The Correlation Between Scatter Detector Performance and Spatial Resolution in a Ring-Shaped Compton Imaging System

Abstract

Whole gamma imaging (WGI) is a new concept that combines PET and Compton imaging by inserting a scatter detector ring into a PET ring. We developed a WGI prototype and demonstrated three-dimensional tomographic Compton imaging of a 89Zr-injected mouse. However, improving the scatter detector performance, i.e. crystal pitch and energy resolution, was required for high resolution Compton imaging. Therefore, we investigated the correlation in Geant4 simulations using 909 keV gamma rays emitted from 89Zr. Three WGI geometries consisting of the scatter detector ring with different GAGG crystal sizes (0.9 W 0.9 W 6 mm3, 1.45 W 1.45 W 6 mm3 and 2 W 2 W 6 mm3) were modeled. The energy resolutions were obtained experimentally for application to the simulation data. Also, the values were virtually halved and 1.5 times degraded. Simulations of a point-like source were carried out varying the pixel pitch in the scatter detector and images were reconstructed. For a source placed at the center of the system, a spatial resolution of 1.7, 1.9 and 2.3 mm was obtained with pixel pitches of 0.9, 1.45 and 2 mm, respectively. The spatial resolution of the system obtained with the 0.9 mm scatterer was improved to 1.4 mm for the halved energy resolution and it was deteriorated to 2.1 mm for the 1.5 times degraded energy resolution. In the hot-rod-like phantom simulation, 2 mm diameter rods were separated in the reconstructed images with the 0.9 mm scatterer. The separation became clearer for the halved energy resolution.