Whole-body dual-ring OpenPET for in-beam particle therapy imaging

The OpenPET is our original idea that realizes the world´s first open-type 3D PET scanner for PET-image guided particle therapy such as in situ dose verification and direct tumor tracking. Even with a full-ring geometry, the OpenPET has an open gap between its 2 detector rings through which the treatment beam passes. Following our initial 2008 proposal, we developed a small prototype in 2010 to show a proof-of-concept. Now, we report the development of a prototype whole-body OpenPET. The key technology which enabled the OpenPET realization is our original, 4-layered depth-of-interaction detector. In order to measure a radiation from the limited activity produced though fragmentation reactions, Zr-doped GSO (GSOZ), which contains less natural radioactivity, was chosen for the scintillators instead of Lu-based scintillators although timing performance was compromised. In order to compensate for the limited light yield, on the other hand, we used 64-channel flat-panel PMTs with a super-bialkali photocathode, which had a 30% higher quantum efficiency. In order to enable stable in-beam PET measurement even under high background radiations, voltage divider circuits were designed to provide 5 times higher linearity. Additionally, to avoid severe radiation damage, we did not use gain control ASICs in the front-end circuits, and position analyzer circuits were placed with a 15-m cable extension. The prototype consists of 2 detector rings, and each detector ring has 2 sub-rings of 40 detectors. Each detector consists of 16 × 16 × 4 array of GSOZ (2.8 × 2.8 × 7.5mm³). The portable gantry has a compact design; each detector ring has a 940 mm outer diameter and 171 mm thickness for the detector inner bore of 640 mm diameter and 113 mm thickness. The system was tested with a carbon beam irradiation at a clinical intensity. Phantom images were obtained by applying a GPGPU-based, list mode iterative reconstruction algorithm with geometrical detector response modeling.