A prototype of a novel transformable single-ring OpenPET

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.