillator PET detectors.

In the field of biomedical X-ray imaging, novel techniques, such as phase-contrast and dark-field imaging, have the potential to enhance the contrast and provide complementary structural information about a specimen. In this paper, a first prototype of a preclinical X-ray phase-contrast CT scanner based on a Talbot-Lau interferometer is characterized. We present a study of the contrast-to-noise ratios for attenuation and phase-contrast images acquired with the prototype scanner. The shown results are based on a series of projection images and tomographic data sets of a plastic phantom in phase and attenuation-contrast recorded with varying acquisition settings. Subsequently, the signal and noise distribution of different regions in the phantom were determined. We present a novel method for estimation of contrast-to-noise ratios for projection images based on the cylindrical geometry of the phantom. Analytical functions, representing the expected signal in phase and attenuation-contrast for a circular object, are fitted to individual line profiles of the projection data. The free parameter of the fit function is used to estimate the contrast and the goodness of the fit is determined to assess the noise in the respective signal. The results depict the dependence of the contrast-to-noise ratios on the applied source voltages, the number of steps of the phase stepping routine, and the exposure times for an individual step. Moreover, the influence of the number of projection angles on the image quality of CT slices is investigated. Finally, the implications for future imaging purposes with the scanner are discussed.

Monolithic scintillator blocks provide an efficient way to significantly increase the sensitivity of high-resolution positron emission tomography (PET) systems. Using statistical or neural network-based positioning schemes, the measured intrinsic detector resolution for perpendicular incident photons is 1.7 mm full-width at half-maximum (FWHM) for 10 mm thick LSO blocks and 2.0 mm FWHM for 20 mm thick LSO blocks. Because the positioning algorithms determine the incidence position as opposed to the combination of interaction position and interaction depth, a very good parallax correction can be achieved. Energy and detector time resolution were 11.5% FWHM and 1.6 ns FWHM, respectively.

The absence of very small crystal pixels in monolithic scintillation detectors has a number of potential advantages such as higher sensitivity, better energy resolution and continuous coordinates. In such detectors, the photon incidence position on the detector surface is derived from the measured scintillation light distribution using artificial neural networks (NNs). To this end, each detector module has to be position-calibrated by training the NNs. An automated procedure to simultaneously obtain the calibration data to train NNs for all detector modules in a fully assembled PET system has been developed and evaluated on a simulator set-up.

We have compared the performance of PET detector modules using continuous scintillator blocks with modules based on a large number of small crystal pixels. We show that a good spatial resolution can be obtained in undivided scintillators. One of the main advantages of the undivided scintillators is the excellent depth of interaction identification capability. There is almost no degradation of the spatial resolution upto incidence angles of 40° away from normal incidence.

The Crystal Clear Collaboration has designed and is building a high-resolution small animal PET scanner. The design is based on the use of the Hamamatsu R7600-M64 multi-anode photomultiplier tube and a LSO/LuYAP phoswich matrix with one to one coupling between the crystals and the photo-detector. The complete system will have 80 PM tubes in four rings with an inner diameter of 137 mm and an axial field of view of 110 mm. The PM pulses are digitized by free-running ADCs and digital data processing determines the gamma energy, the phoswich layer and even the pulse arrival time. Single gamma interactions are recorded and coincidences are found by software. The gantry allows rotation of the detector modules around the field of view. Simulations, and measurements a 2×4 module test set-up predict a spatial resolution of 1.5 mm in the centre of the field of view and a sensitivity of 5.9% for a point source in the centre of the field of view.

LuPO/sub 4/:Nd and YPO/sub 4/:Nd represent new and very fast potential scintillator materials. For most types of ionizing excitation, the luminescence of these materials is dominated by an emission band whose maximum intensity occurs at about 192 nm. The origin of this band lies in the fast 5d-4f transitions of the Nd/sup 3+/ ions. An additional contribution due to host-defect emission varies for different samples. These defects appear to be responsible for the limited light yield of LuPO/sub 4/:Nd.

