Development of a Whole-Body Dual Ring OpenPET for in-Beam PET

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 previously proposed the world's first open-type geometries which we named OpenPET. For in-beam PET, careful designing and testing of detectors are important because secondary particles generated in a target degrade detector performance in the OpenPET geometries. As well, we have developed small prototypes and identified their promising potential for in-beam PET and succeeded in visualizing a 3D distribution of beam stopping positrons inside a phantom. In this work, we developed a whole-body dual ring OpenPET (WBDROP) with a singles-based data acquisition (DAQ) system. This scanner design has two separated rings. Each ring has two detector rings of 40 detector blocks, and each detector block consists of a 4-layered depth-of-interaction (DOI) detector. The ring diameter is 660 mm, and the distance between separated rings, which can be controlled, is 94 mm. Each DOI detector consists of 1024 GSOZ crystals which are arranged in four layers of 16 × 16 arrays, coupled to a 64-ch flat panel position sensitive photomultiplier tube. In order to increase scalability and maintain the possibility for future development of complicated coincidence algorithms to reject background signals, our designed DAQ system has no coincidence circuit and uses only singles list-mode DAQ circuits. The DAQ system makes a judgment on coincidence based on software, where the singles list-mode data include 6-bit energy markers and 500-ps tags for timing information. The system sensitivity measured from a 22Na point source was 4.4%. The average spatial resolutions were calculated as about 3.5 mm FWHM. For the singles count rate, the dead time at 250 MBq was about 8%. The singles-based DAQ system had significant performance in singles list-mode data acquisition. For in-beam PET, imaging performance of the WBDROP was confirmed through phantom experiments. Phantom study results with 11C and 10C beam irradiations of about 3 Gy showed that the beam stopping position in the target could be measured with a precision of better than 2 mm. The WBDROP promises high performance for not only conventional PET studies but also in-beam PET measurements.