Positron emission tomography (PET) detectors with depth-of- interaction (DOI) capability

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Visualization of biologic processes at molecular and cellular levels has revolutionized the understanding and treatment of human diseases. However, no single biomedical imaging modality provides complete information, resulting in the emergence of multimodal approaches. Combining state-of-the-art PET and MRI technologies without loss of system performance and overall image quality can provide opportunities for new scientific and clinical innovations. Here, we present a multiparametric PET/MR imager based on a smallanimal dedicated, high-performance, silicon photomultiplier (SiPM) PET system and a 7-T MR scanner. Methods: A SiPM-based PET insert that has the peak sensitivity of 3.4% and center volumetric resolution of 1.92/0.53 mm 3 (filtered backprojection/ordered-subset expectation maximization) was developed. The SiPM PET insert was placed between the mouse body transceiver coil and gradient coil of a 7-T small-animal MRI scanner for simultaneous PET/MRI. Mutual interference between the MRI and SiPM PET systems was evaluated using various MR pulse sequences. A cylindric corn oil phantom was scanned to assess the effects of the SiPM PET on the MR image acquisition. To assess the influence of MRI on the PET imaging functions, several PET performance indicators including scintillation pulse shape, flood image quality, energy spectrum, counting rate, and phantom image quality were evaluated with and without the application of MR pulse sequences. Simultaneous mouse PET/MRI studies were also performed to demonstrate the potential and usefulness of the multiparametric PET/MRI in preclinical applications. Results: Excellent performance and stability of the PET system were demonstrated, and the PET/MRI combination did not result in significant image quality degradation of either modality. Finally, simultaneous PET/MRI studies in mice demonstrated the feasibility of the developed system for evaluating the biochemical and cellular changes in a brain tumor model and facilitating the development of new multimodal imaging probes. Conclusion: We developed a multiparametric imager with high physical performance and good system stability and demonstrated its feasibility for small-animal experiments, suggesting its usefulness for investigating in vivo molecular interactions of metabolites, and cross-validation studies of both PET and MRI.Key Words: PET/MRI; silicon photomultiplier (SiPM); hybrid imaging; multi-parametric imaging; dual-modality imaging probe Asubst antial role of small-animal imaging has been pinpointed in numerous studies in terms of understanding the underlying mechanism of human diseases and elucidating the efficacy of new therapeutic approaches. Among the in vivo small-animal imaging modalities, which are scaled down to dedicated devices from clinical ones, PET is the most-sensitive technique that is readily translatable to the clinic (1). Spatial and temporal distributions of compounds labeled with a positron-emitting radionuclide are noninvasively measured by the PET scanner. Consequently, the PET scanner p...

Section: Discussionmentioning

confidence: 99%

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

Yoon

Kim

et al. 2016

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“…To enhance sensitivity by elongating crystals, depth-of-interaction determination within crystals will be necessary to avoid the deterioration in spatial resolution uniformity (31). Several groups have shown that depth-ofinteraction encoding with SiPM is feasible (13).…”

Section: Discussionmentioning

confidence: 99%

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

Kwon¹,

Lee²,

Yoon³

et al. 2011

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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.

“…More available space inside the MRI bore than is allowed in dedicated animal MRI scanners is another merit of the clinical MRI system. The larger space allows the use of longer scintillation crystals and various depth-ofinteraction measurement techniques to improve PET system performance (29).…”

Section: Discussionmentioning

confidence: 99%

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

Yoon¹,

Ko²,

Kwon³

et al. 2012

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165

102

The most investigated semiconductor photosensor for MRIcompatible PET detectors is the avalanche photodiode (APD). However, the silicon photomultiplier (SiPM), also called the Geiger-mode APD, is gaining attention in the development of the next generation of PET/MRI systems because the SiPM has much better performance than the APD. We have developed an MRI-compatible PET system based on multichannel SiPM arrays to allow simultaneous PET/MRI. Methods: The SiPM PET scanner consists of 12 detector modules with a ring diameter of 13.6 cm and an axial extent of 3.2 cm. In each detector module, 4 multichannel SiPM arrays (with 4 · 4 channels arranged in a 2 · 2 array to yield 8 · 8 channels) were coupled with 20 · 18 Lu 1.9 Gd 0.1 SiO 5 :Ce crystals (each crystal is 1.5 · 1.5 · 7 mm) and mounted on a charge division network for multiplexing 64 signals into 4 position signals. Each detector module was enclosed in a shielding box to reduce interference between the PET and MRI scanners, and the temperature inside the box was monitored for correction of the temperature-dependent gain variation of the SiPM. The PET detector signal was routed to the outside of the MRI room and processed with a field programmable gate array-based data acquisition system. MRI compatibility tests and simultaneous PET/MRI acquisitions were performed inside a 3-T clinical MRI system with 4-cm loop receiver coils that were built into the SiPM PET scanner. Interference between the imaging systems was investigated, and phantom and mouse experiments were performed. Results: No radiofrequency interference on the PET signal or degradation in the energy spectrum and flood map was shown during simultaneous PET/MRI. The quality of the MRI scans acquired with and without the operating PET showed only slight degradation. The results of phantom and mouse experiments confirmed the feasibility of this system for simultaneous PET/MRI. Conclusion: Simultaneous PET/MRI was possible with a multichannel SiPM-based PET scanner, with no radiofrequency interference on PET signals or images and only slight degradation of the MRI scans. A hybrid PET/MRI scanner has many potential advantages, including a reduced radiation dose, better soft-tissue contrast on MRI than CT, an almost unlimited combination of functional and molecular information, and possible motion correction of the PET image using MRI data (1-4). However, simultaneous PET/MRI with a conventional photomultiplier tube (PMT)-based PET camera is technically challenging, because the PMT is highly sensitive to the magnetic field. Almost every property of the PMT PET signal is distorted within the magnetic field. For example, the energy spectrum of the PET detector is quite diminished because of the loss of PMT signal output, and the peak position of the scintillation crystal cannot be distinguished in the flood maps of block detectors (1). Therefore, if relatively long optical fiber bundles are not used, the PMT PET camera should be placed a distance from the MRI machine (5). Consequently, a longer scan time is...