Detailed results are given for the study of the spatial resolution obtained with different configurations of lutetium oxyorthosilicate (LSO) scintillation crystals read out by a Hamamatsu 2 /spl times/ 8 avalanche photodiode (APD) array. Block detectors as well as small-sized individual LSO crystals have been coupled to the APD matrix. Both one-to-one coupling and light-sharing schemes were investigated. The spatial resolution is derived for each configuration. Two different configurations with two layers of LSO crystals shifted by half a crystal size with respect to each other are used for investigations on the depth-of-interaction information. Prior to the spatial resolution measurements, a comparison of the light output for various crystal surface treatments and different optical coupling materials has been made.

The grating based approach to phase contrast imaging is rather inefficient in the use of the available x-ray flux due to the presence of two absorption gratings and it requires longer scan times compared to conventional CT because multiple images are needed at each projection angle. To avoid these drawbacks, a proof-of-principle experiment was developed to obtain absorption, phase contrast (DPC) and dark field images (DCI) in a single exposure using only a non-absorbing phase grating, a micro-focus source in cone-beam geometry and a highresolution x-ray detector.

The system has been shown to be a promising scintillator for medical imaging devices. Recently, efforts were focused on the improvement of its scintillating properties. Several large crystals with various cerium concentrations were grown. Absorption and excitation spectra were measured in a range extending from the visible to the vacuum ultraviolet (VUV). Emission spectra, fluorescence decay times and light yields, both under -ray and x-ray excitation, were measured under various experimental conditions. A reabsorption process is shown to take place in this material. This process is responsible for the observed decrease of the light yield when increasing the size of the sample.

A 3D small animal PET scanner, designed and built at the University of Brussel, has been operational since the beginning of 1996. The scanner has a transaxial field of view (FOV) of 110 mm and an axial FOV of 52 mm. The absolute sensitivity is around 35,000 coincidences s−1 MBq−1 for a point source at the center of the scanner and the time resolution is 29 ns FWHM. To measure the achievable spatial resolution, a thin 22Na source was scanned at various distances from the scanner axis. The resolution in a reconstructed image for a source close to the center is 3.0 mm FWHM. This figure can be improved at the expense of the sensitivity by lowering the voltage on the anodes in the wire chamber. Finally, to assess the overall image quality, scans were made of a cylindrical phantom with holes of varying diameter.

Novel radiography approaches based on the wave nature of x-rays when propagating through matter have a great potential for improved future x-ray diagnostics in the clinics. Here, we present a significant milestone in this imaging method: in-vivo multi-contrast x-ray imaging of a mouse using a compact scanner. Of particular interest is the enhanced contrast in regions related to the respiratory system, indicating a possible application in diagnosis of lung diseases (e.g. emphysema).

We have developed a compact grating-based in-vivo phase-contrast micro-CT system with a rotating gantry. The 50 W microfocus x-ray source operates with 20 to 50 kV peak energy. The length of the rotating interferometer is around 47 cm. Pixel size in the object is 30 micron; the field of view is approx. 35 mm in diameter, suited to image a mouse. The interferometer consists of three gratings: an absorption grating close to the x-ray source, a phase grating to introduce a π/2 phase shift and an absorption analyzer grating positioned at the first fractional Talbot distance. Numerous drives and actuators are used to provide angular and linear grating alignment, phase stepping and object/gantry precision positioning. Phantom studies were conducted to investigate performance, accuracy and stability of the scanner. In particular, the influences of gantry rotation and of temperature fluctuations on the interferometric image acquisition were characterized. Also dose measurements were performed. The first imaging results obtained with the system show the complementary nature of phase-contrast micro-CT images with respect to absorption-based micro-CT. Future improvements, necessary to optimize the scanner for in-vivo small-animal CT scanning on a regular and easy-to-use basis, are also discussed.

To explore the future clinical potential of improved soft-tissue visibility with grating-based X-ray phase contrast (PC), we have developed a first preclinical computed tomography (CT) scanner featuring a rotating gantry. The main challenge in the transition from previous bench-top systems to a preclinical scanner are phase artifacts that are caused by minimal changes in the grating alignment during gantry rotation. In this paper, we present the first experimental results from the system together with an adaptive phase recovery method that corrects for these phase artifacts. Using this method, we show that the scanner can recover quantitatively accurate Hounsfield units in attenuation and phase. Moreover, we present a first tomography scan of biological tissue with complementary information in attenuation and phase contrast. The present study hence demonstrates the feasibility of grating-based phase contrast with a rotating gantry for the first time and paves the way for future in vivo studies on small animal disease models (in the mid-term future) and human diagnostics applications (in the long-term future).

