Soonseok Kim

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.

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.

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.

An improved quadrant-sharing BGO detector for a low cost rodent-research PET (RRPET)

This is a study to improve the detector engineering of a low-cost high-sensitivity rodent-research PET camera (RRPET). The detector system is a solid ring of BGO made of tapered-pentagon blocks, each with 8/spl times/8 crystals and an average crystal pitch of 2.0 mm. The detector system used a variation of the photomultiplier-quadrant-sharing design (PQS). The first version of the RRPET blocks used white-paint reflectors of different shapes/sizes to control light distribution to the 4 decoding PMT. In this improved version, the white paint was replaced by a new ESR-mirror film. The film is 0.06 mm thick and mechanically robust. The construction provides a very high crystal-packing fraction (96% linearly) with a gap of only 0.08 mm between crystals. The film construction is more suitable for the slab-sandwich-slice construction we developed to make PQS blocks as the film reduces construction time. Average light output increased by 32% compared to the painted version. The light-output ratio (signal uniformity) between the worst crystal group (in the gap between 4 round PMTs) and the best crystal group (near the middle of a PMT) improved from 53% from 63%. The crystal-decoding resolution has also improved. The detection-efficiency uniformity has improved as well. The energy resolution for individual crystals has improved from 24% to 19% for the corner crystals and from 30% to 22% for the central crystals. The film also has better mechanical precision than paint, thereby positioning the small crystals more accurately relative to each other.

Gantry design with accurate crystal positioning for a high-resolution transformable PET camera

A positron emission tomography (PET) camera capable of transforming its geometric configuration is being developed. This high-resolution oncologic transformable PET (HOTPET) can be modified from a large detector ring of 83 cm to a small diameter ring of 54 cm. The system consists of 12 rectangular detector modules arranged in a polygon. The detector gap between modules remains constant in both configurations because each module is rotated around its own axis and displaced radially, bringing together adjacent modules. HOTPETs detectors are highly pixilated (crystal pitch 2.6 mm), requiring accurate placement of the modules relative to each other to ensure alignment of crystals within the same detector ring. We have designed a precise detector bank holder with keyways and complementary keys built onto its sides to allow interlocking with each other to form a polygon and maintain crystal coplanarity. Consequently, we were able to design the gantry supporting the modules using wider tolerances and so reduce its construction cost. The module provides support to 77 photomultiplier tubes (PMTs), the analog front-end electronics, and an automated PMT-gain control, all enclosed within a controlled environment. Potential development of light leaks was minimized with only two parting surfaces throughout the modules box, and tortuous-path air ducts inside the walls. Internal airflow allows temperature control. Simple removal of a back cover and a motherboard gives access to any part of the electronic components or a PMT with minimal disturbance to other components.

A gain-programmable transit-time-stable and temperature-stable PMT Voltage divider

A gain-programmable, transit-time-stable, temperature-stable photomultiplier (PMT) voltage divider design is described in this paper. The signal-to-noise ratio can be increased by changing a PMT gain directly instead of adjusting the gain of the pre-amplifier. PMT gain can be changed only by adjusting the voltages for the dynodes instead of changing the total high voltage between the anode and the photo-cathode, which can cause a significant signal transit-time variation that cannot be accepted by an application with a critical timing requirement, such as positron emission tomography (PET) or time-of-flight (TOF) detection/PET. The dynode voltage can be controlled by a digital analog converter (DAC) isolated with a linear optocoupler. The optocoupler consists of an infra-red light emission diode (LED) optically coupled with two phototransistors, and one is used in a servo feedback circuit to control the LED drive current for compensating temperature characteristics. The results showed that a 6 times gain range could be achieved; the gain drift was < 0.5% over a 20/spl deg/C temperature range; 250 ps transit-time variation was measured over the entire gain range. A compact print circuit board (PCB) for the voltage divider integrated with a fixed-gain pre-amplifier has been designed and constructed. It can save about

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30 per PMT channel compared with a commercial PMT voltage divider along with a variable gain amplifier. The pre-amplifier can be totally disabled, therefore in a system with large amount of PMTs, only one channel can be enabled for calibrating the PMT gain. This new PMT voltage divider design is being applied to our animal PET camera and time-of-flight/PET research.

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Our objectives were to develop two 9times9 and 10times10 array high resolution position sensitive block detectors using low cost photomultiplier-quadrant-sharing (PQS) technology. These two blocks can decode 4.3 mm or 3.9 mm BGO crystals with 39 mm size regular round PMT. By using ESR film as reflector between crystals, we achieved 96% packing fractions for these two blocks. Compared with conventional block design using 19 mm or 26 mm PMT, Phi39 mm size PMT can reduce the number of PMT in a camera by 55%-75%. Use of BGO crystal can reduce crystal cost by more than 70% compared to LSO or GSO crystals that are used in some of the newest PET cameras. The BGO crystal sizes-4.3 mm or 3.9 mm-in these two PQS blocks were similar to or smaller than BGO, LSO, or GSO crystals (4-6.3 mm) in commercial human PET cameras. The spatial resolution of the PQS blocks was expected to be similar or better. List mode data of these two blocks were acquired with Na- 22 source. Crystal-decoding map of blocks and individual crystal spectra were derived. All 9times9 and 10times10 crystals were clearly decoded on the block decoding map. These two PQS detectors decoded 81 or 100 BGO crystals per PMT. The average peak to valley ratio of decoding map was 3.4 and 1.8 respectively. The light collection efficiency for all crystals in the blocks were 71%-100% and 66%-100% respectively. The average energy resolutions of all crystals were 16.8% and 17.2% respectively. Our data suggest that even at lower cost the new PQS PET camera may outperform recent conventional PET cameras.

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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.

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GSO position-sensitive block detectors have been developed using the PMT-Quadrant -Sharing (PQS) technology for animal PET imaging. Two prototype block detectors with dimensions of 1.51 mm/spl times/1.51 mm/spl times/10 mm (12/spl times/12 array) and 1.66 mm/spl times/1.66 mm/spl times/10 mm (11/spl times/11 array) were built using 19 mm round PMTs. The Enhanced Specular Reflector mirror-film (98% reflectance, 0.065 mm thickness, 3M Co.) was used to control the light collection and distribution in the PQS design. The detector pitches are 1.59 (12/spl times/12) and 1.74 mm (11/spl times/11) and the crystal-packing fractions 95.0 and 95.4%, respectively. List-mode measurements with Cs-137 sources were carried out to investigate the crystal decoding and analyzed to extract the individual crystal spectra. All the crystals in both the detectors were clearly identified and well isolated. The light-collection efficiency is lowest in the central crystals and highest in the crystals of the four corners, with a ratio of 0.76 for the 11/spl times/11 array. The overall energy resolution of the 11/spl times/11 array is 13.5% (15.4% at 511 keV) with a standard deviation of 1.2%, which indicates that the individual energy resolutions of the detector are high and uniformly distributed. From this study, we achieved a high decodable crystals/PMT ratio of 144:1 for the 19 mm PQS GSO detector technology with crystals 10 mm deep. It is believed that these detectors would be useful for high resolution animal PET imaging.