Shitao Liu

Ultrahigh-Resolution L(Y)SO Detectors Using PMT-Quadrant-Sharing for Human & Animal PET Cameras

The goal of this study is to develop lower-cost ultrahigh resolution detectors for PET systems, using the PMT-quadrant-sharing (PQS) decoding technology on L(Y)SO scintillation crystals. For this work, L(Y)SO PQS block detectors for both animal and human PET cameras were developed and studied. Both simulation and experimental detector design studies were carried out to achieve efficient light distribution and crystal decoding. The effects of crystal finishes and reflector patterns on light distribution, light output and energy resolution were investigated and used to derive the highest resolution PQS-L(YSO) block-detector. The PQS-L(Y)SO detector performance was measured on the best performing blocks. For performance evaluation, list-mode data from the detectors were acquired and analyzed to extract light-collection efficiency, energy-resolution distribution, and pulse height distribution for individual crystals. The potential PET imaging resolution performance was investigated using Monte Carlo simulation studies with the GEANT4/GATE software for both detectors developed for small animal PET and human PET applications. From these studies, we have the following findings: 1) For light distribution studies on the crystal surface finish, 4 mum lapping was found to be the preferred finish for achieving the best position decoding together with good overall light output for all the crystals in both, the human and animal detector arrays; 2) Intricate reflector patterns between crystals can be made from the ESR mirror film (3M Inc.) for optimally controlling the light sharing between crystals and to the four decoding PMTs, with high packing fractions on the PQS-blocks; 3) For the PQS-LYSO detector block for animal PET systems, using 19-mm circular photomultipliers (PMT), we achieved decoding a 14 times 14 arrays with a crystal pitch of 1.27 times 1.27 mm2. This animal detector has a packing fraction of 95.6%, an energy resolution ranging between 12.9%~15.8% for individual crystals (average energy resolution of 14%), the pulse height for the least favorable crystal is 63.5% of the most favorable crystal; 4) For PQS-LSO detector block for human PET systems using very large circular 51-mm PMT, we achieved decoding a 15 times 15 array with a crystal pitch of 3.25 times 3.25 mm2. The human PET detector has packing fraction of 98.2%, an energy resolution range 12.9%~15.8% (average energy resolution 14%). The pulse height of least favorable crystals is 80% of the most favorable crystal. 5) From Monte Carlo simulations for LSO small animal PET, a spatial resolution of 1.1-1.2 mm may potentially be achieved using low cost 19-mm circular PMT. For human PET systems, 3-mm spatial resolution may potentially be achieved using very large 51-mm circular PMT for cost reduction.We developed high resolution L(Y)SO detectors for human and animal PET applications using Photomulti- plier-quadrant-sharing (PQS) technology. The crystal sizes were 1.27 times 1.27 times 10 mm3 for the animal PQS-blocks and 3.25 times 3.25 times 20 mm3 for human ones. Polymer mirror film patterns (PMR) were placed between crystals as reflector. The blocks were assembled together using optical grease and wrapped by Teflon tape. The blocks were coupled to regular round PMTs of 19/51 mm in PQS configuration. List-mode data of Ga-68 source (511 keV) were acquired with our high yield pileup-event recovery (HYPER) electronics and data acquisition software. The high voltage bias was 1100 V. Crystal decoding maps and individual crystal energy resolutions were extracted from the data. To investigate the potential imaging resolution of the PET cameras with these blocks, we used GATE (Geant4 Application for Tomographic Emission) simulation package. GATE is a GEANT4 based software toolkit for realistic simulation of PET and SPECT systems. The packing fractions of these blocks were found to be 95.6% and 98.2%. From the decoding maps, all 196 and 225 crystals were clearly identified. The average energy resolutions were 14.1% and 15.6%. For small animal PET systems, the detector ring diameter was 16.5 cm with an axial field of view (AFOV) of 11.8 cm. The simulation data suggests that a reconstructed radial (tangential) spatial resolution of 1.24 (1.25) mm near the center is potentially achievable. For the whole-body human PET systems, the detector ring diameter was 86 cm. The simulation data suggests that a reconstructed radial (tangential) spatial resolution of 3.09(3.38) mm near the center is potentially achievable. From this study we can conclude that the PQS design could achieve high spatial resolutions and excellent energy resolutions on human and animal PET systems with substantially lower production costs and inexpensive readout devices.

