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Author(s): Huh, Yoonsuk; Yang, Jaewon; Dim, Odera U; Cui, Yonggang; Tao, Weijie; Huang, Qiu; Gullberg, Grant T; Seo, Youngho | Abstract: PurposeSingle photon emission computed tomography (SPECT) scanners using cadmium zinc telluride (CZT) offer compact, lightweight, and improved imaging capability over conventional NaI(Tl)-based SPECT scanners. The main purpose in this study is to propose a full-ring SPECT system design with eight large-area CZT detectors that can be used for a broad spectrum of SPECT radiopharmaceuticals and demonstrate the performance of our system in comparison to the reference conventional NaI(Tl)-based two-head Anger cameras.MethodsA newly designed full-ring SPECT system is composed of eight large-area CZT cameras (128nmmn×n179.2nmm effective area) that can be independently swiveled around their own axes of rotation independently and can have radial motion for varying aperture sizes that can be adapted to different sizes of imaging volume. Extended projection data were generated by conjoining projections of two adjacent detectors to overcome the limited field-of-view (FOV) by each CZT camera. Using Monte Carlo simulations, we evaluated this new system design with digital phantoms including a Derenzo hot rod phantom and a Zubal brain phantom. Comparison of performance metrics such as spatial resolution, sensitivity, contrast-to-noise ratio (CNR), and contrast-recovery ratio was made between our design and conventional SPECT scanners having different pixel sizes and radii of rotation (one clinically well-known type and two arbitrary types matched to our proposed CZT-SPECT geometries).ResultsThe proposed scanner could result in up to about three times faster in acquisition time over conventional scan time at same acquisition time per step. The spatial resolution improvement, or deterioration, of our proposed scanner compared to the clinical-type scanner was dependent upon the location of the point source. However, there were overall performance improvements over the three different setups of the conventional scanner particularly in volume sensitivity (approximately up to 1.7 times). Overall, we successfully reconstructed the phantom image for both 99m Tc-based perfusion and 123 I-based dopamine transporter (DaT) brain studies simulated for our new design. In particular, the striatal/background contrast-recovery ratio in 3-to-1 reference ratio was over 0.8 for the 123 I-based DaT study.ConclusionsWe proposed a variable-aperture full-ring SPECT system using combined pixelated CZT and energy-optimized parallel-hole collimator modules and evaluated the performance of this scanner using relevant digital phantoms and MC simulations. Our studies demonstrated the potential of our new full-ring CZT-SPECT design, showing reduced acquisition time and improved sensitivity with acceptable CNR and spatial resolution.

Purpose Radiation dose calculated on tumors for radiopharmaceutical therapy varies significantly from tumor to tumor and from patient to patient. Accurate estimation of radiation dose requires multiple time point measurements using radionuclide imaging modalities such as SPECT or PET. In this report, we show our technical development of reducing the number of scans needed for reasonable estimation of tumor and normal organ dose in our pretherapy imaging and dosimetry platform of 124 I-metaiodobenzylguanidine (MIBG) positron emission tomography/computed tomography (PET/CT) for 131 I-MIBG therapy of neuroblastoma. Methods We analyzed the simplest kinetic data, areas of two-time point data for five patients with neuroblastoma who underwent 3 or 4 times of 124 I-MIBG PET/CT scan prior to 131 I-MIBG therapy. The data for which we derived areas were percent of injected activity (%IA) and standardized uptake value of tumors. These areas were correlated with time-integrated activity coefficients (TIACs) from full data (3 or 4 time points). TIACs are direct correlates with radiation dose as long as the volume and the radionuclide are known. Results The areas of %IAs between data obtained from all the two-time points with time points 1 and 2 (day 0 and day 1), time points 2 and 3 (day 1 and day 2), and time points 1 and 3 (day 0 and day 2) showed reasonable correlation (Pearson's correlation coefficient |r| > 0.5) with not only tumor and organ TIACs but also tumor and organ absorbed doses. The tumor and organ doses calculated using %IA areas of time point 1 and time point 2 were our best fits at about 20% individual percent difference compared to doses calculated using 3 or 4 time points. Conclusions We could achieve reasonable accuracy of estimating tumor doses for subsequent radiopharmaceutical therapy using only the two-time point imaging sessions. Images obtained from these time points (within the 48-h after administration of radiopharmaceutical) were also viewed as useful for diagnostic reading. Although our analysis was specific to 124 I-MIBG PET/CT pretherapy imaging data for 131 I-MIBG therapy of neuroblastoma and the number of imaging datasets was not large, this feasible methodology would generally be applicable to other imaging and therapeutic radionuclides with an appropriate data analysis similar to our analysis to other imaging and therapeutic radiopharmaceuticals.

