Spatial Resolution Improvement of a Gamma Camera with a Diverging Collimator Using a Tapered Crystal Array

Abstract

Diverging collimators are used to obtain reduced images of an object or detect a wide field-of-view (FOV) by using a small gamma camera. In gamma cameras equipped with diverging collimators, a monolithic scintillator, and a pixel scintillator array, gamma rays are obliquely incident on the scintillator surface when considering a source that is located near the periphery of the FOV. In such a situation, the spatial resolution is reduced due to oblique detection along the depth direction. In this study, we designed a novel system that improves the spatial resolution of the periphery of the FOV. A tapered crystal array is used to configure the scintillation pixels so that they coincide with the angle relative to the collator hole. This allows imaging of individual scintillation pixel locations, even if the events to be detected occur at different distances. That is, even if an event is detected at various points along the diagonal direction, the gamma rays interact with only a single crystal pixel, thus, the resolution is not degraded. The resolutions of the monolithic scintillator and the tapered crystal array were compared and evaluated using Geant4 Application for Tomographic Emission (GATE) simulations. The dimensions of the monolithic scintillator are 69.4 mm × 69.4 mm × 10.0 mm while those of the tapered crystal array are 49.4 mm × 49.4 mm at the small end and 69.4 mm × 69.4 mm at the large end, with a length of 20 mm. The tapered array contains 45 × 45 elements, and the pixel size is 1 mm × 1 mm at the small end and 1.4 mm × 1.4 mm at the large end. The diverging collimator has dimensions of 69.4 mm × 69.4 mm at the small end and 109.4 mm × 109.4 mm at the large end, with a length of 20 mm. The spatial resolutions of the obtained images were 3.22, 3.77, and 4.63 mm in the monolithic scintillator and 3.25, 3.29, and 3.12 mm in the tapered crystal array. A 16.8% spatial resolution improvement was seen with the tapered crystal array as compared to the monolithic scintillation crystal for all gamma source positions.