Head-to-head comparison of performance characteristics between two helmet-type PET prototypes

[Purpose] Brain PET imaging is a powerful tool to visualize various functions and metabolism of the human brain. We proposed a helmet-shape detector arrangement for increasing sensitivity. For proof-of-concept, we developed the first prototype, which had depth-of-interaction (DOI) detectors without time-of-flight (TOF) capability. Based on the experience of the first prototyping, we have developed the second prototype which had TOF capability of a 245-ps timing resolution without DOI capability. The detector was composed of 12×12 lutetium fine silicate (LFS) scintillation crystals (4×4×10 mm3) connected to a 12×12 silicon photomultiplier (SiPM) array with one-to-one coupling. The crystal thickness was optimized through simulation so as to reduce the parallax error while keeping the gain in image quality. In this work, we evaluated and compared the performance characteristics of the two helmet-type PET prototypes based on standard procedures.[Methods] We evaluated spatial resolution, sensitivity, count rate characteristics and image quality based on the NEMA NU-2 standards. Some measurements were partially modified to be applicable for the helmet-type PET. [Results] The values measured with the first (DOI) and second (TOF) prototypes were: spatial resolution of 2.8 and 3.1 mm at the central position (1-cm offset) by the FBP reconstruction; sensitivities of 13.4 and 4.1 kcps/MBq; scatter fraction at the low activity of 31% and 31%; and peak noise-equivalent count rates (NECR) of 20.0 kcps at 2.6 kBq/mL and 21.1 kcps at 9.0 kBq/mL. With a TOF sensitivity gain of 5.4 (= 20-cm-diameter / 3.675 cm TOF localization), effective sensitivity and effective peak NECR were 22.3 kcps/MBq and 115 kcps at 9.0 kBq/mL for the second prototype. In the image quality assessment, the second prototype outperformed the first prototype. [Conclusion] Although the first prototype had better performance for intrinsic sensitivity, the second prototype showed higher effective sensitivity, higher effective count rate performance and superior image quality. The fast timing properties of the detector configuration (10-mm-thick LFS and SiPM with one-to-one coupling) and its TOF capability significantly improved the imaging performance.第119回日本医学物理学会学術大