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We describe a long axial field-of-view (FOV) PET scanner for high-sensitivity and total-body imaging of nonhuman primates and present the physical performance and first phantom and animal imaging results. Methods: The mini-EXPLORER PET scanner was built using the components of a clinical scanner reconfigured with a detector ring diameter of 43.5 cm and an axial length of 45.7 cm. National Electrical Manufacturers Association (NEMA) NU-2 and NU-4 phantoms were used to measure sensitivity and count rate performance. Reconstructed spatial resolution was investigated by imaging a radially stepped point source and a Derenzo phantom. The effect of the wide acceptance angle was investigated by comparing performance with maximum acceptance angles of 14°-46°. Lastly, an initial assessment of the in vivo performance of the mini-EXPLORER was undertaken with a dynamic 18F-FDG nonhuman primate (rhesus monkey) imaging study. Results: The NU-2 total sensitivity was 5.0%, and the peak noise-equivalent count rate measured with the NU-4 monkey scatter phantom was 1,741 kcps, both obtained using the maximum acceptance angle (46°). The NU-4 scatter fraction was 16.5%, less than 1% higher than with a 14° acceptance angle. The reconstructed spatial resolution was approximately 3.0 mm at the center of the FOV, with a minor loss in axial spatial resolution (0.5 mm) when the acceptance angle increased from 14° to 46°. The rhesus monkey 18F-FDG study demonstrated the benefit of the high sensitivity of the mini-EXPLORER, including fast imaging (1-s early frames), excellent image quality (30-s and 5-min frames), and late-time-point imaging (18 h after injection), all obtained at a single bed position that captured the major organs of the rhesus monkey. Conclusion: This study demonstrated the physical performance and imaging capabilities of a long axial FOV PET scanner designed for high-sensitivity imaging of nonhuman primates. Further, the results of this study suggest that a wide acceptance angle can be used with a long axial FOV scanner to maximize sensitivity while introducing only minor trade-offs such as a small increase in scatter fraction and slightly degraded axial spatial resolution.

Background: Simultaneous positron emission tomography (PET)/magnetic resonance imaging (MRI) scanners and inserts are valuable tools for accurate diagnosis, treatment planning, and monitoring due to their complementary information. However, the integration of a PET system into an MRI scanner presents technical challenges for a distortion‐free operation. Purpose: We aim to develop a PET insert dedicated to breast imaging in combination with the 3T PET/MRI scanner Biograph mMR (Siemens Healthineers) as well as a brain PET insert for the 7T MRI scanner MAGNETOM Terra (Siemens Healthineers). For this development, we selected as a basis the C13500 series PET modules (Hamamatsu Photonics K.K.) as they offer an all‐in‐one solution with a scalable, modular design for compact integration with state‐of‐the‐art performance. The original PET modules were not designed to be operated with an MRI scanner, therefore we implemented several modifications such as signal transmission via plastic optical fiber, radio frequency (RF) shielding of the front‐end electronics, and filter for the power supply lines. In this work, we evaluated the mutual MRI compatibility between the modified PET modules and the 3T and 7T MRI scanner. Methods: We used a proof‐of‐concept setup with two detectors to comprehensively evaluate a potential distortion of the performance of the modified PET modules whilst exposing them to a variety of MR sequences up to the peak operation conditions of the Biograph mMR. A method using the periodicity of the sequences to identify distortions of the PET events in the phase of RF pulse transmission was introduced. Vice versa, the potential distortion of the Biograph mMR was evaluated by vendor proprietary MRI compatibility test sequences. Afterwards, these studies were extended to the MAGNETOM Terra. Results: No distortions were introduced by gradient field switching (field strength up to 20 mT/m at a slew rate of 66.0 T/ms−1). However, RF pulse transmission induced a reduction of the single event rate from 33.0 kcounts/s to 32.0 kcounts/s and a degradation of the coincidence resolution time from 251 to 299 ps. Further, the proposed method revealed artifacts in the energy and timing histograms. Finally, by using the front‐end filters it was possible to prevent any RF pulse induced distortion of event rate, energy, or time stamps even for a 700° flip angle (45.5 μT) sequence. The evaluations to assess potential distortions of the MRI scanner showed that carefully designed RF shielding boxes for the PET modules were required to prevent distortion of the RF spectra. The increase in B0 field inhomogeneity of 0.254 ppm and local changes of the B1 field of 12.5% introduced by the PET modules did not qualitatively affect the MR imaging with a spin echo and MPRAGE sequence for the Biograph mMR and the MAGNETOM Terra, respectively. Conclusion: Our study demonstrates the feasibility of using a modified version of the PET modules in combination with 3T and 7T MRI scanners. Building upon the encouraging MRI compatibility results from our proof‐of‐concept detectors, we will proceed to develop PET inserts for breast and brain imaging using these modules. [ABSTRACT FROM AUTHOR]

