NEMA NU2-like performance evaluation of a helmet-type brain TOF-PET prototype

Objectives: Brain PET imaging plays important roles in many fields such as neurology, neuro-oncology, and molecular imaging research. To fulfill a growing demand for a high sensitivity, high spatial resolution and low-cost brain PET scanner, we have proposed a helmet-type PET scanner. In the last year, we reported the latest version of a prototype which had time-of-flight (TOF) measurement capability. In this work, we evaluated the performance of the helmet-type TOF-PET prototype basically using the NEMA standards. Methods: The helmet-type TOF-PET prototype was formed by 54 detector modules. The 45 detectors were arranged to form a hemisphere and the other 9 detectors were placed at the back of the neck. The detector module itself was composed of 12×12 lutetium fine silicate (LFS) scintillation crystals (4.1×4.1×10 mm3) connected to a 12×12 (4 mm pitch, 144 channels) multi-pixel photon counter (MPPC) array with a one-to-one coupling. After fine timing calibration, a 245-ps TOF timing resolution was obtained as a system average for all possible lines-of-response. We evaluated the performance such as spatial resolution, sensitivity and count rate characteristics (with TOF gain), and image quality based on the NEMA NU2 standard. Some measurements were partially modified to be applicable for the helmet-type PET prototype. In addition, the hemispherical 3D Hoffman phantom [Akamatsu G, et al. Biomed Phys Eng Express. In press] was measured for an image contrast evaluation. Results: FBP spatial resolution was 4.0 mm near the center of field-of-view. With a TOF sensitivity gain of 5.4 (= 20 cm diameter / 3.675 cm TOF localization accuracy), effective sensitivity and effective peak noise equivalent count rate (NECR) was 49.9 kcps/MBq and 560 kcps @ 15.4 kBq/mL, respectively. PET images of the brain-size image quality phantom and the hemispherical 3D Hoffman phantom showed a high image contrast. The PET image quality was clearly improved with TOF information. Conclusions: The helmet-type TOF-PET prototype demonstrated a promising performance for brain imaging. Even for a small detector ring diameter, it was shown that the 245-ps timing resolution has a high potential to significantly improve the performance of brain PET imaging.SNMMI Annual Meetin