The initial design and feasibility study of an affordable high-resolution 100-cm long PET

This is a design and feasibility study of an affordable high-resolution 100 cm long PET covering the entire body (EB-PET) for imaging head-&-torso in one fixed bed position. Our design studies show that EB-PET may image the entire body in 2-4 minutes with a low 2.5 mCi FDG dose. The high patient throughput may lower the cost of wholebody imaging and the low dose would allow more frequent cancer-management monitoring. EB-PET can capture dynamic wholebody time-activity images and arterial (cardiac) input function concurrently to yield quantitative metabolic images for the wholebody to improve diagnosis and to measure wholebody systemic side effects of therapy. Dynamic imaging using EB-PET may also unshackle wholebody PET imaging from the static FDG-type of tracers required by current PET to new classes of more dynamic tracers. The EB-PET detection system is based on the latest generation of the low-cost BGO detector prototypes developed in our laboratory which can decode 121 BGO crystals per PMT (39 mm diameter), thereby enabling this very large system to use only 1768 PMT for its 205,700 high resolution crystals (3.5 x 3.5 x 20 mm). The system resolution and NES characteristics were also calculated with Monte Carlo (MC) simulations (GATE/GEANT) for point sources, NEMA NES phantom and wholebody Turkington phantoms. Prototype detectors achieved a 15% energy resolution and clearly decoded 3.5 x 3.5 mm detectors. With such data, MC simulations show that the central transaxial image resolution is 3.2 mm (4.4 mm) for 5 cross-ring coincidences (274 cross-ring coincidences), while at 10 cm transaxial radius, the image resolution is 4.2 mm (5.1 mm).

Engineering and performance (NEMA and animal) of a lower-cost higher-resolution animal PET/CT scanner using photomultiplier-quadrant-sharing detectors.

The dedicated murine PET (MuPET) scanner is a high-resolution, high-sensitivity, and low-cost preclinical PET camera designed and manufactured at our laboratory. In this article, we report its performance according to the NU 4-2008 standards of the National Electrical Manufacturers Association (NEMA). We also report the results of additional phantom and mouse studies. Methods: The MuPET scanner, which is integrated with a CT camera, is based on the photomultiplier-quadrant-sharing concept and comprises 180 blocks of 13 × 13 lutetium yttrium oxyorthosilicate crystals (1.24 × 1.4 × 9.5 mm3) and 210 low-cost 19-mm photomultipliers. The camera has 78 detector rings, with an 11.6-cm axial field of view and a ring diameter of 16.6 cm. We measured the energy resolution, scatter fraction, sensitivity, spatial resolution, and counting rate performance of the scanner. In addition, we scanned the NEMA image-quality phantom, Micro Deluxe and Ultra-Micro Hot Spot phantoms, and 2 healthy mice. Results: The system average energy resolution was 14% at 511 keV. The average spatial resolution at the center of the field of view was about 1.2 mm, improving to 0.8 mm and remaining below 1.2 mm in the central 6-cm field of view when a resolution-recovery method was used. The absolute sensitivity of the camera was 6.38% for an energy window of 350–650 keV and a coincidence timing window of 3.4 ns. The system scatter fraction was 11.9% for the NEMA mouselike phantom and 28% for the ratlike phantom. The maximum noise-equivalent counting rate was 1,100 at 57 MBq for the mouselike phantom and 352 kcps at 65 MBq for the ratlike phantom. The 1-mm fillable rod was clearly observable using the NEMA image-quality phantom. The images of the Ultra-Micro Hot Spot phantom also showed the 1-mm hot rods. In the mouse studies, both the left and right ventricle walls were clearly observable, as were the Harderian glands. Conclusion: The MuPET camera has excellent resolution, sensitivity, counting rate, and imaging performance. The data show it is a powerful scanner for preclinical animal study and pharmaceutical development.

