A Small Prototype for a Proof-of-Concept of OpenPET Imaging

The OpenPET geometry is our new idea to visualize a physically opened space between two detector rings. The OpenPET is expected to enable 1) PET image-guided radiation therapy by letting the beams pass though the gap, 2) real-time multimodal imaging by inserting another imaging device in the gap, and 3) extension of an axial field-of-view (FOV) with the limited number of detectors. In this paper, we developed the first small prototype to show a proof-of-concept of OpenPET imaging. The prototype was designed as a compact system so as to be easily carried between PET areas and therapy areas such as the Heavy Ion Medical Accelerator in Chiba (HIMAC). Two detector rings of 110 mm in diameter composed of 8 block-detectors were placed with a gap of 42 mm. Actual gap was limited to 27 mm by the gantry structure. Each block-detector, which had 4-layer depth-of-interaction capability, was composed of 2.9 x 2.9 x 5 mm3 LGSO crystals and a Hamamatsu H8500 PMT. For a proof-of-concept of PET imaging during radiation therapy, we carried out the in-beam tests in the HIMAC to visualize in situ dose distribution. The OpenPET prototype was positioned so that the beam passes through the gap. A PMMA cylindrical phantom of 4 cm in diameter, placed in the center of the PET FOV, was irradiated along radial direction by a 11C pencil beam. We showed that PET images corresponding to dose distribution were obtained. For an initial proof-of-concept of real-time multimodal imaging, we measured a tumor-inoculated mouse with FDG, and an optical image of the surface was taken during PET measuring by inserting a digital camera in the gap. We confirmed that the tumor in the gap was clearly visualized. The result also showed the effect of extension of an axial FOV, and the large axial FOV of 126 mm was obtained with the detectors originally covering only an 84 mm axial FOV. In conclusion, our initial imaging studies showed promising performance of the OpenPET prototype.2010IEEE NSS&MI