Performance characteristics of a high resolution oncologic transformable PET in brain/breast and whole-body modes

A high resolution transformable PET camera for human research has been developed. It can be transformed from whole-body mode (83 cm detector ring diameter, 13 cm axial FOV) to brain/breast mode (54 cm diameter, 21 cm axial FOV). This flexibility allows more optimal imaging for more dedicated human research studies, compared to commercial systems that are designed primarily for whole-body cancer imaging. The camera has 12 rectangular detector modules with gaps between modules which are small and remain constant for every configuration. Each module has a large detection area of 13 times21 cm² and 3,168 BGO detector crystals of 2.68 times 2.68 times 18 mm³ with built-in front-end electronics. The system has 38,016 BGO detectors and 924 PMT in total. The low-cost high-resolution PMT-quadrant-sharing (PQS) detector design was used for the detection system. A fast LED cross-reference calibration method is used to maintain the PMT gain balance. To improve the spatial sampling, the gantry rotates 30" with a fine step of 1"along with the high-speed electronics and detector modules. The whole system (detectors, electronics, mechanics, and software) was designed and fabricated inhouse. The performance characteristics are measured following the NEMA NU 2-2001 standard. 3D acquisitions were collected without interplane septa in the system. The sensitivity is 4.2% for whole-body mode and 9.2% for brain/breast mode with a Na-22 point source placed at FOV center. For brain/breast mode, the transaxial image resolutions at 0, 10 cm are 2.7 and 4.0 mm; while for whole-body mode, the resolutions are 3.3 and 3.9 mm. The slice thickness resolution is 2.6 to 3.3 mm in brain mode. DOI reduction technique is applied into the sinogram with the model- based Point Spread Function (PSF) obtained by Monte Carlo simulations to achieve high-definition (HD), high uniform PET images. The improved image resolutions are 2.9, 3.3 mm in whole-body mode at the location of 0, 10 cm o- ffset. A coincidence window of 13 ns collects 93% of the trues. The scatter fraction is 48% in brain mode. The maximum noise- equivalent count rate (NECR) is 34 Kcps at 4.6 KBq/cc for the brain mode and 20 Kcps at 5.6 KBq/cc for whole-body mode with the 70-cm NEMA NECR phantom. The convertible lead shield is being redesigned for higher NECR in whole-body mode.