The metaiodobenzylguanidine (MIBG) scan is one of the most sensitive noninvasive lesion detection modalities for neuroblastoma. Unlike (123)I-MIBG, (124)I-MIBG allows high-resolution PET. We evaluated (124)I-MIBG PET/CT for its diagnostic performance as directly compared with paired (123)I-MIBG scans. Methods: Before (131)I-MIBG therapy, standard (123)I-MIBG imaging (5.2 MBq/kg) was performed on 7 patients, including whole-body (anterior–posterior) planar imaging, focused-field-of-view SPECT/CT, and whole-body (124)I-MIBG PET/CT (1.05 MBq/kg). After therapy, 2 of 7 patients also completed (124)I-MIBG PET/CT as well as paired (123)I-MIBG planar imaging and SPECT/CT. One patient underwent (124)I-MIBG PET/CT only after therapy. We evaluated all 8 patients who showed at least 1 (123)I-MIBG–positive lesion with a total of 10 scans. In 8 pairs, (123)I-MIBG and (124)I-MIBG were performed within 1 mo of each other. The locations of identified lesions, the number of total lesions, and the curie scores were recorded for the (123)I-MIBG and (124)I-MIBG scans. Finally, for 5 patients who completed at least 3 PET/CT scans after administration of (124)I-MIBG, we estimated the effective dose of (124)I-MIBG. Results: (123)I-MIBG whole-body planar scans, focused-field-of-view SPECT/CT scans, and whole-body (124)I-MIBG PET scans found 25, 32, and 87 total lesions, respectively. There was a statistically significant difference in lesion detection for (124)I-MIBG PET/CT versus (123)I-MIBG planar imaging (P < 0.0001) and (123)I-MIBG SPECT/CT (P < 0.0001). The curie scores were also higher for (124)I-MIBG PET/CT than for (123)I-MIBG planar imaging and SPECT/CT in 6 of 10 patients. (124)I-MIBG PET/CT demonstrated better detection of lesions throughout the body, including the chest, spine, head and neck, and extremities. The effective dose estimated for patient-specific (124)I-MIBG was approximately 10 times that of (123)I-MIBG; however, given that we administered a very low activity of (124)I-MIBG (1.05 MBq/kg), the effective dose was only approximately twice that of (123)I-MIBG despite the large difference in half-lives (100 vs. 13.2 h). Conclusion: The first-in-humans use of low-dose (124)I-MIBG PET for monitoring disease burden demonstrated tumor detection capability superior to that of (123)I-MIBG planar imaging and SPECT/CT.

A technology for imaging extremely low photon flux is an unmet need, especially in targeted alpha therapy (TAT) imaging, which requires significantly improved sensitivity to detect as many photons as possible while retaining a reasonable spatial resolution. In scintigraphy using gamma cameras, the radionuclide collimator rejects a large number of photons that are both primary photons and scattered photons, unsuitable for photon-starved imaging scenarios like imaging TAT. In this paper we develop a min-min weighted robust least squares (WRLS) algorithm to solve a general reconstruction problem with uncertainties and validate it with the extreme scenario: collimatorless scintigraphy. Ra-223, a therapeutic alpha emitting radionuclide whose decay chain includes x-ray and gamma-ray photons, is selected for an exploratory study. Full Monte Carlo simulations are performed using Geant4 to obtain realistic projection data with collimatorless scintigraphy geometry. The results show that our proposed min-min WRLS algorithm could successfully reconstruct point sources and extended sources in the collimatorless scintigraphy with a resolution close to its system resolution and figures of merit (FOM) better than the collimator-based scintigraphy for extremely low activity TAT. This approach could be expanded as a 3D algorithm, which could lead to 3D collimatorless SPECT.

