Compact Positron Emission Tomography Research Articles

A Preclinical Positron Emission Tomography (PET) and Electron-Paramagnetic-Resonance-Imaging (EPRI) Hybrid System: PET Detector Module.

We report the design and experimental validation of a compact positron emission tomography (PET) detector module (DM) intended for building a preclinical PET and electron-paramagnetic-resonance-imaging hybrid system that supports sub-millimeter image resolution and high-sensitivity, whole-body animal imaging. The DM is eight detector units (DU) in a row. Each DU contains 12×12 lutetium-yttrium oxyorthosilicate (LYSO) crystals having a 1.05 mm pitch read by 4×4 silicon photomultipliers (SiPM) having a 3.2 mm pitch. A small-footprint, highly-multiplexing readout employing only passive electronics is devised to produce six outputs for the DM, including two outputs derived from SiPM cathodes for determining event time and active DU and four outputs derived from SiPM anodes for determining energy and active crystal. Presently, we have developed two DMs that are 1.28×10.24 cm2 in extent and approximately 1.8 cm in thickness, with their outputs sampled at 0.7 GS/s and analyzed offline. For both DMs, our results show successfully discriminated DUs and crystals. With no correction for SiPM nonlinearity, the average energy resolution for crystals in a DU ranges from 14% to 16%. While not needed for preclinical imaging, the DM may support 300-400 ps time-of-flight resolution.

Development of a compact and cost-effective PET detector module for a small-animal PET system with FESA ASIC

Due to the high cost and large channel number in positron emission tomography (PET) systems, the integration level and cost performance must be considered in the construction of commercial PET systems. In this paper, a highly integrated readout electronic system was designed and implemented based on a self-developed FESA (Front End for SiPM Arrays) readout ASIC; a compact cost-effective detector module with four lined up detector heads was built, and its performance was evaluated. Each detector head consisted of a 24× 24 LYSO crystal array (each crystal element had dimensions of 0.99 × 0.99 × 15 mm3), one end coupling to an 8 × 8 silicon photomultiplier array (SiPM array with dimensions of 3.2 × 3.2 mm2 each), and the other end covered with a 1 mm thick light guide layer. Since the front-end FESA readout chip employed a row-column summing circuit to reduce the readout channels from 64 to 16, we tried a new depth encoding method based on a single-end readout with the structure. The test results showed that the average coincidence time resolution (CTR), energy resolution, and depth of interaction (DOI) resolution of the detector are 413 ps, 15.7%, and 5.2 mm, respectively, while the crystal array in the detector head with 9-to-1 coupling can be clearly separated in the flood image. Compared with the detector head without the depth encoding ability, the clearness of the flood image was slightly decreased, but the energy resolution and time performance remained the same. We also implemented a central control board that connected all detector modules via fiber connections. The data from the detector modules could be collected over the connections, and the system central clock could be precisely distributed; therefore, the compactness of the electronics system was further enhanced.

High-resolution and high-sensitivity PET for quantitative molecular imaging of the monoaminergic nuclei: A GATE simulation study.

