Positron Emission Tomography Detectors Based on Silicon Photomultiplier

Abstract

Silicon photomultiplier (SiPM) is a promising semi-conductor photo-sensor in positron emission tomography (PET), simultaneous PET/MRI, and time-of-flight PET because it is intrinsically MR-compatible and has comparable internal gain and timing properties to a photomultiplier tube (PMT). Their compact size enables one-to-one coupling between the crystal and SiPM to yield the best timing performance. However, huge and complex electronics are required for operating each individual SiPM. The aim of this study was to investigate into the use of SiPM for PET and to develop small animal PET scanner and time-of-flight PET block detector. The SiPM PET consists of 8 detectors, each of which is composed of 2 × 6 SiPMs and 4 × 13 LGSO crystals. Each crystal has dimensions of 1.5 × 1.5 × 7 mm3. The crystal face-to-face diameter and axial FOV are 6.0 cm and 6.5 mm, respectively. Bias voltage is applied to each SiPM using a finely-controlled voltage supply because the gain of the SiPM strongly depends on the supply voltage. Reconstructed PET images using a maximum-likelihood expectation-maximization (MLEM) algorithm were co-registered with animal X-ray CT images. All individual LGSO crystals within the detectors were clearly distinguishable in flood images obtained by irradiating the detector using a 22Na point source. The energy resolution for individual crystals was 25.8±2.6% on average for 511 keV photo-peaks. The spatial resolution measured with the 22Na point source in a warm background was 1.0 mm (2 mm off center) and 1.4 mm (16 mm off center) when the MLEM algorithm was applied. A myocardial 18F-FDG study in mice and a skeletal 18F study in rats demonstrated the fine spatial resolution of the scanner. The feasibility of the SiPM PET was also confirmed in the tumor images of mice using 18F-FDG and 68Ga-RGD, and the brain images of rats using 18F-FDG. Proposed time-of-flight PET block detector using SiPM array are comprised of several M × N SiPM array blocks with signal encoding method and a field-programmable gate array (FPGA) board, which had developed digital time-to-digital converters (TDCs). Each SiPM was directly coupled with an LGSO crystal (3 × 3 × 20 mm3). Each output signal of SiPM is connected to each row and column that reduces the output signals from M × N to M + N. These row and column signals were used to measure energy and timing information of each incident γ-ray event, respectively. To verify the signal encoding methods for TOF PET block detector, various sizes of SiPM array (4 × 4, 8 × 8, and 12 × 12) were tested using standard methods for energy and time measurement. In the front-end electronics board for 4 × 4 SiPM array, comparators were used to convert row and column signals from analog to digital signals. These converted row and column digital signals transferred to a FPGA board. In FPGA, these row and column signals were used to measured energy and time information only using developed digital TDCs. Time-over-threshold (TOT) method was used to measure energy using time duration without traditional energy measurement method. By applying signal encoding method, coincidence timing resolution slightly increased in large array, but the resolutions from the 12 × 12 SiPM array showed sufficient results for the use of TOF PET scanners. The developed digital TDCs on FPGA had the least square bit of ~15 ps. The intrinsic timing resolutions of ~130 ps were measured using developed TDC and two front-end boards without SiPMs. Dependence of TOT value on γ-ray energy was obtained with various radiation sources and the results showed clearly distinguishable. Coincidence γ-ray measurements were performed using 22Na. The coincidence timing resolution of ~450 ps was obtained, which was comparable with conventional TOF PET scanners. In conclusion, these results indicate that it is possible to develop a PET using a promising semi-conductor photo-sensor, which yielded reasonable PET performances in phantom and animal studies. Moreover, proposed time-of-flight PET detector can enable the development of a simple and cost-effective block detector for human time-of-flight PET scanner without traditional energy measurement method and individual SiPM readout in the block detector.