The focus of this paper is the investigation of a new dynode-timing technique optimized for PET block detectors. This method allows utilization of dynode signals from single but especially multiple photo-multiplier tubes (PMTs), operated with negative high-voltage. The technique will provide an event-timing trigger without deteriorating the anode signal. A printed circuit board has been developed and built for this investigation. Benchmark measurements have been performed, comparing timing of the anode signal with timing of the inverted last-dynode signal and timing of the dynode signal extracted via a newly developed LVPECL-logic based board. Timing measurements were performed with plastic as well as LSO scintillators. From single PMT measurements we find a 30 ps improvement with the dynode-timing method compared to the standard anode timing with two Photonis XP2020Q PMTs with LSO (10 mm x 10 mm x 10 mm). For a quad-PMT block detector, assembled of four Hamamatsu R9800 with a Hi-Rez block, the timing-resolution improves ~10%, by 43 ps compared to the standard anode timing. Index Terms--Detectors, dynode-timing, high resolution, positron emission tomography (PET), small animal, SPECT.

The focus of this paper is the evaluation of the new fast 51 mm-diameter, 8-stage Hamamatsu R9779 photomultipliers (PMTs) with an acceleration-ring at the front-end and the Photonis XP20D0 PMTs with a screening grid in front of the anode. The following performance characteristics are presented: Timing resolution, anode-scan-uniformity and transit-time spread. The unfolded timing resolution for two R9779 was 192 ps and 210 ps using plastic scintillators. The individual timing resolutions for two XP20D0 using plastic scintillators are 181 ps and 154 ps, respectively. The variation in time resolution across the windows of the two R9779 ranged between 117 ps and 171 ps, and 79 ps and 73 ps for the two XP20D0 PMTs. Index Terms--PET, photomultiplier, time-of-flight.

Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (~70% for APD vs. ~20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (~200 for APD vs. ~ [10.sup.6] for PMT), and a smaller photoactive area (~ 5 mm x 5 mm for APD vs. 10-52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2 x 2 APD array reading out an 8 x 8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2 mm x 2 mm x 20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmission through long twisted-pair cables. Index Terms--Avalanche-photodiode, multiplexing circuit, positron emission tomography, RF transformer.