GATE Monte Carlo Simulation of a High-Sensitivity and High-Resolution LSO-Based Small Animal PET Camera

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

A lower-cost high-resolution LYSO detector development for positron emission mammography (PEM)

We have developed several positron emission tomography (PET) cameras using photomultiplier-quadrant-sharing (PQS) geometry; in which each detector block is optically coupled to four round PMTs, and each PMT is shared by four blocks. Although PQS design reduces the cost of high-resolution PET systems, when the camera consists of detector panels that are made up of square blocks, half of the PMT’s sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel, which minimize the unused portion of the PMTs while maintaining the low cost, high resolution, and high sensitivity of positron emission mammography (PEM) camera. Our plan was to modify PQS design by using elongated blocks at panel edges and square blocks in the inner area. For elongated blocks, symmetric and asymmetrical reflector patterns were developed, and PQS and PMT-half-sharing (PHS) arrangements were implemented in order to obtain a suitable decoding. The performance of our blocks was good, producing good crystal-decoding and average energy resolution. Using a modified PQS geometry and asymmetric block design, we reclaimed the unused PMT region at detector panel edges, thereby increasing field-of-view and overall detection sensitivity and minimizing undetected breast region near the chest wall. This lower cost design using regular round PMT allowed us to use larger detector panels and hence to build a lower-cost, high-resolution, high-sensitivity PEM camera.

Monte Carlo simulation study on the time resolution of a PMT-quadrant-sharing LSO detector block for time-of-flight PET

We developed a detailed Monte Carlo simulation method to study the time resolution of detector for time-of-flight positron emission tomography (TOF PET). The process of gamma ray reaction in detector, scintillation light emission and transport inside the detector, the photoelectron generation and anode signal generation in the photomultiplier tube (PMT), and the electronics process of discriminator are simulated. We tested this simulation method using published experimental data, and found that it can generate reliable results. Using this method, we simulated the time resolution for a 13×13 detector block of 4×4×20 mm3 lutetium orthosilicate (LSO) crystals coupled to four 2-inch PMTs using PMT-quadrant-sharing (PQS) technology. We analyzed the effects of several factors, including the number of photoelectrons, light transport, transit time spread (TTS), and the depth of interaction (DOI). The simulation results indicated that system time resolution of 300–350ps should be possible with currently available fast PMTs. This simulation method can also be used to simulate the time resolution of other detector design method.

A Lower-Cost High-Resolution LYSO Detector Development for Positron Emission Mammography (PEM)

In photomultiplier-quadrant-sharing (PQS) geometry for positron emission tomography applications, each PMT is shared by four blocks and each detector block is optically coupled to four round PMTs. Although this design reduces the cost of high-resolution PET systems, when the camera consists of detector panels that are made up of square blocks, half of the PMTs sensitive window remains unused at the detector panel edge. Our goal was to develop a LYSO detector panel which minimizes the unused portion of the PMTs for a low-cost, high-resolution, and high-sensitivity positron emission mammography (PEM) camera. We modified the PQS design by using elongated blocks at panel edges and square blocks in the inner area. For elongated blocks, symmetric and asymmetrical reflector patterns were developed and PQS and PMT-half-sharing (PHS) arrangements were implemented in order to obtain a suitable decoding. The packing fraction was 96.3% for asymmetric block and 95.5% for symmetric block. Both of the blocks have excellent decoding capability with all crystals clearly identified, 156 for asymmetric and 144 for symmetric and peak-to-valley ratio of 3.0 and 2.3 respectively. The average energy resolution was 14.2% for the asymmetric block and 13.1% for the symmetric block. Using a modified PQS geometry and asymmetric block design, we reduced the unused PMT region at detector panel edges, thereby increased the field-of-view and the overall detection sensitivity and minimized the undetected breast region near the chest wall. This detector design and using regular round PMT allowed building a lower-cost, high-resolution and high-sensitivity PEM camera.