Quantitative in vivo molecular imaging of fine brain structures requires high-spatial resolution and high-sensitivity. Positron emission tomography (PET) is an attractive candidate to introduce molecular imaging into standard clinical care due to its highly targeted and versatile imaging capabilities based on the radiotracer being used. However, PET suffers from relatively poor spatial resolution compared to other clinical imaging modalities, which limits its ability to accurately quantify radiotracer uptake in brain regions and nuclei smaller than 3 mm in diameter. Here we introduce a new practical and cost-effective high-resolution and high-sensitivity brain-dedicated PET scanner, using our depth-encoding Prism-PET detector modules arranged in a conformal decagon geometry, to substantially reduce the partial volume effect and enable accurate radiotracer uptake quantification in small subcortical nuclei. Two Prism-PET brain scanner setups were proposed based on our 4-to-1 and 9-to-1 coupling of scintillators to readout pixels using mm3 and mm3 crystal columns, respectively. Monte Carlo simulations of our Prism-PET scanners, Siemens Biograph Vision, and United Imaging EXPLORER were performed using Geant4 application for tomographic emission (GATE). National Electrical Manufacturers Association (NEMA) standard was followed for the evaluation of spatial resolution, sensitivity, and count-rate performance. An ultra-micro hot spot phantom was simulated for assessing image quality. A modified Zubal brain phantom was utilized for radiotracer imaging simulations of 5-HT1A receptors, which are abundant in the raphe nuclei (RN), and norepinephrine transporters, which are highly concentrated in the bilateral locus coeruleus (LC). The Prism-PET brain scanner with 1.5 mm crystals is superior to that with 1 mm crystals as the former offers better depth-of-interaction (DOI) resolution, which is key to realizing compact and conformal PET scanner geometries. We achieved uniform 1.3 mm full-width-at-half-maximum (FWHM) spatial resolutions across the entire transaxial field-of-view (FOV), a NEMA sensitivity of 52.1 kcps/MBq, and a peak noise equivalent count rate (NECR) of 957.8 kcps at 25.2 kBq/mL using 450-650 keV energy window. Hot spot phantom results demonstrate that our scanner can resolve regions as small as 1.35 mm in diameter at both center and 10 cm away from the center of the transaixal FOV. Both 5-HT1A receptor and norepinephrine transporter brain simulations prove that our Prism-PET scanner enables accurate quantification of radiotracer uptake in small brain regions, with a 1.8-fold and 2.6-fold improvement in the dorsal RN as well as a 3.2-fold and 4.4-fold improvement in the bilateral LC compared to the Biograph Vision and EXPLORER, respectively. Based on our simulation results, the proposed high-resolution and high-sensitivity Prism-PET brain scanner is a promising cost-effective candidate to achieve quantitative molecular neuroimaging of small but important brain regions with PET clinicallyviable.

Performance comparison of dual-ended readout depth-encoding PET detectors based on BGO and LYSO crystals

The performance of dual-ended readout depth-encoding positron emission tomography (PET) detectors based on bismuth germanate (BGO) coupled to silicon photomultipliers (SiPM) arrays was measured for the first time and compared to lutetium-yttrium oxyorthosilicate (LYSO)-based detectors using the same readout. The BGO and LYSO crystal arrays all had a crystal pitch of 2.2 mm and were coupled to 8 × 8 SiPM arrays with a matching pitch of 2.2 mm, using a one-to-one coupling configuration. Three types of crystals with Toray reflector were used: polished LYSO, polished BGO, and unpolished BGO, and for two different crystal thicknesses of 20 mm and 30 mm. All the crystal elements in the BGO arrays were clearly resolved in the flood histogram. Better flood histograms were obtained using the LYSO arrays for a selected crystal thickness, and better flood histograms were obtained using the 20 mm thick crystal arrays for a selected crystal type. The average crystal level energy resolution and timing resolution for 20 mm polished LYSO, polished BGO and unpolished BGO crystals at their optimal SiPM bias voltage were 18.6 ± 1.3% and 1.19 ± 0.20 ns, 17.8 ± 0.8% and 4.43 ± 0.47 ns, and 18.0 ± 1.0% and 4.68 ± 1.0 ns, respectively. Depth-of-interaction (DOI) resolution of the 20 mm polished LYSO array was 2.31 ± 0.17 mm and for the 20 mm unpolished BGO array was 3.53 ± 0.25 mm. However, polished BGO arrays with Toray reflector did not provide DOI information. Our key conclusion is that dual-ended readout depth-encoding 20 mm thick unpolished BGO detectors are good candidates for low-activity PET systems with small field-of-view and low timing performance requirements, such as preclinical or compact organ-dedicated PET systems, with the advantage over LYSO of having no background radiation and significantly lower cost.

Evaluation of quantitative, efficient image reconstruction for VersaPET, a compact PET system.