High spatial resolution is essential to image small lesions in positron emission tomography. Traditional methods suggest using small crystals to obtain high resolution. In this study, a new method is introduced to achieve high spatial resolution by using scintillators with different decay times arranged in a checkerboard pattern. Pulse shape discrimination was implemented to determine which crystal the gamma energy was deposited. In this work, a 13 x 13 LSO crystal array and a 13 x 13 LSO-GSO checkerboard crystal array using 4 x 4 x 20 [mm.sup.3] crystals were built and optimized using light sharing techniques. Similarly, using 2.5 x 2.5 x 19 [mm.sup.3] LSO and LYSO crystals, a 17 x 17 LSO-LYSO checkerboard crystal array was built as a high-resolution application. The average pixel energy resolution for the 13 x 13 LSO was measured to be 14.5%. Similarly, the average pixel resolutions for the GSO part of the 13 x 13 LSO-GSO crystal array and the LSO part of the same array were measured to be 13.9% and 18.5%, respectively. The average crystal energy resolutions for the LSO and LYSO parts of the 17 x 17 LSO-LYSO crystal array were measured to be 23.5% and 24.2%, respectively. The average peak-to-valley ratios in the position profiles were measured to be 2.1 for the 13 x 13 LSO crystal array, 7.2 for the LSO part of the 13 x 13 LSO-GSO crystal array and 4.3 for the GSO part of the 13 x 13 LSO-GSO crystal array. Similarly, the average peak-to-valley ratios in the position profiles were measured to be 2.5 for the LSO part of the 17 x 17 LSO-LYSO crystal array and 2.7 for the LYSO part of the 17 x 17 LSO-LYSO crystal array. Index Terms--Decay time, GSO, high resolution, LSO, LYSO, positron emission tomography (PET).

An APD-based, light-sharing detector has been evaluated for use in PET. The detector configuration is a 2 x 2 array of 5 mm x 5 mm Hamamatsu $8664-55 APDs used to readout multi-crystal LSO blocks. Initially, a basic performance study was undertaken with a single APD coupled to a chemically etched 4 mm x 4 mm x 10 mm LSO crystal (teflon wrapped) using a custom, single-channel fast ASIC preamplifier. Timing and energy resolution measurements with a 22Na source were performed using the monolithic LSO crystal coupled to the APD. The timing resolution of the APD channel in coincidence with a plastic scintillator coupled to a PMT was 870 ps FWHM. The energy resolution of the 511 keV photopeak was 12.1% FWHM. Based on these initial results, a 9 x 9 array of 2 mm x 2 mm x 20 mm LSO crystals was assembled and evaluated with a 2 x 2 APD array. The LSO block had an average energy resolution of 20.9% and a timing resolution of 2.47 ns FWHM. These results show that APDs are promising photodetectors for high-resolution and cost-effective PET systems utilizing light-sharing block detectors. Index Terms--Avalanche photodiodes (APDs), light-sharing, lutetium oxyorthosilicate (LSO), positron emission tomography (PET).

The design of a new scanner for use in small animal PET imaging is described. The goal is to achieve 1 mm FWHM resolution in each of three orthogonal directions throughout a volume suitable for whole body mouse imaging, roughly 40 mm diameter x 80 mm long. Simultaneously, the design should achieve a sensitivity of greater than 5% of all decays from a point source located at the center of the scanner. The scanner uses 12, plane detector banks mounted in a 160 mm diameter ring on a rotating gantry. Each detector bank consists of a 48 x 108 array of 20 mm long LSO crystals with an array pitch of 0.87 mm. Each bank uses two Hamamatsu H8500 large-area, multi-anode photomultiplier tubes for fluorescence detection. The detector banks are divided into two sets with the respective lines of response offset by one quarter of the array pitch to give increased sampling density. Tests using a prototype crystal array demonstrate that individual crystals can be resolved. Simulations have been performed to evaluate the performance expected in the complete scanner. With F-18 point sources, the FWHM resolutions in the radial, tangential, and axial directions are less than 1 mm for source positions throughout the desired field of view (FOV). Simultaneously, the detector sensitivity is greater than 7% of all decays for a point source located at the center of the FOV. Results are also presented for simulations using different PET isotopes to investigate the effect of positron range, and for a phantom containing hot spots added to a uniform background to evaluate the scanner performance for an extended object. Index Terms--Positron emission tomography (PET), positron emission tomography (PET) instrumentation, small animal imaging.

