Detector size and geometry optimization for the helmet-chin PET

Abstract

High sensitivity, high spatial resolution and low cost dedicated brain PET scanners are required for early diagnosis of Alzheimer's disease and brain function studies. As an optimal geometry, we proposed and developed the first prototype of the helmet-chin PET scanner. The scanner was constructed from 4-layer DOI detectors constructed from GSO crystals which were originally developed for our OpenPET. The helmet part of the helmet-chin PET consisted of three ring detectors with different radii arranged on a surface of a hemisphere and a top cover. In this study, for our next development, we optimized the size of the detectors to be arranged on the helmet, and compared two types of geometrical arrangements of the detectors on the hemisphere: a spherical arrangement in which the center of each detector faces toward the center of the hemisphere, and a multi-ring arrangement which has a similar detector arrangement to that of the first prototype. Geant4 simulation toolkit was used to model the scanners. The simulated scanners were constructed from LYSO crystals with a size of 1 × 1 × 5 mm3 (transaxial × axial × DOI). A dead-space of 2 mm was assumed in the axial and transaxial directions of the detector such as for wrapping. While fixing the number of the DOI layers to 4, the size of the detectors was varied by changing the number of crystals in the transaxial and axial directions. The results showed that, for the helmet detector, a detector with a size of 42 × 42 × 20 mm3 had the highest sensitivity evaluated using a hemispherical phantom. Then, the helmet-chin PET was modeled based on a detector whose size is 42 × 42 × 20 mm3 with the two geometrical arrangements and their sensitivities were compared. The respective sensitivities for hemispherical and multi-ring arrangements were 3.4% and 2.9% for a hemispherical phantom.