Vrain: a brain-dedicated hemispherical PET system evolved from a lab bench prototype

[Objectives] A PET system can be more efficient in terms of size and cost when it is designed as an organ-dedicated or application-specific system. Considering the potential need for brain PET, we proposed the concept of a hemispherical detector arrangement, which enables higher sensitivity with fewer number of detectors than a conventional cylindrical geometry has. A laboratory benchtop prototype has been evolved into a complete brain PET system, which has been commercialized in Japan under the product name of VrainTM. In this work, we characterize the system and carry out its first human imaging tests. [Methods] The VrainTM has 54 detectors. A hemispherical part is formed by 45 detectors and the other 9 detectors are located at the backside of the neck. Each detector is a one-to-one combination of lutetium-based scintillators (crystal element size: 4.1×4.1×10 mm3) and silicon photomultipliers (SiPMs) (active area size: 4×4 mm2). The inner diameter of the bottom detector ring is 279 mm. The crystal thickness is optimized by simulation to reduce the parallax error and to improve the TOF performance. The energy window is also optimized to 450–590 keV to get better image quality. We evaluated the physical performance of the VrainTM according to the NEMA NU 2-2018 standards. Some measurements were added and others were modified to suit the hemispherical detector arrangement. Finally, we carried out the first healthy volunteer imaging. 18F-FDG of 3.7 MBq/kg was intravenously injected to a male subject (42 years old). The PET measurement duration was 10 min starting at 45 min after injection. PET images were reconstructed using the OSEM with 4 iterations and 8 subsets. A Gaussian filter of 4 mm FWHM was applied to PET images. CT images, which were obtained by another PET/CT scanner, were used for attenuation and scatter corrections. [Results] The 2.2 mm rods of a small rod phantom were resolved. The TOF resolution at the peak noise-equivalent count ratio (NECR) (9.8 kBq/mL) was 256 ps and the TOF gain in sensitivity was calculated as 5.2 (20 cm diameter / 3.8 cm localization accuracy). The effective sensitivity was 21.8 kcps/MBq and the effective peak NECR was 129 kcps at 9.8 kBq/mL. The sensitivity was higher at the upper position in the axial direction. For the brain-sized image quality phantom evaluation, the percent contrast of the 10 mm sphere was 48% with the percent background variability of 8.6%. For the human FDG PET imaging, the whole brain was successfully imaged without any artifacts. Gray and white matters were clearly visualized with a high contrast. [Conclusion] The helmet-type brain PET system VrainTM showed 2.2 mm spatial resolution, 256 ps TOF resolution, and excellent image quality. The brain FDG PET images of the healthy volunteer were high contrast and low noise. Further studies are warranted to assess the potential clinical impact of PET imaging with the VrainTM.
SNMMI 2022 Annual Meeting