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