OpenPET (2). Development of a small prototype for a proof-of-concept

We have proposed an OpenPET geometry which consists of two axially separated detector rings. The open gap would be suitable for in-beam positron emission tomography (PET), which is a method for in situ monitoring of charged particle therapy. In this work, we developed the first prototype of the OpenPET, especially for in-beam experiments using phantoms. The OpenPET prototype was designed with 2 detector rings of 8 detector blocks. Ring diameter was 110 mm and the minimum axial field-of-view was 99 mm. Distance between detector rings could be controlled. The depth-of-interaction (DOI) detector consisted of 784 LGSO crystals which were arranged in 4-layer of 14 x 14 arrays, coupled to a 64-ch FP-PMT. Each crystal element was 2.9 x 2.9 x 5 mm³. Front-end circuits were separated from the detector head by 1.2 m coaxial cables. Front-end circuits were set under the detector rings and surrounded by lead and paraffin shields. The system sensitivities were measured from a ²²Na point source. The system sensitivity without open gap was 7.2%. On the other hand, the system sensitivity with open gap kept to 5.8%. Count rate performance test utilized 8-cm cylinder phantom. Maximum count rate of prompt was 29 kcps at 0.1 mCi. The small OpenPET prototype was succeed to get fully 3D images under the in-beam experiment. The OpenPET prototype has promises high sensitivity and enough performance for in-beam experiments. (author)