Previously we developed a high-resolution positron emission tomography(PET) system-VersaPET-characterized by a block geometry with relatively large axial and transaxial interblock gaps and a compact geometry susceptible to parallax blurring effects. In this work, we report the qualitative and quantitative evaluation of a graphic processing unit (GPU)-accelerated maximum-likelihood by expectation-maximization (MLEM) image reconstruction framework for VersaPET which features accurate system geometry and projection space point-spread-function (PSF) modeling. We combined the ray-tracing module from software for tomographic image reconstruction (STIR),an open-source PET image reconstruction package, with VersaPET's exact block geometry for the geometric system matrix. Point-spread-function modeling of crystal penetration and scattering was achieved by a custom Monte-Carlo simulation for projection space blurring in all dimensions. We also parallelized the reconstruction in GPU taking advantage of the system's symmetry for PSF computation. To investigate the effects of PSF width, we generated and studied multiple kernels between one that reflects the true LYSO density in the MC simulation and another that reflects geometry only (no PSF). GATE simulations of hot and cold-sphere phantoms with spheres of different sizes, real microDerenzo phantom, and human blood vessel data were used to characterize the quantitative and qualitative performances of the reconstruction. Reconstruction with an accurate system geometry effectively improved image quality compared to STIR (version 3.0) which assumes an idealized system geometry. Reconstructions of GATE-simulated hot-sphere phantom data showed that all PSF kernels achieved superior performance in contrast recovery and bias reduction compared to using no PSF, but may introduce edge artifact and lumped background noise pattern depending on the width of PSF kernels. Cold-sphere phantom simulation results also indicated improvement in contrast recovery and quantification with PSF modeling (compared to no PSF) for 5 and 10mm cold spheres. Real microDerenzo phantom images with the PSF kernel that reflects the true LYSO density showed degraded resolving power of small sectors that could be resolved more clearly by underestimated PSF kernels, which is consistent with recent literature despite differences in scanner geometries and in approaches to system model estimation. The human vessel results resemble those of the hot-sphere phantom simulation with the PSF kernel that reflects the true LYSO density achieving the highest peak in the time activity curve (TAC) and similar lumped noise pattern. We fully evaluated a practical MLEM reconstruction framework that we developed for VersaPET in terms of qualitative and quantitative performance. Different PSF kernels may be adopted for improving the results of specific imaging tasks but the underlying reasons for the variation in optimal kernel for the real and simulation studies requires further study.

Modularized compact positron emission tomography detector for rapid system development.

We report the development of a modularized compact positron emission tomography (PET) detector that outputs serial streams of digital samples of PET event pulses via an Ethernet interface using the UDP/IP protocol to enable rapid configuration of a PET system by connecting multiple such detectors via a network switch to a computer. Presently, the detector is [Formula: see text] in extent (excluding I/O connectors) and contains an [Formula: see text] array of [Formula: see text] one-to-one coupled lutetium-yttrium oxyorthosilicate/silicon photomultiplier pixels. It employs cross-wire and stripline readouts to merge the outputs of the 216 detector pixels to 24 channels. Signals at these channels are sampled using a built-in 24-ch, 4-level field programmable gate arrays-only multivoltage threshold digitizer. In the computer, software programs are implemented to analyze the digital samples to extract event information and to perform energy qualification and coincidence filtering. We have developed two such detectors. We show that all their pixels can be accurately discriminated and measure a crystal-level energy resolution of 14.4% to 19.4% and a detector-level coincidence time resolution of 1.67ns FWHM. Preliminary imaging results suggests that a PET system based on the detectors can achieve an image resolution of [Formula: see text].

Performance evaluation of a LYSO-based PET scanner for monitoring of dose delivery in hadrontherapy

The DoPET scanner is a compact positron emission tomography (PET) device. It has been developed for monitoring the range of charged particles during therapy with hadron beams. Previous works have focused on the development and upgrade of the device and on data analysis. In this paper, a full performance characterization of the DoPET system in terms of the energy resolution, spatial resolution, sensitivity, uniformity, and noise equivalent count rate is reported. All measurements refer to an adapted version of the National Electrical Manufacturers Association (NEMA) NU 4 - 2008 protocol, which was written originally for small animal PET systems. Since DoPET is a dual head planar system, it requires a modified characterisation procedure with respect to those described for ring geometries as in the NEMA NU 4 - 2008 protocol. The presented procedure may be of interest for any other PET system with a similar geometry as DoPET.

