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

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 3-D tomographic Compton imaging of a ⁸⁹ Zr-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 ⁸⁹ Zr. Three WGI geometries consisting of the scatter detector ring with different GAGG crystal sizes ( $0.9\times 0.9\times6$ mm ³ , $1.45\times 1.45\times6$ mm ³ , and $2\times 2\times6$ mm ³ ) 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.