In order to study light transport in LSO crystals for PET, we simulated single crystals and block detectors using Detect2000. We devised basic experiments to evaluate fundamental physics parameters of the simulation and benchmarked the simulation with a set of experiments on single crystals. The block detectors that were studied were composed of Lutetium Oxyortho-Silicate crystals, a light guide and Photo Multiplier Tubes. The results of the simulation were compared to an experimental detector block's position profile. By refining our simulation to model actual detectors, a tool was created that can be used to optimize the performance of detector blocks. Index Terms--Detect2000, light transport, lutetium oxyorthosilicate (LSO), Monte Carlo simulation, scintillator.

The combination of PET and SPECT in one device may have clinical and economic advantages. The PET/SPECT camera developed as a joint venture between CPS Innovations, Knoxville, TN, and Siemens Medical Systems, Hoffmann Estates, IL, is a fully 3-D system with 511 keV transmission sources. The detector is based on a NaI-LSO phoswich scintillator. The individual LSO and NaI crystals have a size of 4.3 x 4.3 10 [mm.sup.3]. The 12 x 12 crystal blocks are read out by 2 x 2 in photomultiplier tubes, arranged in a low cost quadrant sharing system. The purpose of this paper is to illustrate phoswich detector setup and to evaluate the detector performance for both modes, PET, and SPECT. The quadrant sharing method and the two crystal layers require special algorithms to find the individual crystal regions and distinguish them from light cross talk caused by adjacent blocks. The base for this procedure is a position profile acquired with a flood source. The mean energy resolution for the NaI(TI) crystal layer was 13.1% (FWHM) at 140 keV and 15.7% (FWHM) at 511 keV for the LSO crystal layer. For the PET setup the average time resolution was 3.4 ns (FWHM). A daily quality check showed a very stable detector condition. Peak shifts due to room temperature changes were very little for both modes and both crystal layers. Index Terms--NaI-LSO detector, PET/SPECT, quadrant-sharing.

We investigated the scintillation characteristics of 2186 LSO crystals that were cut from over 400 crystal boules. We measured the light output and energy resolution of all crystals and also measured the decay times of a broad sampling of the crystals. We observed a significant reduction in the variation of both light output and decay time in this population of crystals compared to the results previously published in 1998. The reduced variation in scintillation properties may be attributed to better control of the populations of the two Ce scintillation centers in LSO, as well as better control of the effect of quenching centers and the effects of Ce segregation during crystal growth. Index Terms--Crystals, decay time, energy resolution, light output, LSO, scintillators.

A daily quality check (DQC) plays an important role for clinical medical systems like a positron emission tomograph (PET). To guarantee image quality a constant monitoring of the scanner integrity is essential. On present PET scanners, the built-in transmission sources are being used for the measurement of a uniform blank scan and the computation of detector-block efficiencies. These efficiencies can be compared to reference data and deviations indicate changes of the scanner condition. On the high-resolution research tomograph (ECAT HRRT) the design of the transmission source makes this method impractical. For that reason we investigated the use of the natural background radioactivity from the new scintillator material LSO as a uniform source. A procedure was developed to measure detector-block sensitivities and energy spectra directly. An initial quality check (QC) scan is the basis for comparison of daily scans, so that gain shifts of photomultipliers and specific hardware defects can be detected. With an improved list mode based data acquisition it was possible to reduce the acquisition time to around 3 h despite the low LSO count rate, so that this routine is as practical as previous procedures in clinical routine. Index Terms--Daily quality check (DQC), lutetium-oxy-orthosilicate (LSO), positron emission tomography (PET).