Development of PET/MRI with insertable PET for simultaneous PET and MR imaging of human brain

The purpose of this study was to develop a dual-modality positron emission tomography (PET)/magnetic resonance imaging (MRI) with insertable PET for simultaneous PET and MR imaging of the human brain. The PET detector block was composed of a 4 × 4 matrix of detector modules, each consisting of a 4 × 4 array LYSO coupled to a 4 × 4 Geiger-mode avalanche photodiode (GAPD) array. The PET insert consisted of 18 detector blocks, circularly mounted on a custom-made plastic base to form a ring with an inner diameter of 390 mm and axial length of 60 mm. The PET gantry was shielded with gold-plated conductive fabric tapes with a thickness of 0.1 mm. The charge signals of PET detector transferred via 4 m long flat cables were fed into the position decoder circuit. The flat cables were shielded with a mesh-type aluminum sheet with a thickness of 0.24 mm. The position decoder circuit and field programmable gate array-embedded DAQ modules were enclosed in an aluminum box with a thickness of 10 mm and located at the rear of the MR bore inside the MRI room. A 3-T human MRI system with a Larmor frequency of 123.7 MHz and inner bore diameter of 60 cm was used as the PET/MRI hybrid system. A custom-made radio frequency (RF) coil with an inner diameter of 25 cm was fabricated. The PET was positioned between gradient and the RF coils. PET performance was measured outside and inside the MRI scanner using echo planar imaging, spin echo, turbo spin echo, and gradient echo sequences. MRI performance was also evaluated with and without the PET insert. The stability of the newly developed PET insert was evaluated and simultaneous PET and MR images of a brain phantom were acquired. No significant degradation of the PET performance caused by MR was observed when the PET was operated using various MR imaging sequences. The signal-to-noise ratio of MR images was slightly degraded due to the PET insert installed inside the MR bore while the homogeneity was maintained. The change of gain of the 256 GAPD/scintillator elements of a detector block was <3% for 60 min, and simultaneous PET and MR images of a brain phantom were successfully acquired. Experimental results indicate that a compact and lightweight PET insert for hybrid PET/MRI can be developed using GAPD arrays and charge signal transmission method proposed in this study without significant interference.

Development of a MPPC-based prototype gantry for future MRI-PET scanners

We have developed a high spatial resolution, compact Positron Emission Tomography (PET) module designed for small animals and intended for use in magnetic resonance imaging (MRI) systems. This module consists of large-area, 4 × 4 ch MPPC arrays (S11830-3344MF; Hamamatsu Photonics K.K.) optically coupled with Ce-doped (Lu,Y)2(SiO4)O (Ce:LYSO) scintillators fabricated into 16 × 16 matrices of 0.5 × 0.5 mm2 pixels. We set the temperature sensor (LM73CIMK-0; National Semiconductor Corp.) at the rear of the MPPC acceptance surface, andapply optimum voltage to maintain the gain. The eight MPPC-based PET modules and coincidence circuits were assembled into a gantry arranged in a ring 90 mm in diameter to form the MPPC-based PET system. We have developed two types PET gantry: one made of non-magnetic metal and the other made of acrylonitrile butadiene styrene (ABS) resins. The PET gantry was positioned around the RF coil of the 4.7 T MRI system. We took an image of a point }22Na source under fast spin echo (FSE) and gradient echo (GE), in order to measure the interference between the MPPC-based PET and MRI. The spatial resolution of PET imaging in a transaxial plane of about 1 mm (FWHM) was achieved in all cases. Operating with PET made of ABS has no effect on MR images, while operating with PET made of non-magnetic metal has a significant detrimental effect on MR images. This paper describes our quantitative evaluations of PET images and MR images, and presents a more advanced version of the gantry for future MRI/DOI-PET systems.