The international organizations, including IAEA, have established a diagnostic reference levels (DRLs) for diagnostic xray examinations in order to control doses. The DRLs are mostly expressed as an entrance surface dose (ESD) to quantify dose on the patients. However, there are significant uncertainties associated with the measurements of ESD, and most of them are results from back scatter factor (BSF). The accurate BSF could be determined only by Monte Carlo simulations. In addition, as the human body is much more complicate compared with a commonly used ANSI phantom, for realistic simulation the computational anthropomorphic extended cardiac-torso (XCAT) phantom has to be used in order to obtain accurate BSF. However, to our knowledge, a few BSF values have previously been reported in the literature in x-ray examinations considering patients’ age with XCAT phantom with Monte Carlo methods. The aim of this study was to quantitatively analyze the BSFs using XCAT phantom with Monte Carlo simulation in various x-ray examinations considering the patient’s age. For chest examinations, the BSFs were 1.21, 1.29, and 1.35 for one-year old, seven-year old, and adult patient’s protocol, respectively. For adult patient, the BSFs were 1.35, 1.26, and 1.10 for chest, abdomen, and extremity examinations, respectively. In addition, the ESDs in chest examinations were 36.30 μGy, 52.69 μGy, and 111.80 μGy for one-, seven-year old, and adult patient’s protocol, respectively. These results demonstrated that accurate BSF should be chosen considering the diagnostic examination in order to estimate the ESD properly.

This paper presents a compensation method to stabilize the temperature dependent performance of Geiger-mode Avalanche Photodiode (G-APD) arrays for Positron Emission Tomography (PET). The compensation method is used to identify the bias voltage range that provides stable performance even at different temperatures using the G-APD’s characteristics, and to control the photo-peak variation as a function of temperature using the preamplifier gain within the identified bias voltage range. A pair of G-APD detectors and temperature sensors were located in the temperature chamber and the preamplifiers which can control the gain of the detectors using the digital potentiometer were positioned outside the chamber. The performance of the G-APD detector, especially energy resolution and coincidence timing resolution, was characterized as a function of bias voltage at different temperatures from 20 °C to 40 °C at 5 °C increments; the energy resolution, coincidence timing resolution, and photo-peak position of all channels of G-APD PET detectors before and after the preamplifier gain correction were then measured and compared. The results of this study demonstrated that the optimal bias voltage range providing the good energy and coincidence timing resolution, 12.1±1.2% and 1.30±0.09 ns, respectively, could be identified at the temperature range and the photo-peak variation and the performance at different temperatures could be stabilized by adjusting the preamplifier gain within the identified bias voltage range. We concluded the proposed method to be reliable and useful for the development of the PET system using G-APD arrays.

Photon-counting (PC) technique has been paid attention to digital radiography applications due to its potential in lowdose operation and multi-energy imaging capability. In this study, we theoretically investigate the performance gain in digital radiography when the PC detectors are used instead of the conventional energy-integrating (EI) detectors. We use the Monte Carlo technique for estimating energy-absorption distributions in detector materials such as CdTe for the PC detector and CsI for the EI detector. To estimate the signal and noise transfers through the two different detectoroperation schemes, we use the cascaded linear-systems approach. In the Monte Carlo simulations, the square and rectangle focal spots are considered to mimic the advanced carbon nanotube (CNT) and conventional filament cathodes, respectively. From the simulation results, the modulation-transfer functions of the PC detector are more sensitive to asymmetric focal spot geometry than those of the EI detector. On the other hand, the PC detector shows better image signal-to-noise ratio than the EI detector; hence better dose efficiency with the PC detector. The dose efficiency of the PC detector in comparison with the EI detector is however marginal for the filament x-ray beam whereas the dose efficiency is not negligible for the CNT x-ray beam. The theoretical upper limits of the imaging performance of the advanced digital radiography technology are reported in this study.

In the development of a mobile X-ray imaging apparatus, it is needed to pre-calculate the ambient leakage and stray radiation doses of the apparatus because the interest in exposure damage is increasing. This issue is strongly related to the additional radiation damage of the patients, caregivers, and radiologists around X-ray emission areas. Monte Carlo simulation with well-applied X-ray device’s characteristics is a powerful method to estimate the leakage and stray radiation distribution. The first purpose of this study is to verify an MC simulation model with leakage radiation and stray radiation distribution for accurate scattered dose distribution. The second purpose is to optimize the lead shield part of the X-ray tube and predict various scattered dose distributions with the default and low-dose conditions in three clinical protocols (ChestAP, Abdomen Supine, and Pelvis AP). An X-ray mobile device was imported using a CAD import method for applying its complicated structures easily. X-ray photons were emitted on the anode with skipping the step of electrons hitting the anode for reducing the simulation time. We compared the air-kerma values between the experimental and simulation results under the leakage and stray radiation regulation. Then, we analyzed the dose distribution maps as a function of the size of the lead shield or the distance from the anode focal spot. The experimental and simulation results show similar leakage and stray radiation dose values as functions of the direction and the height, respectively. The analysis of the leakage and stray dose distribution maps shows that the maps strongly depend on the lead shield structures of the X-ray tube. Furthermore, all dose reduction rates of the three clinical protocols were over 40% and similar to the exposure dose reduction rates and remained constant as a function of the distance. This study demonstrates that our proposed simulation model can be used to develop other applications as well as a mobile X-ray imaging device and to protect against additional radiation damage with diverse clinical protocols.

