A prototype of a novel transformable single-ring OpenPET

Abstract

We are developing the world's first, open-type 3D PET scanner “OpenPET” for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Following our first idea of a dual-ring Open PET (DROP), we proposed our second-generation geometry, single-ring OpenPET (SROP), which is more efficient than DROP in terms of manufacturing cost and sensitivity. In this paper, we have developed a SROP prototype based on a novel detector arrangement, in which block detectors originally forming a conventional PET scanner were axially shifted little by little. Sixteen detector units each of which consists of two depth-of-interaction detectors are arranged to form a perfect circle, 25cm in diameter. Detector units have an axial shifting mechanism so that they can be transformed into the SROP; adding this mechanism to the units allows us to use the scanner as a conventional (i.e., non-open) PET when in-beam PET measurements are not required. After confirming its basic imaging performance using a phantom filled with 18F solution, we carried out in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC). In addition to the usual carbon (12C) beam, we applied RI beams of 11C and 10C. Stopping positions of primary particles were directly imaged with the RI beam irradiation, while the stopping position distribution of secondary particles was imaged with the 12C beam irradiation. Phantom study results with pencil beam irradiation of about 2.5Gy showed that beam stopping positions can be measured with the precision better than 2mm with the 11C beam irradiation followed by 20 min PET measurement. With the 10C beam, PET measurement time could be reduced to 1/10 while still maintaining the precision. For both 11C and 10C, there is room for further reduction of PET measurement time.