A Feasibility Study on the Use of Optical Fibers for the Transfer of Scintillation Light to Silicon Photomultipliers

ArticleinIEEE Transactions on Nuclear Science 58(3):579 - 589 · July 2011with10 Reads
DOI: 10.1109/TNS.2011.2112778 · Source: IEEE Xplore
Integrated PET/MRI units with simultaneous acquisition capability are set to play an important role in studies of human breast and prostate imaging and brain function. However, to take advantage of existing MRI units in hospitals and institutions, minimally modified combined PET MRI is highly desirable. In addition, the current MRI trend is to utilize powerful body coils to transmit radio-frequency (RF) waves and local RF coils to receive signals. The authors propose a silicon photomultiplier (SiPM) PET equipped with optical fiber bundles that transfer photons from scintillation crystal to SiPM. To investigate the feasibility of SiPM PET using optical fiber bundles, the authors studied the performances of SiPM/scintillator couplings using single optical fibers and a fiber bundle. GEANT4 Monte-Carlo simulation was used to study scintillation photon transfer from scintillation crystals to the SiPM. This simulation showed that light loss, due to the bending of an optical fiber, is not significant for a fiber with a diameter of 2.0 mm and a bending radius of greater than 25 mm. To validate the GEANT4 Monte-Carlo simulation, several simple detectors were assembled and tested. Simulation results agreed reasonably well with experimental results. Two Hamamatsu multi-pixel photon counters (MPPCs) were tested using double clad optical fibers of 1.5 mm and 2.0 mm diameter, and 25 mm and 50 mm bending radius, respectively. When two MPPCs were directly attached to 2.0 × 2.0 × 10.0 mm<sup>3</sup> LYSO crystals, a ~14% energy and a ~1.3 ns coincidence timing resolution were obtained at full width half maximum (FWHM). With one of the MPPCs attached to an optical fiber of 1.5 mm diameter, 50 mm bending radius, and length 300 mm, energy and coincidence timing resolutions were 27% and 2.2 ns, respectively. With an optical fiber bundle made of bare fibers with 1.5 mm diameter and length 100 mm, an ~ 26% energy resolution was obtained. Even though the Monte-Carlo simulat- - ion showed light loss was not significant for a single 90° turn of bending, the mechanical integrity of the optical fiber, especially the absence of cracks which can be caused by sharp bending, seemed to be a far more important constraint on sharp bending. These initial results are encouraging with respect to the use of combined SiPM PET using optical fibers.
    • "Based on the study of effects of copper shielding on MR images and on previous work on the SiPM-PET with a short optical fiber bundle bent 90 @BULLET (Hong et al 2011), we proposed a SiPM-PET configuration with short optical fiber bundles. The relatively short length of optical fiber bundles is expected not to deteriorate PET performance significantly while optical fiber bundles provide an increased spacing between RF-shielded electronics boxes facilitating RF passage to imaging objects as shown in figure 2. The length of an optical fiber can be varied to optimize PET performance and MR imaging. "
    [Show abstract] [Hide abstract] ABSTRACT: For positron emission tomography (PET) inserts to magnetic resonance imaging (MRI) applications, optical fibers have been used for some time to transfer scintillation photons to photomultiplier tubes positioned outside the fringe magnetic field. We previously proposed a novel utilization of an optical fiber for good radio frequency (RF) transmission from body coils to an imaging object. Optical fiber bundles between silicon photomultipliers (SiPM) and scintillation crystals provide an increased spacing between RF-shielded electronics boxes, facilitating RF passage from the body RF coils to imaging objects. In this paper, we present test results of a SiPM-PET system with a short optical fiber bundle for simultaneous PET-MR imaging. We built the SiPM-PET system which consisted of 12 SiPM-PET modules; each module was assembled with a lutetium yttrium oxyorthosilicatecrystal block, a 31 mm optical fiber bundle, a Hamamatsu multi-pixel photon counter S11064-050P and a signal processing box shielded with copper. The SiPM-PET system, with a face-to-face distance of 71 mm, was placed inside a 3 T MRI. A small surface coil placed inside the SiPM-PET system was used to receive the signal from phantoms while the body RF coil transmitted the RF pulses. The SiPM-PET system showed little performance degradation during the simultaneous PET-MR imaging and it caused no significant degradation of MR images with turbo spin echo (TSE), gradient echo or 3D spoiled gradient recalled sequences. Echo planar imaging MR images with and without the SiPM-PET inside the MR scanner were significantly worse than the images obtained with the TSE sequence.
    Full-text · Article · May 2012
  • [Show abstract] [Hide abstract] ABSTRACT: PurposeA coincidence detector is an essential unit to acquire distribution of radioisotopes in positron emission tomography (PET) systems. The coincidence pairs rapidly increase as the number of detector modules increase. To make this system compact, flexible, and conveniently controllable, a field programmable gated array (FPGA)-based coincidence detector was developed. MethodsThe unit targeted PET systems have eight detector modules. The coincidence detector module needs veto function for controlling the data transfer rate of the output signal because the output is connected to the band-limited ADC and data transfer unit. The coincidence detector module has eight inputs connected to the detector modules, one coincidence output signal connected to the trigger input of the ADC, and three control signals for setting system parameters, such as the coincidence window time, output pulse length, and veto time. ResultsThe performance of the unit was restricted by system clock frequency. The maximum clock frequency was 250 MHz when the unit was implemented to XILINX Spartan 3 FPGA. The unit had a minimum 2 ns of coincidence timing window when it runs in the double data rate (DDR) mode. The unit was also applied to PET image acquisition and the PET/CT and PET/MR images were successfully acquired. ConclusionsThe coincidence unit developed in this study was useful for PET image acquisition with PMT- and SiPM-based PET prototypes. The unit can be easily extended to larger systems that have many more detector modules without extra components. KeywordsPET–FPGA–Coincidence–Acquisition
    Article · Feb 2011
  • [Show abstract] [Hide abstract] ABSTRACT: Purpose Adding time-of-flight (TOF) information to positron emission tomography (PET) can lead to an improvement in image quality because TOF information allows the estimation of gamma ray annihilation positions in three dimensions. For accurate position estimation, precise time resolution is the most important characteristic required of TOF-PET detectors. The aims of this study were to establish the optimal setup for timing measurements and to evaluate the timing performance of a fast photomultiplier tube (PMT) (Hamamatsu R9800) coupled with a lutetium yttrium orthosilicate (LYSO) scintillation crystal for TOF-PET detector development. Methods Performance of a fast PMT (diameter, 25 mm; length, 55 mm) coupled with a LYSO crystal (4 × 4 × 10 mm3) was measured in coincidence experiments with 511 keV annihilation photons from a 22Na gamma source. Timing measurement optimization was carried out with a leading-edge discriminator (LED) and a constant fraction discriminator (CFD). Results Results showed the optimal timing resolution could be obtained with a CFD time delay of 1 ns; timing resolutions (FWHM) of the LYSO crystal-coupled fast PMT were measured to be 198 and 254 ps for two different R9800 PMTs. The energy resolutions at 511 keV were 10.8% and 10.6%, respectively. Additional experiments to determine the position uncertainty were performed at three different source positions and the position uncertainty (FWHM) in the TOF measurement using these PMTs was estimated to be 4.50 cm. Conclusions The performance of fast R9800 PMT coupled with a LYSO crystal showed good potential for development as a TOF-PET detector module.
    Article · Aug 2011
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