This study examined the intrinsic performance of silicon drift detector (SDD)-based gamma detectors under a variety of conditions. The prototype detector consisted of an array of seven hexagon-shaped SDDs optically coupled to a single slab of a scintillator. The active area of the SDD sensor was 15.2 mm in diameter, as measured from one vertex to another. The detector unit (SDD array, scintillator and preamplifier circuits) was operated in a cooling chamber with a typical operating temperature of -20 °C. Nitrogen gas was supplied to the detector unit to prevent condensation. The drift time was measured using a LED pulse generation device and the longest drift time was measured to be 4.6 µsec from the edge of the sensor. The intrinsic energy resolution with a 55Fe source for direct X-ray conversion was 3% at the 5.9 keV peak. For indirect conversion, i.e. photon detection, the energy resolution for CsI(Tl) and NaI(Tl) was 7.9% and 8.2% with a 13 µsec and 2.71 µsec shaping time, respectively. For this indirect conversion measurement, the temperature was set to -20 °C and a 1 x 1 x 1 cm³ cube scintillator was coupled directly to the sensor. For the intrinsic spatial resolution measurement with a hole-phantom (3 x 2 mm diameter holes), the x and y directional profiles at a center hole were 2.2 and 2.1 mm in FWHM, respectively. Overall, the intrinsic performance of the SDD prototype is quite promising and advantages of this technology makes it highly feasible for use as a gamma ray detector. [ABSTRACT FROM AUTHOR]

Avalanche-photodiodes (APDs) as photosensors in positron emission tomography (PET) detectors have been extensively investigated in this field. Compared with conventional photosensors such as the photomultiplier tubes (PMTs), most APDs have advantages of higher quantum efficiency (∼70% for APD vs. ∼20% for PMT), robust packaging and very low magnetic susceptibility. However, it usually has very low gain (∼200 for APD vs. ∼10e for PMT), and a smaller photoactive area (∼5 mm × 5 mm for APD vs. 10–52 mm diameter for PMT). The proposal described in this paper was based on a previous APD block detector design, in which each block consists of a 2 × 2 APD array reading out an 8 × 8 array of Lutetium Oxyorthosilicate (LSO) crystals. Each crystal is 2 mm × 2 mm × 20 mm. Due to the small block size, in order to build an APD PET system with similar axial field-of-view of a conventional PET scanner, substantially more APD detectors would be needed. Consequently, more electronics processing channels would be required. To simplify the detector electronics, we initiate a multiplexing concept based on RF transformers. This approach may reduce the signal-processing channels by a factor of 16 (from 64 channels to four). The circuits would work from both current and voltage sources, as opposed to resistor networks which map signals only from current sources. We built prototype printed-circuit-boards (PCBs) to evaluate different multiplexing schemes. The initial measurements demonstrate that the multiplexing Circuits can be implemented in the detector electronics to reduce signal output channels, without increasing signal rise-time and degrading signal-to-noise ratio (SNR). The detector maintains an energy resolution of 19% and timing resolution of about 2 ns (block to single crystal). Moreover, the transformer can function as a single-ended (pseudo-differential) to true-differential converter; this would facilitate retaining signal integrity in transmission through long twisted-pair cables. [ABSTRACT FROM AUTHOR]

Positron emission tomography (PET)-based on time-of-flight (TOF) is desired to enhance the image quality by improving the noise variance in the reconstruction. This benchtop study evaluates the timing performance of the current Hi-Rez detector which is based on a 13 × 13 LSO array of 4 mm × 4 mm × 20 mm crystals and not optimized for TOF PET applications. A Hamamatsu R9779, fast, 51-mm-diameter PMT was used for some experiments in this study as a reference PMT and its time resolution was measured to be 160 ps with a plastic scintillator. The average crystal time resolution of two Hi-Rez detectors, coupled to a 2 × 2 array of Hamamatsu 25-mm-diameter R8619 PMTs at + 1050 V, in coincidence was measured to be 733 ps. The average crystal energy resolutions for two detectors were measured 13.1% and 13.2%. The average crystal time resolution of two Hi-Rez detectors improved from 733 ps to 642 ps when the PMT voltage was increased to + 1300 V. The Hi-Rez detector was originally optimized for good energy resolution to reduce scatter and high packing fraction to obtain high sensitivity and improved spatial resolution for clinical PET. [ABSTRACT FROM AUTHOR]