Performance evaluation of a transformable axial-shift type single-ring OpenPET

One of the challenging applications of PET is implementing it for in-beam PET, which is an in situ monitoring method for charged particle therapy. For this purpose, we have previously proposed two geometries for our open-type PET scanners named OpenPET. The original OpenPET had a physically opened field-of-view (FOV) between two detector rings through which irradiation beams pass. This dual-ring OpenPET (DROP) had a wide axial FOV including the gap. But, this geometry is not necessarily the most efficient for application to in-beam PET in which only a limited FOV around the irradiation field is required. Therefore, we proposed a single-ring OpenPET (SROP) geometry which can provide an accessible and observable open space with higher sensitivity and a reduced number of detectors compared to DROP. Also, we proposed two arrangement types, a slanted ellipse (SE) type where oval detector rings are slanted and stacked and an axial shift (AS) type where block detectors originally forming a conventional PET scanner are axially shifted little by little. Previously, we developed the SE-SROP prototype, and showed its good performance for in-beam PET. However, the SE-SROP decreased sensitivity slightly relative to the ring-type PET and needed a large gantry in the axial direction for conventional PET studies. On the other hand, AS-SROP can transform conventional ring-type PET scanners into stacked detectors with a gradual axial shift. In this work, we developed and evaluated a small AS-SROP prototype for proof-of-concept. The AS-SROP prototype was designed with 2 cylinder-shaped detector rings of 16 DOI detectors. The system sensitivity and spatial resolution were measured and were 5.1% and 2.6 mm, respectively, for the OpenPET mode. For the conventional PET mode, the system sensitivity and spatial resolution were 7.3% and 2.2 mm, respectively. We concluded that the AS-SROP geometry has a good potential for not only in-beam imaging but also conventional PET studies.