Qualification test of a MPPC-based PET module for future MRI-PET scanners

We have developed a high-resolution, compact Positron Emission Tomography (PET) module for future use in MRI-PET scanners. The module consists of large-area, 4×4 ch MPPC arrays (Hamamatsu S11827-3344MG) optically coupled with Ce:LYSO scintillators fabricated into 12×12 matrices of 1×1mm2 pixels. At this stage, a pair of module and coincidence circuits was assembled into an experimental prototype gantry arranged in a ring of 90mm in diameter to form the MPPC-based PET system. The PET detector ring was then positioned around the RF coil of the 4.7T MRI system. We took an image of a point 22Na source under fast spin echo (FSE) and gradient echo (GE), in order to measure interference between the MPPC-based PET and the MRI. We only found a slight degradation in the spatial resolution of the PET image from 1.63 to 1.70mm (FWHM; x-direction), or 1.48–1.55mm (FWHM; y-direction) when operating with the MRI, while the signal-to-noise ratio (SNR) of the MRI image was only degraded by 5%. These results encouraged us to develop a more advanced version of the MRI-PET gantry with eight MPPC-based PET modules, whose detailed design and first qualification test are also presented in this paper.

Accelerating Image Reconstruction in Dual-Head PET System by GPU and Symmetry Properties

Positron emission tomography (PET) is an important imaging modality in both clinical usage and research studies. We have developed a compact high-sensitivity PET system that consisted of two large-area panel PET detector heads, which produce more than 224 million lines of response and thus request dramatic computational demands. In this work, we employed a state-of-the-art graphics processing unit (GPU), NVIDIA Tesla C2070, to yield an efficient reconstruction process. Our approaches ingeniously integrate the distinguished features of the symmetry properties of the imaging system and GPU architectures, including block/warp/thread assignments and effective memory usage, to accelerate the computations for ordered subset expectation maximization (OSEM) image reconstruction. The OSEM reconstruction algorithms were implemented employing both CPU-based and GPU-based codes, and their computational performance was quantitatively analyzed and compared. The results showed that the GPU-accelerated scheme can drastically reduce the reconstruction time and thus can largely expand the applicability of the dual-head PET system.

Sci-Fri AM: Imaging - 03: Temperature dependence of a SiPM detector for an MR compatible PET system.

Silicon photomultiplier (SiPM) detectors are rapidly becoming the detector of choice for research and development of new detectors for positron emission tomography (PET) due to their combination of high gain, fast timing, compact form factor and ability to function in a magnetic field. We are investigating using SiPM based detectors in a compact PET system designed to be inserted into a 7T animal MRI system and enable simultaneous PET/MRI imaging. In order to understand the level of thermal stability required for this PET system, we examined the stability of a prototype SiPM detector vs. temperature. A detector was constructed using a SensL SPMArray4 SiPM array coupled to a LYSO scintillator crystal array. The temperature of the detector was varied between 23 and 60°C in 5°C steps. At each temperature setting data were collected to characterize the detector flood histogram, photopeak amplitude and energy resolution at 511 keV, timing resolution and signal arrival time. While the flood image showed no noticeable changes with temperature, the 511 keV photopeak amplitude showed a linear decrease of 1.5%/°C and the energy resolution degraded by 0.08%/°C. The timing resolution degraded by 1.5 ns, from 3.5 ns to 5 ns when the temperature changed from 23 to 60°C. Over this temperature range there was a shift in the signal arrival time of approximately 3 ns. These results demonstrate that the detector can be operated over a wide range of temperature, giving a large degree of flexibility in choosing an operating temperature set-point for our PET system.

Calibration of Dual-Ended Readout of Axially Oriented 100-mm-Long LYSO Crystals for Use in a Compact PET System

