Shuping Xie

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.

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

High resolution GSO block detectors using PMT-quadrant-sharing design for small animal PET

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.

Performance evaluation of the low-cost high-sensitivity rodent research PET (RRPET) camera using Monte Carlo simulations

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.

Electronics design for a low-cost high-sensitivity rodent-research PET (RRPET)

BGO pentagonal blocks are used to build a solid detector cylinder for a rodent research PET (RRPET). With the PMT-quadrant-sharing design, this camera is low-cost and has higher sensitivity than commercial animal PET. The performance parameters of RRPET are obtained using Monte Carlo simulations by GATE, a newly developed toolkit based on GEANT4 for PET/SPECT. Two models of the RRPET system, one 30-sided-polygon geometry model and one 10-sided-polygon electronics model with different time-response constants, are used in the simulations to get the accurate results for both geometry-related and time-related parameters, including the sensitivity, count rate and resolution. Several types of sources are simulated, including a point source, a linear source, and several uniform sources simulating the mouse and the rat. The simulations give the noise equivalent count rate (NECR) curves with the maximum NECR of 2.2 Mcps @ 1.5 mCi for point source, 750 kcps @ 1.5 mCi for mouse-like source and 290 kcps @ 1.0 mCi for rat-like source; the sensitivity is 10.3%; the spatial resolution at the center of the field of view (FOV) is 1.6 mm for an 18F source inside water phantom; the radial and tangential resolution at the edge of the FOV (10 cm) is 3.2 mm and 2.2 mm, respectively. Preliminary experiments showed a sensitivity of 8.4% to 10.2% (depending on electronics) and a resolution of 1.8 mm with a 22Na source. By comparing the simulation and experiment results, the resolution degradation from the mechanical misalignment and the block-decoding blurring is found to be 1.1 mm

An instantaneous photomultiplier gain calibration method for PET or gamma camera detectors using an LED network

We designed and implemented electronics for a low-cost high-sensitivity positron emission tomography camera for research involving rodents. To reduce cost and increase sensitivity, we used continuous full-ring photomultiplier tube (PMT) with quadrant sharing (PQS) detector design. In this prototype camera, 168 PMTs decode 144 scintillation detector blocks consisting of 9216 crystal elements. An Anger position matrix board weight sums the 144 detector blocks as eight individual gamma camera zones. The full-ring detector decoding is performed by eight fixed local zones. However, in the PMT-quadrant-sharing design, every two adjacent zones share seven axial PMTs. A boundary processing technique has been developed for the PMT-quadrant-sharing detector blocks so that the decoding of the full-ring detector can be performed by individual zones. A high-yield-pileup-event-recovery decoding board, a module-based coincidence processing system and a data acquisition computer, which were originally developed for a whole-body PET, can still be used by this rodent PET camera. The camera needs only eight decoding boards, and each board decodes 18 detector blocks of one detector zone. The entire decoding electronics need only 24 ADCs and can handle about six million events/second of single-rate. A motherboard decodes the control commands from the data acquisition computer, performs the real-time boundary processing and distributes DC power signals to all the eight decoding boards. To further reduce the cost and size of the camera, we have developed a new compact PMT voltage divider with adjustable PMT gain that can be controlled by programming the dynode high voltages directly. The very front-end preamplifier is also integrated into this divider board to increase the signal-to-noise ratio. A new instantaneous light-emitting diode automatic PMT gain calibration method is also used in this camera for better quality control; the gains of 168 PMTs can be equalized within 1 minute.

An instantaneous photomultiplier tube gain-tuning method for PET or gamma camera detectors using an LED network

In current clinical positron emission tomography (PET) cameras, there are about 1000 photomultiplier tubes (PMTs) in the detector system. Even a less-complicated gamma camera has many dozens of PMTs. Image quality and resolution of a camera is dependent on the proper equalization of all the PMT gains. However, a PMT gain can change with many environmental factors, such as room temperature, patient load, short-term or long-term radiation exposure, and time. Hence, an instantaneous automated PMT gain calibration method is especially important for an ultrahigh-resolution PET camera. We have developed a new PMT gain auto-tuning method using a blue light-emitting diode (LED) network. Each LED shines directly into the center of a scintillation crystal block from the PMT side, and the light is collected by the surrounding PMTs. The effects of crystal optical transferring efficiency and PMT optical coupling efficiency have been considered. The calibration is done by changing the gains of these surrounding PMTs or their following amplifiers to have the same signal output. An LED has well known problems of large light-yield varieties and is very sensitive to temperature. To overcome these problems, the light outputs of two neighboring LEDs are aligned first by a shared PMT. Each LED flashes individually and is driven by a 250 KHz pulse generator. At such a high pulse rate, the data acquisition for the gain calibration can be finished within a very short time so the LED temperature effect can be ignored. The amount of LED light output is set as close as possible to the amount of scintillation light by programming the width or height of the pulses; therefore, the same system electronics can be used for both purposes. Our proposed high-resolution whole-body PET camera with 924 PMTs in a PMT-quadrant-sharing (PQS) design can be calibrated in 1 minutes or less.