After successful demonstrations of soft-tissue phase-contrast imaging with grating interferometers at synchrotron radiation sources and at laboratory based x-ray tubes, a first preclinical CT scanner with grating based phase contrast imaging modality has been constructed. The rotating gantry is equipped with a three-grating interferometer, a 50 watt tungsten anode source and a Hamamatsu flat panel detector. The total length of the interferometer is 45 cm, and the bed of the scanner is optimized for mice, with a scanning diameter of 35 mm. From one single scan both phase-contrast and standard attenuation based tomography can be attained, providing an overall gain in image contrast.

Item does not contain fulltext Cramér-Rao theory can be used to derive the lower bound on the spatial resolution achievable with position-sensitive scintillation detectors as a function of the detector geometry and the pertinent physical properties of the scintillator, the photosensor and the readout electronics. Knowledge of the Cramér-Rao lower bound (CRLB) can for example be used to optimize the detector design and to test the performance of the method used to derive position information from the detector signals. Here, this approach is demonstrated for monolithic scintillator detectors for positron emission tomography. Two detector geometries are investigated: a 20 × 10 × 10 mm(3) and a 20 × 10 × 20 mm(3) monolithic LYSO:Ce(3+) crystal read out by one or two Hamamatsu S8550SPL avalanche photodiode (APD) arrays, respectively. The results indicate that in these detectors the CRLB is primarily determined by the APD excess noise factor and the number of scintillation photons detected. Furthermore, it is shown that the use of a k-nearest neighbor (k-NN) algorithm for position estimation allows the experimentally obtained spatial resolution to closely approach the CRLB. The approach outlined in this work can in principle be applied to any scintillation detector in which position information is encoded in the distribution of the scintillation light over multiple photosensor elements.

To explore the potential of grating-based x-ray phase-contrast imaging for clinical applications, a first compact gantry system was developed. It is designed such that it can be implemented into an in-vivo small-animal phase-contrast computed tomography (PC-CT) scanner. The purpose of the present study is to assess the accuracy and quantitativeness of the described gantry in both absorption and phase-contrast.A phantom, containing six chemically well-defined liquids, was constructed. A tomography scan with cone-beam reconstruction of this phantom was performed yielding the spatial distribution of the linear attenuation coefficient μ and decrement δ of the complex refractive index. Theoretical values of μ and δ were calculated for each liquid from tabulated data and compared with the experimentally measured values. Additionally, a color-fused image representation is proposed to display the complementary absorption and phase-contrast information in a single image.Experimental and calculated data of the phantom agree well confirming the quantitativeness and accuracy of the reconstructed spatial distributions of μ and δ. The proposed color-fused image representation, which combines the complementary absorption and phase information, considerably helps in distinguishing the individual substances.The concept of grating-based phase-contrast computed tomography (CT) can be implemented into a compact, cone-beam geometry gantry setup. The authors believe that this work represents an important milestone in translating phase-contrast x-ray imaging from previous proof-of-principle experiments to first preclinical biomedical imaging applications on small-animal models.

This paper presents a study of possible models to describe the relation between the scintillation light point-of-origin and the measured photo detector pixel signals in monolithic scintillation crystals. From these models the X, Y and depth of interaction (DOI) coordinates can be estimated simultaneously by nonlinear least-square fitting. The method depends only on the information embedded in the signals of individual events, and therefore does not need any prior position training or calibration. Three possible distributions of the light sources were evaluated: an exact solid-angle-based distribution, an approximate solid-angle distribution and an extended approximate solid-angle-based distribution which includes internal reflection at side and bottom surfaces. The performance of the general model using these three distributions was studied using Monte Carlo simulated data of a 20 × 20 × 10 mm lutetium oxyorthosilicate (Lu 2 SiO 5 or LSO) block read out by 2 Hamamatsu S8550 avalanche photo diode arrays. The approximate solid-angle-based model had the best compromise between resolution and simplicity. This model was also evaluated using experimental data by positioning a narrow 1.2 mm full width at half maximum (FWHM) beam of 511 keV photons at known positions on the 20 × 20 × 10 mm LSO block. An average intrinsic resolution in the X-direction of 1.4 mm FWHM was obtained for positions covering the complete block. The intrinsic DOI resolution was estimated at 2.6 mm FWHM.

