Development of Small-Animal PET Prototype Using Silicon Photomultiplier (SiPM): Initial Results of Phantom and Animal Imaging Studies

Abstract

Silicon photomultiplier (SiPM; also called a Geiger-mode avalanche photodiode) is a promising semiconductor photosensor in PET and PET/MRI because it is intrinsically MRI-compatible and has internal gain and timing properties comparable to those of a photomultiplier tube. In this study, we have developed a smallanimal PET system using SiPMs and lutetium gadolinium oxyorthosilicate (LGSO) crystals and performed physical evaluation and animal imaging studies to show the feasibility of this system. Methods: The SiPM PET system consists of 8 detectors, each of which comprises 2 · 6 SiPMs and 4 · 13 LGSO crystals. Each crystal has dimensions of 1.5 · 1.5 · 7 mm. The crystal face-toface diameter and axial field of view 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. The physical characteristics were studied by measuring energy resolution, sensitivity, and spatial resolution. Various mouse and rat images were obtained to study the feasibility of the SiPM PET system in in vivo animal studies. Reconstructed PET images using a maximum-likelihood expectation maximization algorithm were coregistered with animal CT images. Results: All individual LGSO crystals within the detectors were clearly distinguishable in flood images obtained by irradiating the detector using a 22 Na point source. The energy resolution for individual crystals was 25.8% 6 2.6% on average for 511-keV photopeaks. The spatial resolution measured with the 22 Na point source in a warm background was 1.0 mm (2 mm off-center) and 1.4 mm (16 mm off-center) when the maximum-likelihood expectation maximization algorithm was applied. A myocardial 18 F-FDG study in mice and a skeletal 18 F study in rats demonstrated the fine spatial resolution of the scanner. The feasibility of the SiPM PET system was also confirmed in the tumor images of mice using 18 F-FDG and 68 Ga-RGD and in the brain images of rats using 18 F-FDG. Conclusion: These results indicate that it is possible to develop a PET system using a promising semiconductor photosensor, which yielded reasonable PET performance in phantom and animal studies.

Simultaneous Multiparametric PET/MRI with Silicon Photomultiplier PET and Ultra-High-Field MRI for Small-Animal Imaging

“…However, the relatively low signal amplification gain and slow time response of avalanche photodiodes compared with photomultiplier tubes have stressed the demand for more advanced semiconductor photosensors in nextgeneration simultaneous PET/MRI systems. In recent years, several groups, including us, have demonstrated the feasibility of smallanimal PET and PET/MR imaging based on silicon photomultipliers (SiPMs); SiPMs are composed of an array of micro-avalanche photodiode cells operating in Geiger mode, which provides a much higher signal amplification gain and faster timing properties than the avalanche photodiode (15)(16)(17)(18)(19)(20).…”

Initial Results of Simultaneous PET/MRI Experiments with an MRI-Compatible Silicon Photomultiplier PET Scanner

“…We chose LGSO because it has a fast scintillation decay time (t 5 41 ns), good stopping power (m 5 0.85 cm 21 at 511 keV), and high light output. It has yielded outstanding performance in our previous investigations (18,20), and its MRI compatibility was confirmed (21). The LGSO array consists of 360 (20 · 18) crystals (Supplemental Fig.…”

“…During the PET experiments, a 25-g BALB/c mouse was anesthetized by continuous administration of isoflurane in oxygen. 18 F-FDG (3.7 MBq) was injected through the tail vein, and 30 min later a 60-min image dataset was acquired. T2-weighted turbo spin echo MRI scans were acquired during the PET scan (15 slices).…”

“…Alternatively, the silicon photomultiplier (SiPM)-also called the Geiger-mode APD, solid-state photomultiplier, multipixel photon counter (MPPC)-is gaining much attention as a promising photosensor for future PET/MRI systems, because it is insensitive to the magnetic field and has amplification gain and timing resolution equivalent to those of the PMT (10)(11)(12). Therefore, several groups, including us, have worked on the development of SiPM PET detectors and systems (13)(14)(15)(16)(17)(18)(19).…”

“…Recently, we have shown the feasibility of our first SiPM PET prototype for small-animal imaging (18). The first prototype system consisted of 8 SiPM detector modules composed of 4 · 13 Lu 1.9 Gd 0.1 SiO 5 :Ce (LGSO) crystals and 2 · 6 SiPMs, and it had a ring diameter of 6.0 cm and an axial field of view (FOV) of 0.65 cm.…”