We are investigating the use of 100-mm-long axially oriented scintillator crystals read out at either end by position-sensitive photomultiplier tubes (PSPMTs) for application to a small-animal positron emission tomography (PET) system. A system design of this type requires a robust and efficient calibration of the detector response in the light-sharing direction in order to accurately decode the location of photon interaction. In this paper, we evaluate methods for determining the axial positioning calibration of dual-ended readout of lutetium yttrium oxyorthosilicate (LYSO) crystals wrapped in either Teflon or 3M Enhanced Specular Reflector (ESR). We compare a collimated irradiation method to a uniform irradiation method recently proposed by Shao. In the collimated irradiation method, an electronically collimated beam of 511-keV photons aimed perpendicular to the long axis of the crystal is used to irradiate the crystal at nine locations in steps of 1 cm. The ratio of the two PMT signals at each irradiation position is used to give the axial-positioning calibration relating ratio signal to position. In the method by Shao, the crystal was irradiated using either a uniform Ge line source, a Na point source, or the intrinsic background activity in the LYSO. To check the validity of both calibration methods for unbalanced detectors, three voltage variations ranging from 750-800 V were used for the two PMTs. For cases with balanced detector gains, the calibration curves obtained using uniform irradiation and Shao's method agreed very well with the reference curve obtained from the collimated irradiation measurements. For the case with detectors that had unbalanced gain, agreement between uniform irradiation and collimated irradiation methods was only achieved if the trigger threshold energies of the two PMTs were adjusted to give similar single-event rates in both PMTs. These findings suggest that Shao's calibration method using uniform irradiation will provide a simple, accurate, and rapid method for calibration of 100-mm-long dual-ended readout detectors.

A low-noise CMOS front-end for TOF-PET

An analogue CMOS front-end for triggering and amplification of signals produced by a silicon photomultiplier (SiPM) coupled to a LYSO scintillator is proposed. The solution is intended for time-of-flight measurement in compact Positron Emission Tomography (TOF-PET) medical imaging equipments where excellent timing resolution is required ( ≈ 100ps). A CMOS 0.13μm technology was used to implement such front end, and the design includes preamplification, shaping, baseline holder and biasing circuitry, for a total silicon area of 500x90 μm. Waveform sampling and time-over-threshold (ToT) techniques are under study and the front-end provides fast and shaped outputs for time and energy measurements. Post layout simulation results show that, for the trigger of a single photoelectron, the time jitter due to the pre-amplifier noise can be as low as 15 ps (FWHM), for a photodetector with a total capacitance of 70 pF. The very low input impedance of the pre-amplifier ( ≈ 5Ω) allows 1.8 ns of peaking time, at the cost of 10 mW of power consumption.

Patient self‐attenuation and technologist dose in positron emission tomography

Positron emission tomography (PET), with 511-keV radiation and long patient-uptake times, presents unique radiation safety concerns. This two-part study considers aspects of PET radiation safety as they relate to PET suite design, dose to the public, and technologist occupational dose. In the first part of the study, the self-attenuation of radiation by patients' bodies was quantified. The radiation exposure was measured at three positions from 64 patients injected with fluorine-18 fluorodeoxyglucose (FDG) during the uptake period. Compared with an in vitro control used as a point source, a significant decrease in exposure (>40% at 1 m) was observed due to nonuniform distribution of FDG and attenuation within the patients. The attenuation data are consistent with results from simulations [M. E. Phelps, "Comments and Perspectives," J. Nucl. Med. 45, 1601 (2004)] that treat the body as a uniform, water-filled cylinder. As distance is often the principal source of protection for 511-keV radiation, the considerable self-attenuation may allow for more compact PET suites. However, despite high patient self-attenuation, shielding, and standard precautionary measures, PET technologist occupational doses can remain quite high (approximately 12 mSv/year). The second part of this study tracked the daily dose received by PET technologists. Close technologist-patient interaction both during and following FDG administration, as much as 20 min/study, contribute to the high doses and point to the need for a more innovative approach to radiation protection for PET technologists.

Performance measurements of a depth-encoding PET detector module based on position-sensitive avalanche photodiode read-out

We are developing a high-resolution, high-efficiency positron emission tomography (PET) detector module with depth of interaction (DOI) capability based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends to position-sensitive avalanche photodiodes (PSAPDs). In this paper we present the DOI resolution, energy resolution and timing resolution results for complete detector modules. The detector module consists of a 7 × 7 matrix of LSO scintillator crystals (1 × 1 × 20 mm3 in dimension) coupled to 8 × 8 mm2 PSAPDs at both ends. Flood histograms were acquired and used to generate crystal look-up tables. The DOI resolution was measured for individual crystals within the array by using the ratio of the signal amplitudes from the two PSAPDs on an event-by-event basis. A measure of the total scintillation light produced was obtained by summing the signal amplitudes from the two PSAPDs. This summed signal was used to measure the energy resolution. The DOI resolution was measured to be 3–4 mm FWHM irrespective of the position of the crystal within the array, or the interaction location along the length of the crystal. The total light signal and energy resolution was almost independent of the depth of interaction. The measured energy resolution averaged 14% FWHM. The coincidence timing resolution measured using a pair of identical detector modules was 4.5 ns FWHM. These results are consistent with the design goals and the performance required of a compact, high-resolution and high-efficiency PET detector module for small animal and breast imaging applications.