This paper presents the development of filtering methods for positron emission tomography (PET) signals contaminated by radio frequency (RF) pulses for combined PET and clinical 3-T magnetic resonance imaging (MRI). The filtering methods include software, hardware, and hybrid correction methods. In the software correction method, PET signals are assessed, and valid signals are identified based on the characteristics of a typical PET signal using Field-Programmable Gate Array (FPGA)-based programming. The hardware correction method makes use of differential-to-single-ended and low-pass filter circuits for PET analog signals. The hybrid correction method involves the sequential application of both the hardware and software methods. Both valid and contaminated PET signals are measured with an oscilloscope. An evaluation is then made of the performance (energy resolution, photopeak channel, total counts, and coincidence timing resolution) of the PET detector modules with and without various MR sequences (gradient echo, spin echo T1 sequence). For all correction methods, the energy resolution, photopeak position, and coincidence timing resolution with MR sequences are similar (

Purpose In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging. Methods The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored. Results The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged. Conclusions Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.

The improvement of sensitivity in neuro-PET is crucial because it can reduce the scan time and/or the radiation dose. In this study, we proposed a novel PET detector design that combined conical shape detector with cylindrical one (conical PET) to obtain high sensitivity. The sensitivity as a function of the oblique angle and the ratio of the conical to cylindrical portion was estimated to optimize the design of conical PET using Monte Carlo simulation tool, GATE v6.2. An axial sensitivity and misplacement rate by penetration of γ rays were also estimated to evaluate the performance of the conical PET. The sensitivity was improved by 36% at the center of axial FOV. This value was similar to the analytically calculated value. The misplacement rate of conical PET was about 5% higher than the conventional PET. The results of this study demonstrated the conical detector proposed in this study could provide subsequent improvement in sensitivity which could allow to design high sensitivity PET for brain imaging.

The aim of this study was to develop a prototype magnetic resonance (MR)-compatible positron emission tomography (PET) that can be inserted into a MR imager and that allows simultaneous PET and MR imaging of the human brain. This paper reports the initial results of the authors' prototype brain PET system operating within a 3-T magnetic resonance imaging (MRI) system using newly developed Geiger-mode avalanche photodiode (GAPD)-based PET detectors, long flexible flat cables, position decoder circuit with high multiplexing ratio, and digital signal processing with field programmable gate array-based analog to digital converter boards.A brain PET with 72 detector modules arranged in a ring was constructed and mounted in a 3-T MRI. Each PET module was composed of cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled to a tileable GAPD. The GAPD output charge signals were transferred to preamplifiers using 3 m long flat cables. The LYSO and GAPD were located inside the MR bore and all electronics were positioned outside the MR bore. The PET detector performance was investigated both outside and inside the MRI, and MR image quality was evaluated with and without the PET system.The performance of the PET detector when operated inside the MRI during MR image acquisition showed no significant change in energy resolution and count rates, except for a slight degradation in timing resolution with an increase from 4.2 to 4.6 ns. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI. Rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. The activity distribution patterns between the white and gray matter in the Hoffman brain phantom were well imaged. The hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences were observed without any noticeable artifacts, although MR image quality requires some improvement.These results demonstrate that the simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