A comparison of four-image reconstruction algorithms for 3-D PET imaging of MDAPET camera using phantom data

A photomultiplier tube (PMT) gain can change with many environmental factors, such as room temperature, patient load, short-term or long-term radiation exposure, and time. Unbalanced PMT gains degrade the image resolution and quality in a positron emission tomography (PET) camera or a gamma camera. This paper presented a new method to instantaneously recover the original manufacture PMT gain setting using a blue light-emitting diode (LED) network. Each LED shines directly into the center of a scintillation crystal block from the PMT side, and the light is collected by the surrounding PMTs. The gain tuning is done by changing the gains of these surrounding PMTs or their following amplifiers to have the same signal output. An LED has well-known problems of large light-yield varieties and is very sensitive to temperature. To overcome these problems, the light outputs of two neighboring LEDs are aligned first by a shared PMT. Each LED flashes at 250-KHz pulse rate, the data acquisition for the gain tuning can be finished within a very short time so the LED temperature effect can be ignored. The amount of LED light output is set as close as possible to the amount of scintillation light by programming the width or height of the pulses; therefore, the same electronics can be used for data acquisition and tuning. We estimated a 12 module PET camera with 924 PMTs in a PMT-quadrant-sharing design can be tuned in 1 min.

High resolution GSO block detectors using PMT-quadrant-sharing design for human whole body and breast/brain PET applications

We compared two fully three-dimensional (3-D) image reconstruction algorithms and two 3-D rebinning algorithms followed by reconstruction with a two-dimensional (2-D) filtered-backprojection algorithm for 3-D positron emission tomography (PET) imaging. The two 3-D image reconstruction algorithms were ordered-subsets expectation-maximization (3D-OSEM) and 3-D reprojection (3DRP) algorithms. The two rebinning algorithms were Fourier rebinning (FORE) and single slice rebinning (SSRB). The 3-D projection data used for this work were acquired with a high-resolution PET scanner (MDAPET) with an intrinsic transaxial resolution of 2.8 mm. The scanner has 14 detector rings covering an axial field-of-view of 38.5 mm. We scanned three phantoms: 1) a uniform cylindrical phantom with inner diameter of 21.5 cm; 2) a uniform 11.5-cm cylindrical phantom with four embedded small hot lesions with diameters of 3, 4, 5, and 6 mm; and 3) the 3-D Hoffman brain phantom with three embedded small hot lesion phantoms with diameters of 3, 5, and 8.6 mm in a warm background. Lesions were placed at different radial and axial distances. We evaluated the different reconstruction methods for MDAPET camera by comparing the noise level of images, contrast recovery, and hot lesion detection, and visually compared images. We found that overall the 3D-OSEM algorithm, especially when images post filtered with the Metz filter, produced the best results in terms of contrast-noise tradeoff, and detection of hot spots, and reproduction of brain phantom structures. Even though the MDAPET camera has a relatively small maximum axial acceptance (/spl plusmn/5 deg), images produced with the 3DRP algorithm had slightly better contrast recovery and reproduced the structures of the brain phantom slightly better than the faster 2-D rebinning methods.

A comparison of four image reconstruction algorithms for detection of small lesions in brain phantom

High resolution GSO block detectors have been developed using the PMT-quadrant-sharing (PQS) design for human whole body and breast/brain PET applications. Segmented GSO (Ce 0.5 mol%) crystals of 3.02 mmtimes3.02 mmtimes20 mm (13times13 array) and 3.58 mmtimes3.58 mmtimes20 mm (11times11) were assembled together to build two prototype detectors for whole body PET imaging with 39 mm round PMTs and crystals of 2.04 mmtimes2.04 mmtimes20 mm (9times9) for breast/brain PET imaging with 19 mm round PMTs. The enhanced specular reflector (3M Co.) mirror-film windows with reflectance 98% and thickness 0.065 mm were used to control light collection and distribution. Crystal pitches are 3.1 (13times13), 3.64 (11times11), and 2.12 mm (9times9) and crystal-packing fractions are 95, 97, and 93%, respectively. List-mode measurements with Cs-137 sources were carried out to investigate crystal decoding and were analyzed to extract individual crystal spectra. All the crystals in the detectors were identified. Energy resolution for individual crystals ranges from 13.7 to 18.5% (13times13), 12.2 to 14.2% (11times11), and 20.9 to 28.9% (9times9), with overall energy resolutions of 15.6, 13.2, and 24.3%, respectively. Light collection efficiencies for the central crystals are 60 (13times13), 66 (11times11), and 72% (9times9), compared to the corner crystals. The maximum decodable crystals/PMT ratios at 511 keV are high 121-169 for the 39 mm PQS GSO detector design and 81 for the 19 mm PQS GSO detector design with crystals 20 mm deep. It is anticipated that these PQS GSO detectors will provide high resolution and sensitivity at low cost