Proper selection of modern key components allows eliminating most artifacts in micro-CT and nano-CT systems already during data acquisition. X-ray cameras with direct photon detection allow avoiding ring artifacts. Newly developed fully depleted CCD sensors show an energy response similar to traditional cameras with a thin scintillator, but without any geometrical distortions and flashes from x-ray photons penetrating through the fiber optics. Air-bearing rotation stages and piezo-positioning minimizes mechanical inaccuracies in acquiring angular projections. Beam hardening can be eliminated by energy-selective photon counting imaging.

An integrated microCT/microXRF system has been designed and built at SkyScan. The two sub-systems are aligned. The microCT provides 3D morphological information of the sample, which can be also used for attenuation correction during microXRF reconstruction. The microXRF, based on a pin-hole collimator and a photon-counting energy-sensitive 2Ddetector, obtains 2D projections of 3D chemical composition inside the sample with 50-70 microns spatial resolution. The reconstruction of 3D microXRF scans is challenging because of very low photon counting statistics due to limited power of laboratory x-ray sources and the strong self-absorption of the low-energy fluorescence photons. We have developed a maximum-likelihood expectation-maximization (ML-EM) algorithm based on Poisson model. This algorithm has proven to be rather robust and good reconstructions have been obtained with sample scans. Regularization is necessary to achieve stable reconstruction. One method is to apply smoothing between iterations. Two different smoothing kernels have been evaluated: 3D symmetric Gaussian kernel and minimization of total variation. For further improvement, a multi-ray resolution recovery technique has been evaluated. The self-absorption is currently compensated by a simplified method: the correction coefficients are pre-calculated and obtained by forward-projecting the attenuation map for both the primary X-rays and the fluorescence photons. The attenuation maps at the energy of fluorescence photons are approximated from the CT image.

X-ray fluorescence (XRF) allows imaging of the chemical composition of a specimen. We developed a 2 nd generation prototype laboratory system that can produce 3D chemical maps using microXRF as well as volumetric microCT images. The latter can be used to overlay morphological information on top of the XRF image for co-registration. It is also employed for attenuation correction during the tomographic reconstruction of the XRF images. The new system has various hardware and software changes to improve the performance, stability and flexibility. A deep depleted CCD was employed to improve the detection efficiency for high-energy fluorescence X rays. The use of a deep depleted CCD requires signal-clustering techniques to correct for charge diffusion in the CCD to obtain the correct energy of the fluorescence x rays. Furthermore, energy drift correction techniques were put in place to ensure stability of energy measurement during very long scan times. To minimize the contribution of the long CCD readout times to the total scan time, the exposure frames are dynamically adjust during the scan to the maximum time allowed for operation under photon counting mode. The XRF component has a spatial resolution of 70 μm and an energy resolution of 180 eV at 6.4 keV.

Most X-ray systems are limited in spatial resolution by the x-ray source performance. In laboratory sources, x-rays are generated by the interaction of an electron beam with a metal target. Bulk target sources produce a spot size in the micron range. Thin layer targets allow a spot size improvement down to hundreds of nanometers, but with a significant flux reduction. Until now a spatial resolution under 100 nm could only be achieved by imaging with Fresnel zone plates with limited depth of focus, typically - several microns. This is acceptable for imaging of flat objects, but it creates a problem for tomography, which requires all parts of a bulk object to be in focus. To overcome the limitations, we invented an x-ray source with a new type of target. Because x-ray cameras can only collect photons from a small angle, the new emitter is physically shaped in such way that the camera can see it as a small dot, but it has a big length along the direction perpendicular to the camera creating a significant flux without compromising the resolution. Evaluation shows that structures down to 50 nm can be distinguished while maintaining a significant x-ray flux and infinite depth of focus required for nano-tomographical reconstruction.