There has been great interest in the development of combined PET/MR, a useful tool for both functional and anatomic imaging. We have developed a proof-of-principle MR compatible PET, employing a design concept that uses GAPD arrays as a PET photo-sensor and charge signal transmission method for human brain imaging. The purpose of this study was to design the 2nd version of brain PET with an extended axial field-of-view (FOV) and to evaluate its initial performance. The PET consisted of 18 detector blocks arranged in a ring of 390 mm diameter with 60 mm axial FOV. Each detector block was composed of a 4 × 4 matrix of detector module, each of which consisted of a 4 × 4 array L YSO coupled to a 4-side tileable 4 × 4 GAPD array. The PET gantry was shielded with gold-plated conductive fabric tapes with a thickness of 0.1 mm. PET signals were fed into the position decoder circuit (PDC) generating the digital address and analog pulse of the one interacted channel among the 256 output channels of the detector block, using a 4 m long flat cable. Commercial DAQ modules were used to digitize analog output signals of the PDCs and to store the data in list mode format. The flat cable was shielded with a mesh-type aluminum sheet, which had a thickness of 0.24 mm. All electronics were enclosed in an aluminum box, which had a thickness of 10 mm, located outside the MR bore. Average energy and timing resolutions of the developed PET measured outside the MR room were 18.1±3.2% (n=4,608) and 3.6 ns, respectively. The sensitivity and spatial resolution were 1.2% and 3.1 mm at the center of the field of view, respectively. No significant degradations of PET performance and the uniformity of MR image were observed. Simultaneous PET and MR images of hot-rod phantom and cat brain were successfully acquired. Experimental results indicate that the high performance compact and lightweight PET insert for hybrid PET-MRI can be developed using GAPD arrays and charge signal transmission method.

In recent times, there has been great interest in the use of Geiger-mode avalanche photodiodes (GAPDs) as scintillator readout in positron emission tomography (PET) detectors because of their advantages, such as high gain, compact size, low power consumption, and magnetic field insensitivity. The purpose of this study was to develop a novel PET system based on GAPD arrays for brain imaging.The PET consisted of 72 detector modules arranged in a ring of 330 mm diameter. Each PET module was composed of a 4 × 4 matrix of 3 × 3 × 20 mm(3) cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled with a 4 × 4 array three-side tileable GAPD. The signals from each PET module were fed into preamplifiers using a 3 m long flat cable and then sent to a position decoder circuit (PDC), which output a digital address and an analog pulse of the interacted channel among 64 preamplifier signals transmitted from four PET detector modules. The PDC outputs were fed into field programmable gate array (FPGA)-embedded data acquisition (DAQ) boards. The analog signal was then digitized, and arrival time and energy of the signal were calculated and stored.The energy and coincidence timing resolutions measured for 511 keV gamma rays were 18.4 ± 3.1% and 2.6 ns, respectively. The transaxial spatial resolution and sensitivity in the center of field of view (FOV) were 3.1 mm and 0.32% cps/Bq, respectively. The rods down to a diameter of 2.5 mm were resolved in a hot-rod phantom image, and activity distribution patterns between the white and gray matters in the Hoffman brain phantom were well imaged.Experimental results indicate that a PET system can be developed using GAPD arrays and the GAPD-based PET system can provide high-quality PET imaging.

A PET employing Geiger-mode avalanche photodiode (GAPD) has been designed and developed in our laboratory for simultaneous human brain PET-MR imaging. The purpose of this study is to evaluate the effect of temperature on the performance of the brain PET. The PET gantry including LYSO and GAPD array were placed in a temperature controlled chamber. All electronics including preamplifiers were located outside the chamber to minimize the temperature effect. Energy resolution and photo-peak position of all channels of the brain PET were measured. Count rates of the PET were also measured using 2 different Schemes: fixed energy window and adjusted energy window. All measurement was performed at 10°C ∼ 40°C with 5°C step. Hot-rod phantom was acquired at 10°C, 25°C and 40°C. The energy resolution was degraded from 14% to 30% as the temperature raised. The photo-peak position the coincidence count rates were respectively decreased from 1314 ch to 560 ch, from 2.5 kcps to 0.9 kcps as the temperature raised. When the energy window was adjusted to the photo-peak position, the rates were stable from 10°C to 35°C. In hot-rod phantom images acquired at 10°Cand 25°C, the rods down to a diameter of 3.5 mm were clearly resolved. However the phantom image acquired at 40 °C demonstrated noticeable degradation in image quality. Future study is being conducted to optimize the adjustable window to obtain stable performance of PET system using GAPD as a function of temperature.