Dual APD array readout of LSO crystals: optimization of crystal surface treatment

We are developing a compact positron emission tomography (PET) detector module with a depth of interaction capability (DOI) based on a lutetium oxyorthosilicate (LSO) scintillator array coupled at both ends by avalanche photodiode (APD) arrays. This leads to a detector with high sensitivity that can provide high and uniform image resolution. We report studies on improving the DOI resolution by optimizing the crystal surface treatment. Six 2/spl times/2/spl times/20 mm LSO crystals were treated with different surface finishes along their length: raw saw-cut, polished optical finish, and chemically etched by hot anhydrous phosphoric acid (H/sub 3/PO/sub 4/) with etching times varying from 1 to 5 min. The ratio of the signals from the two APD arrays was used to measure DOI, and the sum of the signals to measure the total light output. Crystals finished by chemical etching for 2-3 min gave the best overall detector performance, with DOI resolutions ranging from 3.1 to 3.9 mm for events above a 150-keV threshold and uniform light output for different DOI positions. The energy resolution ranged between 14% and 18%. This detector design appears promising for PET applications requiring very high resolution and high sensitivity, for example, in small animal imaging and human breast imaging.

A direct sinogram-restoration method for fast image reconstruction in compact DOI-PET systems

In a recent study, we proposed a sophisticated data-processing technique for correcting parallax errors that we believe allows for different, more efficient, and cost-effective design strategies. In particular, we demonstrated with a conceptual design the general feasibility of adopting this approach for the construction of compact positron emission tomography (PET) systems. In our design, we employed the new-generation depth-of-interaction detectors in a circular, highly compact PET system to generate imperfect, but redundant, data. The redundancy was exploited by sinograin restoration (SR) techniques to correct for the data imperfection, thereby obtaining high-quality PET images despite the use of a compact PET configuration. In this paper, we consider the specific task of applying the proposed design concept for building a dedicated, compact, small-animal PET system. We develop a direct SR method that is suitable for use in the small-animal system. We evaluate the performance of this conceptual design with computer simulations. Our results indicate that, with the effects of scatter and randoms ignored, the proposed design can produce images of quality comparable to those generated by a conventional larger ring system.

A 4/spl times/8 APD array, consisting of two monolithic silicon wafers, coupled to a 32-channel LSO matrix for high-resolution PET

Avalanche photodiodes (APDs) offer new design options for high-resolution positron emission tomography (PET) detectors. The goal of this study was the design of a very compact PET detector module with two layers. For this purpose, new monolithic arrays of 4/spl times/8 densely packed APDs (each with a 1.6/spl times/1.6 mm/sup 2/ sensitive area) were developed and evaluated for their suitability to read out 32 small (2/spl times/2/spl times/6 mm/sup 3/) lutetium oxyorthosilicate (LSO) crystals. The gain of the APDs was about 100, its standard deviation within an array was less than 12%. The average dark current was less than 45 nA per pixel, and the detector capacitance was about 12 pF. Energy resolution was 15% for gamma-rays with 511 keV. The time resolution was 2.5 ns (full-width at half-maximum). A high-reflectivity foil was used to hold all 32 individual crystals in a matrix. This matrix was glued directly on the APD array. The light crosstalk between the channels was only 7.9%. Together with a specially developed 16-channel low-noise JFET-CMOS preamplifier chip, a compact front-end detector module was built. The monolithic APD arrays proved to be stable and may, in combination with lutetium oxyorthosilicate (LSO) crystals, facilitate the design of various detector geometries, such as panels or multilayer modules.