The monolithic scintillator block approach for gamma detection in the Positron Emission Tomography (PET) avoids estimating Depth of Interaction (DOI), reduces dead zones in detector and diminishes costs of detector production. Neural Networks (NN) are very efficient to determine the entrance point of a gamma incident on a scintillator block. This paper presents results on the robustness of the spatial resolution as a function of the random fraction in the data, temperature and HV fluctuations. This is important when implementing the method in a real scanner. Measurements were done with two Hamamatsu S8550 APD arrays, glued on a 20×20×10 mm3 monolithic LSO crystal block.

Vsing a nano-focus x-ray source we have been able to visualize structures down to 150 nm in x-ray projection images. Because of their unlimited depth of focus, these sources don't restrict the specimen size in case of 3D tomographic imaging. Simulation studies have shown that optimization of the detector response curve and switching from a reflective x-ray target to a transmission target might allow us to reach sub-100 nm resolutions.

We are currently developing a prototype of monolithic scintillator PET detector module based on neural network position estimators. The detector module comprises of a 25.5mm×25.5mm×10mm LYSO crystal coupled to Hamamatsu 64 channels multi-anode PMT H7546B. Comparing with classical pixelated detectors, the prominent drawbacks of neural network based detector module are lower signal to noise ratio and more complicated signal readout scheme and data processing. The former needs low noise multi-channel readout front-end electronics with high dynamic range and high resolution signal digitization; meanwhile the latter can resort to modern FPGA technology. This paper describes our design and implementation of electronics for single block crystal PET detector modules which we are building. We choose to digitize 16 readout signals compressed from the 64 PMT channels after the simulating optimization of several possible signal readout geometries. Benefiting from the capacity and the flexibility of resource allocation of modern general class of FPGA, we implement all necessary digital nuclear signal processing techniques into a FPGA. Specially, the neural network position calculation is realized also in the FPGA. The validation and performance tests show that the system functions well with 15.3M events on-line processing throughput. The modular detector prototypes based on this electronics system can be a good platform for future physical experiments.

We developed positron emission tomography (PET) detectors based on monolithic scintillation crystals and position-sensitive light sensors. Intrinsic depth-of-interaction (DOI) correction is achieved by deriving the entry points of annihilation photons on the front surface of the crystal from the light sensor signals. Here we characterize the next generation of these detectors, consisting of a 20 mm thick rectangular or trapezoidal LYSO:Ce crystal read out on the front and the back (double-sided readout, DSR) by Hamamatsu S8550SPL avalanche photodiode (APD) arrays optimized for DSR. The full width at half maximum (FWHM) of the detector point-spread function (PSF) obtained with a rectangular crystal at normal incidence equals approximately 1.05 mm at the detector centre, after correction for the approximately 0.9 mm diameter test beam of annihilation photons. Resolution losses of several tenths of a mm occur near the crystal edges. Furthermore, trapezoidal crystals perform almost equally well as rectangular ones, while improving system sensitivity. Due to the highly accurate DOI correction of all detectors, the spatial resolution remains essentially constant for angles of incidence of up to at least 30 degrees . Energy resolutions of approximately 11% FWHM are measured, with a fraction of events of up to 75% in the full-energy peak. The coincidence timing resolution is estimated to be 2.8 ns FWHM. The good spatial, energy and timing resolutions, together with the excellent DOI correction and high detection efficiency of our detectors, are expected to facilitate high and uniform PET system resolution.

This work is part of the design study for a medical positron emission tomography (PET) system based on the use of BaF 2 scintillator and photosensitive wire chambers with TMAE. The number of photoelectrons per MEV (phe/MeV) is measured in the wire chamber and on a photomultiplier for crystals from five different suppliers. For the best crystals 26.7±2.0 phe/MeV in TMAE are observed, corresponding to an average quantum efficiency of TMAE for the fast component of BaF 2 of (7.6±0.7)%. The performance of a PET scanner based on this principle was evaluated with a detailed Monte Carlo simulation. It is shown that the reduced energy resolution with wire chambers has only a marginal effect on the scatter rejection in the scanner. Finally LaF 3 :Nd 3+ as an alternative to BaF 2 was tested and found to give a disappointing photoelectron yield.