The aim of this study is to develop a MR compatible PET that is insertable to MRI and allows simultaneous PET and MR imaging of human brain. The brain PET having 72 detector modules arranged in a ring of 330 mm diameter was constructed and mounted in a 3-T MRI. Each PET module composed of 4 × 4 matrix of 3 mm × 3 mm × 20 mm LYSO crystals coupled to a tileable 4 × 4 array Geiger-mode avalanche photodiode (GAPD) and designed to locate between RF and gradient coils. GAPD output charge signals were transferred to preamplifiers using flat cable of 3 m long, and then sent to position decoder circuit (PDC) identifying digital address and generating an analog pulse of the one interacted channel from preamplifier signals. The PDC outputs were fed into FPGA-embedded DAQ boards. The analog signal was digitized, and arrival time and energy of the signal were calculated and stored. LYSO and GAPD were located inside MR bore and all electronics including preamplifiers were positioned outside MR bore to minimize signal interference between PET and MR. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI using the MR compatible PET system. The rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. Activity distribution patterns between white and gray matter in Hoffman brain phantom were well imaged. No degradation of image quality of the hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences was observed. These results demonstrate that simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

A dual-ended readout PET detector module based on Geiger-mode avalanche photodiode (GAPD) with large-area microcells was proposed to obtain high photon detection efficiency (PDE) and to overcome the energy non-linearity problem. Theoretical analysis and experimental measurement were performed for the single- and dual-ended PET detector modules which were consisted of the two types of GAPDs with 50×50 μm2 and 100×100 μm2 microcell sizes. A Monte Carlo simulation was conducted to predict the number of incident photons impinging on the GAPD entrance surface to estimate the light collection efficiency (LCE) and linearity performance. Also, the depth of interaction (DOI) ratio histogram was obtained. Experimental study was performed to acquire the energy spectra of different γ-rays, and the linearity was evaluated by analyzing the photo-peak channels. The simulation results showed the LCE of dual-ended PET detector modules were improved 9% and 55% comparing to the single-ended one, with 50×50 μm2 and 100×100 μm2 microcells GAPDs, respectively. Also, it was estimated that the proposed method can provide excellent (3–4 mm) and uniform DOI resolution. In the experimental measurement, the 511 keV photo-peak channels of dual-ended PET detector modules was increased 26% and 71% comparing to the single-ended one, with 50×50 μm2 and 100×100 μm2 microcells GAPDs, respectively. The coefficient of determination (R2) was improved from 0.86 to 0.93 with 100×100 μm2 microcells GAPD. The similar improvement in photo-peak channel and linearity was observed in the simulation results. It demonstrated that the dual-ended PET detector configuration could considerably improve the non-linearity properties of GAPD without modification of microcell size and, hence, such configuration could provide high LCE, as well as DOI capabilities, for high PET detector performance.

Light sharing PET detector configuration coupled with thick light guide and Geiger-mode avalanche photodiode (GAPD) with large-area microcells was proposed to overcome the energy non-linearity problem and to obtain high light collection efficiency (LCE). Theoretical evaluations were performed for 90 types of PET detector modules. A Monte Carlo simulation was conducted for the three types of LSO block, 4×4 array of 3×3×20 mm3, 6×6 array of 2×2×20 mm3, and 12×12 array of 1×1×20 mm3 discrete crystals, to investigate the scintillation light distribution after conversion of the γ-rays in LSO. The incident photons were read out by three types of 4×4 array photo-sensors, which were PSPMT of 25% quantum efficiency (QE), GAPD1 with 50×50 μm2 microcells of 30% photon detection efficiency (PDE) and GAPD2 with 100×100 μm2 of 45% PDE. The number of counted photons in each photo-sensor was analytically calculated. The LCE, linearity and flood histogram were examined for each PET detector module as a function of light guide thickness ranging from 1 to 10 mm. The performance of PET detector modules based on GAPDs was significantly improved by using the thick light guide. The LCE was increased from 24 to 30% and from 14 to 41%, and the linearity was also improved from 0.96 to 0.99, from 0.78 to 0.99, for GAPD1 and GAPD2, respectively. In the contrary, the performance was not changed for PSPMT based detector. The flood histogram of 12×12 array PET detector modules using 3 mm light guide coupled with GAPDs was obtained by simulation, and all crystals of 1×1×20 mm3 size was clearly identified. PET detector module coupled with thick light guide and GAPD array with large-area microcells was proposed to obtain high QE and high spatial resolution, and its feasibility was verified. It demonstrates GAPDs could be a competitive and cost-effective photo-sensor respect to the high QE (∼40%) PMT.

