A small prototype of a single-ring OpenPET

The OpenPET geometry is our original idea to visualize a physically opened space even with a full ring geometry. One of our targets is in-beam PET, which is a method for in situ monitoring of charged particle therapy. In our initial idea, the OpenPET had a physically opened field-of-view (FOV) between two detector rings separated by a gap. Originally, the OpenPET was proposed to provide a stress-less brain imaging device. For a dedicated in-beam PET scanner, this dual-ring OpenPET is a good candidate, but it is not necessarily the most efficient geometry because it has a wide FOV (i.e., a gap FOV plus two in-ring FOVs) while only a limited FOV around the irradiation field is required in actual use of in-beam PET. At the last conference, therefore, we proposed a single-ring OpenPET (SROP) dedicated for in-beam PET as our 2nd generation geometry. The detector ring of the SROP geometry was the cylinder both ends of which were cut by parallel aslant planes. In this paper, we developed a small prototype of the SROP for a proof-of-concept. It consisted of 2 ellipse-shaped detector rings, each of which had 16 detectors. Each ellipse-shaped detector ring had a major axis of 281.6 mm and a minor axis of 207.5 mm. The rings were slanted by 45 deg and staggered to obtain an open space of 74.5 mm width. We carried out initial in-beam imaging tests in the Heavy Ion Medical Accelerator in Chiba (HIMAC) using a ¹¹C beam as well as a ¹²C beam. PET measurement started at the beginning of the irradiation, and continued for 20 min after the irradiation. For about 3Gy irradiation, a 6 mm range difference was clearly detected with the ¹¹C beam irradiation. Our initial imaging studies showed promising performance of the SROP prototype.