The optimal pixel size of SiPMT arrays to read out monolithic 20x20x10 mm LSO scintillator block detectors has been studied. The influence of different SiPMT parameters on the achievable spatial resolution was compared. It was concluded that a SiPMT array with 3*3 mm pixel size, pitch 3.4 mm pixel, 30% detection efficiency, 100 ìm *100 ìm micro-cell size and crosstalk smaller than 20% is a good and realistic design that achieves better spatial and energy resolution than our current monolithic scintillator block readout based on Hamamatsu S8550 APD arrays.

We are presently developing a novel PET scanner for human brain functional imaging based on monolithic scintillator crystals read by APD matrices, capable of being inserted into an MRI system. In this work we report on the detailed study that has been made of the design of the detector blocks, aiming at defining the most suitable geometrical and readout configuration for optimizing the overall performance of the entire scanner. Both parallel and trapezoidal geometries have been simulated, featuring two layers of active scintillator material with different or similar thickness and APD readout on the front or back side. Results of this study indicate that a trapezoidal geometry with equal thickness of both layers is the best solution for improving the expected scanner performance.

In this work we present experimental results obtained with a PET prototype module based on monolithic block detectors of LYSO:Ce coupled to 2 Hamamatsu S8550-02 APD arrays and a dedicated ASIC front end readout. This module has been designed to be compatible with strong magnnetic fields and will be used on a research prototype for PET/MRI human brain imaging. The front-end electronics presented is based on a new ASIC: the VATA240. This ASIC implements the sum of the signals provided by the 8 pixels of each column and row of the detector array, composed of two APD modules. Using these 16 summed channels instead of the output of the 64 single pixels. the signal to noise ratio of the overall detector response is enhanced. To estimate the impinging photon direction we use Neural Networks (NN). The use of the summed channels also reduces the complexity of the NN algorithms implemented. The characterization of the VATA240 ASIC and the adjustment of key parameters have been carried out, and both the spatial and energy resolution with an electronically-collimated 22Na radioactive source have been experimentally evaluated.

Neural Networks trained with error back propagation Levenberg-Marquardt training (LM), Neural Networks trained with an algebraic method and Support Vector Machines (SVM) were evaluated to extract the position information from measured light distributions generated by the interactions of 511 keV photons in monolithic scintillator blocks. All three algorithms can achieve a similar average resolution (similar to 1.6 mm FWHM in a 20 x 10 x 10 mm LSO block) but the LM trained neural networks do so most efficiently. When the incidence angle of the photons increases to 30 degrees, the resolution degrades slightly to 2.0 mm FWHM. A small mismatch (

The CRYSTAL CLEAR collaboration, in short CCC, is a consortium of 12 academic institutions, mainly from Europe, joining efforts in the area of developing instrumentation for nuclear medicine and medical imaging. In the framework of the CCC a high performance small animal PET system, called ClearPET, was developed by using new technologies in electronics and crystals in a phoswich arrangement combining two types of lutetium- based scintillator materials: LSO:Ce and LuYAP:Ce. Our next aim will be the development of hybrid image systems. Hybrid MR-PET imaging has many unique advantages for brain research. This has sparked a new research line within CCC for the development of novel MR-PET compatible technologies. MRI is not as sensitive as PET but PET has poorer spatial resolution than MRI. Two major advantages of PET are sensitivity and its ability to acquire metabolic information. To assess these innovations, the development of a 9.4T hybrid animal MR-PET scanner is proposed based on an existing 9.4T MR scanner that will be adapted to enable simultaneous acquisition of MR and PET data using cutting- edge technology for both MR and PET.