A Geiger-mode avalanche photodiode (GAPD) array-based MRI compatible PET is being developed in our laboratory. The purpose of this study is to develop filtering methods for PET event signals contaminated by RF pulses for simultaneous PET-MR imaging. The detector for the MRI compatible PET was composed of LYSO scintillators coupled with GAPD arrays. PET signal acquisition was implemented by a FPGA-based data acquisition (DAQ) system. The noise caused by RF field of MRI has large effect on PET signal acquisition when PET detectors are placed inside MRI. In this study, software, hardware and hybrid filtering methods were evaluated. The software method utilizes a maximum rise window and an integration (energy) window for real time RF noise rejection. The hardware method uses differential to single ended circuit including low-pass filter. The hybrid method combines software and hardware method. Our preliminary results demonstrate that the RF pulse effect on PET DAQ could be reduced by the software or hardware method developed in this study and hybrid method is the best to minimize the interference of RF pulses.

Geiger-mode avalanche photodiode (GAPD) has been demonstrated to be a high performance PET sensor because of high gain, fast response, low excess noise, low bias voltage operation and magnetic field insensitivity. The purpose of this study is to develop a PET for human brain imaging using 4 × 4 array of large size GAPD. PET detector modules were designed and built to develop a prototype PET. The PET consisted of 72 detector modules arranged in a ring with an inner diameter of 330 mm. The LYSO arrays consisted of 4 × 4 array of 3 × 3 × 20 mm3 pixels, which were 1-to-1 coupled to 4 × 4 arrays of 9 mm2 GAPD pixels (SensL, Ireland). The GAPDs were tiled together using flip chip technology on glass and operated at a bias voltage of 32 V for a gain of 3.5 × 106. The signals of the each module were amplified by a 16 channel preamplifier circuit with differential outputs and then sent to a position decoder circuit (PDC), which readout digital address and analog pulse of the one interacted channel from 64 signals of 4 preamplifier boards. The PDC output signals were fed into FPGA-embedded DAQ boards. The analog signal was sampled with 100 MHz, and arrival time and energy of the digitized signal were calculated and stored. The coincidence data were sorted and reconstructed by standard filtered back projection. The energy and time resolution of LYSO-GAPD block detector for 511-keV was 20.4% and 2.4 ns, respectively. The developed PDC could accurately provide the interacted PET signal and reduce the number of the readout channels of PET detector modules based on array type GAPD. The rods down to a diameter of 3.5 mm were resolved in hot-rod phantom image acquired with the brain PET which is similar to the image obtained by Monte Carlo simulation. Activity distribution pattern between white and gray matter in Hoffman brain phantom was well imaged. These results demonstrate that high performance PET could be developed using the GAPD-based PET detectors, analog and digital signal processing methods designed in this work. The prototype brain PET will be tested in a clinical 3T MRI to construct a combined PET-MRI.

A simple and improved digital timing method has been developed for positron emission tomography (PET). The so-called initial rise interpolation method is based on an important characteristic of gamma signal: a properly pre-amplified and sampled gamma signal pulse can be characterized to arrive with an initial rise from baseline and then to go up with a maximum rise. Pulse arrival time is obtained by calculating the intersection of the initial rise line with the baseline for each gamma signal pulse. In this study, a FPGA-based data acquisition (DAQ) card was used for data acquisition and processing. We measured coincidence timing resolution of two types (fast and slow) of recently developed 3 mm × 3 mm Geiger mode avalanche photodiodes (GAPDs) using 3 different digital timing methods: initial rise interpolation (IRI), digital CFD and maximum rise interpolation (MRI). Furthermore, simulation has been performed to evaluate effects of pulse rise time, pulse amplitude and front-end noise level on timing resolution estimated by the three digital timing methods. Measured results show that, IRI method provided the best timing resolution for both types of GAPDs: 0.7 ns FWHM for fast GAPD and 1.5 ns for slow GAPD (digital CFD: 1.5 ns and 2.2 ns; MRI: 1.8 ns and 2.7 ns). In accordance with measured results, simulation results also show that IRI method provided the best timing resolution. Based on these experimental results, we concluded that the developed simple and improved digital timing method is reliable and useful for the development of high performance PET.