The system design, engineering architecture and preliminary results of a lower-cost high-sensitivity high-resolution Positron Emission Mammography camera

A lower-cost high-sensitivity high-resolution Positron Emission Mammography (PEM) camera is developed. It consists of two detector modules with the planar detector bank of 12×20 cm2. Each bank has 60 low-cost PMT-Quadrant-Sharing (PQS) blocks arranged in a 10×6 array with two types of geometries. One is the symmetric 19.36×19.36 mm2 block made of 1.5×1.5×10 mm3 LYSO crystals in 12×12 array. The other is the 12×13 asymmetric elongated block made of 1.5×1.9×10 mm3 crystals. One row (10) of the elongated blocks are used along one side of the bank to reclaim the half empty PMT photocathode in the regular PQS design to reduce the dead area at the edge of the module. The bank has a high overall crystal packing fraction of 88%, which results in a very high sensitivity. Mechanical design and electronics have been developed for low-cost, compactness and stability. Each module has four Anger-HYPER decoding electronics that can handle a count-rate of 3 Mcps for singles. A simple two-module coincidence board with a hardware delay window for accidental events has been developed with an adjustable window of 6–15 ns. Some of the performance parameters have been studied by preliminary tests and simulations, including the decoding quality and energy resolution of the detectors, and the point source sensitivities, spatial resolutions and phantom images of the whole system.

PET resolution and image quality optimization study for different detector block geometries and DOI designs

Detector geometry for PET camera is one of the most important factors that will determine the resolution and image quality of the camera. In this work, the point source resolutions and hot-rod phantom images of the PET systems with different detector geometries are studied using Monte Carlo simulations. The PET systems been studied include a human brain PET and an animal PET with the typical detector ring dimensions but different crystal sizes, number of layers and block geometries. The detector ring diameter is 480 mm for the brain PET and 160 mm for the animal PET. The blocks been studied include 1-layer and 2-layer geometries. Two types of 2- layer blocks are studied, one has a half-crystal-offset (HCO) between the top and bottom layers to double the radial and axial samplings, and the other one is the regular (non-HCO) 2-layer block. The blocks in the brain PET is 40times40times20 mm3 and with a crystal matrix of 10times10, 13times13 and 15times15; the block in the animal PET is 20times20times10 mm3 and with a crystal matrix of 10times10, 12times12 and 14times14. Point source resolution curves (radial, tangential and axial) as the function of off-center distance are obtained with an F-18 source. Two Derenzo-like hot-rod phantoms are used for overall image quality test. The results show the significant improvement on the DOI effect with 2-layer block especially in the radial direction. Using the HCO block, the transaxial resolution is a little better than that of non-HCO block in the center region of the FOV but become worse with some distance from the center because of the sampling rates in different radial offset positions are not the same for these two types of blocks. Another important finding is that the axial resolution with HCO block is much better than that from 1- layer or 2-layer non-HCO block. Therefore using HCO block with larger crystal pitch can still achieve better image quality not only in transaxial plane at the outside region of a large FOV compare to the small crystal pitch with 1-layer block, but also better resolution in axial direction without additional cost, which might be a better choice for high-resolution PET camera design.

Real time digital implementation of the high-yield-pileup-event-recover (HYPER) method

We have proposed a high-yield-pileup-event- recover (HYPER) method that can process scintillation signals in very high count-rate situations where multiple-event pileups are normal, and successfully used this method in our BGO animal PET and human PET systems. In the first generation HYPER electronics, the integration and weight-sum circuits were implemented using analog signal. However, the same idea can be implemented in full digital mode. In the digital HYPER method, the input signal is digitized with a free run ADC, and then processed in a field programmable gate array (FPGA). Recent improvement in integrated circuit technology makes it possible to do digitization and real-time processing with clock frequency over 200 MHz. The dead time is reduced because theres no dead-time for discharging the integration value. The analog delay line used to balance the trigger delay is removed, which will reduce the signal distortion, and in turn increase the measurement resolution. The processing in the FPGA includes digital integration, weight-sum, and dynamic pile-up correction. Simulation shows a possible working frequency of 320 MHz with a low-cost FPGA, the Altera CY2C35F484C6. Energy spectrum of LSO with count rate up to 20 MCPS has been studied. The energy resolution of 1, 2, 4, 8, 12, 16 and 20 MCPS is 10.6%, 11.1%, 12.4%, 14.1%, 16.2%, 18.4% and 23.0%, respectively.