The CRYSTAL CLEAR collaboration, in short CCC, is a consortium of 12 academic institutions, mainly from Europe, joining efforts in the area of developing instrumentation for nuclear medicine and medical imaging. In the framework of the CCC a high performance small animal PET system, called ClearPET, was developed by using new technologies in electronics and crystals in a phoswich arrangement combining two types of lutetium-based scintillator materials: LSO:Ce and LuYAP:Ce. Our next aim will be the development of hybrid image systems. Hybrid MR-PET imaging has many unique advantages for brain research. This has sparked a new research line within CCC for the development of novel MR-PET compatible technologies. MRI is not as sensitive as PET but PET has poorer spatial resolution than MRI. Two major advantages of PET are sensitivity and its ability to acquire metabolic information. To assess these innovations, the development of a 9.4T hybrid animal MR-PET scanner is proposed based on an existing 9AT MRscanner that will be adapted to enable simultaneous acquisition of MR and PET data using cutting-edge technology for both MR and PET.

Several positioning algorithms are tested to extract position information from the measured scintillation light distribution generated in monolithic LSO blocks of various shapes and read out by a Hamamatsu S8550 APD array. The intrinsic detector resolutions of photons impinging at different angles e.g. 0deg, plusmn 10deg, plusmn 20deg, plusmn 30deg are studied. To this end, we evaluate the following positioning algorithms : Neural Networks trained with error back propagation (Levenberg-Marquardt), Neural Networks trained with an algebraic method and Support Vector Machines (SVM).

The performance of a small-animal positron emission tomography (PET) scanner based on monolithic scintillation detectors read-out by avalanche photo-diode arrays has been investigated by simulation. By minimizing dead space, both within and between the modules, these detectors offer increased detection efficiency compared to pixellated detectors. The spatial resolution of the scanner was investigated in 2-D by simulating a point source at various radial distances from the center. To model the detector response, measured detector line-spread functions were used. An optimum value of approximately 1 mm FWHM was found at 10 mm radial distance from the scanner central axis. Point-source sensitivity profiles in the radial and axial directions were simulated at 1 MBq activity using the Monte-Carlo code GATE. They indicated that monolithic designs increase the sensitivity roughly by a factor of two compared to pixellated designs. NECR curves simulated for these scanner designs show no significant degradation of the performance for activities up to 40 MBq.

Experimental evaluation of the imaging system Clear-PEM for positron emission mammography, under development within the framework of the crystal clear collaboration at CERN, is presented in terms of its long-term stability. The detector modules and experimental setup are described. Time evolution results of signal yield, energy resolution, depth-of-interaction and inter-channel crosstalk for a reference detector module are reported.

We propose and evaluate an improved design at the level of PET detector blocks based on monolithic crystals that will eventually be used on a research prototype for human brain PET/MRI imaging - the BrainPET scanner. These new detector blocks, when compared with pixilated designs, feature simpler mechanics, lower cost, larger sensitive volume, better energy and spatial resolutions, all of which contribute to improvements in PET detector technology. Moreover, the magnetic compatibility of all the materials composing the block makes it suitable for operation inside an MRI scanner. Results from both experimental data and Monte Carlo simulations allow an evaluation of the performance of the detector blocks, illustrating their potential for high-sensitivity PET imaging of the human brain.

The Clear-PEM scanner for positron emission mammography under development is described. The detector is based on pixelized LYSO crystals optically coupled to avalanche photodiodes and readout by a fast low-noise electronic system. A dedicated digital trigger (TGR) and data acquisition (DAQ) system is used for on-line selection of coincidence events with high efficiency, large bandwidth and small dead-time. A specialized gantry allows to perform exams of the breast and of the axilla. In this paper we present results of the measurement of detector modules that integrate the system under construction as well as the imaging performance estimated from Monte Carlo simulated data. http://www.sciencedirect.com/science/article/B6TJM-4M942B5-D/1/e8aea93baa1aeae3538ea200a5a54665

Monolithic scintillation detectors are very promising for high resolution and high sensitivity positron emission tomography. These detectors consist of a few cubic centimeters of scintillating material coupled to one or more position-sensitive APD arrays. The entry point of an impinging annihilation photon is estimated from the light distribution on the APD pixels. In this paper, a model will be derived that predicts the line spread function of these detectors. This model includes the influences of the finite width of the measurement beam, scatter of radiation inside the detector, light yield and intrinsic energy resolution of the scintillator, quantum efficiency, gain and excess noise factor of the avalanche photo-diode arrays, and electronic noise. With this model a better understanding of the behavior of the detector is possible, which can make optimization more efficient.

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