Purpose: The aim of this study was to develop a prototype magnetic resonance (MR)-compatible positron emission tomography (PET) that can be inserted into a MR imager and that allows simultaneous PET and MR imaging of the human brain. This paper reports the initial results of the authors’ prototype brain PET system operating within a 3-T magnetic resonance imaging (MRI) system using newly developed Geiger-mode avalanche photodiode (GAPD)-based PET detectors, long flexible flat cables, position decoder circuit with high multiplexing ratio, and digital signal processing with field programmable gate array-based analog to digital converter boards. Methods: A brain PET with 72 detector modules arranged in a ring was constructed and mounted in a 3-T MRI. Each PET module was composed of cerium-doped lutetium yttrium orthosilicate (LYSO) crystals coupled to a tileable GAPD. The GAPD output charge signals were transferred to preamplifiers using 3 m long flat cables. The LYSO and GAPD were located inside the MR bore and all electronics were positioned outside the MR bore. The PET detector performance was investigated both outside and inside the MRI, and MR image quality was evaluated with and without the PET system. Results: The performance of the PET detector when operated inside the MRI during MR image acquisition showed no significant change in energy resolution and count rates, except for a slight degradation in timing resolution with an increase from 4.2 to 4.6 ns. Simultaneous PET/MR images of a hot-rod and Hoffman brain phantom were acquired in a 3-T MRI. Rods down to a diameter of 3.5 mm were resolved in the hot-rod PET image. The activity distribution patterns between the white and gray matter in the Hoffman brain phantom were well imaged. The hot-rod and Hoffman brain phantoms on the simultaneously acquired MR images obtained with standard sequences were observed without any noticeable artifacts, although MR image quality requires some improvement. Conclusions: These results demonstrate that the simultaneous acquisition of PET and MR images is feasible using the MR insertable PET developed in this study.

Positron emission tomography (PET) employing Geiger-mode avalanche photodiodes (GAPDs) and charge signal transmission approach was developed for small animal imaging. Animal PET contained 16 LYSO and GAPD detector modules that were arranged in a 70 mm diameter ring with an axial field of view of 13 mm. The GAPDs charge output signals were transmitted to a preamplifier located remotely using 300 cm flexible flat cables. The position decoder circuits (PDCs) were used to multiplex the PET signals from 256 to 4 channels. The outputs of the PDCs were digitized and further-processed in the data acquisition unit. The cross-compatibilities of the PET detectors and MRI were assessed outside and inside the MRI. Experimental studies of the developed full ring PET were performed to examine the spatial resolution and sensitivity. Phantom and mouse images were acquired to examine the imaging performance. The mean energy and time resolution of the PET detector were 17.6% and 1.5 ns, respectively. No obvious degradation on PET and MRI was observed during simultaneous PET-MRI data acquisition. The measured spatial resolution and sensitivity at the CFOV were 2.8 mm and 0.7%, respectively. In addition, a 3 mm diameter line source was clearly resolved in the hot-sphere phantom images. The reconstructed transaxial PET images of the mouse brain and tumor displaying the glucose metabolism patterns were imaged well. These results demonstrate GAPD and the charge signal transmission approach can allow the development of high performance small animal PET with improved MR compatibility.

The use of a dual-ended readout PET detector module based on Geiger-mode avalanche photodiodes (GAPDs) with large-area microcells was proposed to obtain high photon detection efficiency (PDE) and overcome energy non-linearity problems. A simulation study was performed and experimental measurement were taken for the single- and dual-ended PET detector modules consisting of the two types of GAPDs with 50 × 50 μm2 and 100 × 100 μm2 microcells. A Monte Carlo simulation was conducted to predict the number of incident photons impinging on the GAPD entrance surface to estimate the light collection efficiency (LCE) and energy linearity performance. A depth of interaction (DOI) ratio histogram was also obtained. An experimental study was performed to acquire the spectra of different energy γ-rays, and the energy linearity was evaluated by analyzing the photo-peak channels. The simulation results showed that the LCE and energy linearity of the dual-ended PET detector modules were considerably improved compared to the single-ended one, with 100 × 100 μm2 microcell GAPDs. We also estimated that the proposed method can provide accurate (3–4 mm) and uniform DOI resolution. In the experimental measurement, the 511 keV photo-peak channels of the dual-ended PET detector modules were increased 26% and 71% compared to the single-ended one, with 50 × 50 μm2 and 100 × 100 μm2 microcell GAPDs, respectively. The coefficients of determination (R2) were increased from 0.97 to 0.99 and from 0.86 to 0.93 with 50 × 50 μm2 and 100 × 100 μm2 microcell GAPDs, respectively. The results of this study demonstrate that the dual-ended readout scheme using GAPDs with large-area microcells provides high LCE and DOI information with minimized energy non-linearity. This will enable investigators to configure PET detector modules with high sensitivity and resolution.