Comparison of a newly-designed stack-up collimator with conventional parallel-hole collimators in pre-clinical CZT gamma camera systems: a Monte Carlo simulation study

Abstract

Recently, many studies have been conducted with pixelated semiconductor detectors that use cadmium zinc telluride (CZT) because these detectors have many advantages in pre-clinical gamma imaging. Collimators play an extremely important role in the imaging performance of pixelated semiconductor gamma cameras. In our previous study, based on the pixelated semiconductor gamma camera system we recommended the use of a pixelated parallel-hole collimator with equal hole and pixel sizes; this approach improved both the sensitivity and the spatial resolution. However, the pixelated parallel-hole collimator had two major limitations: (a) Although its sensitivity was higher than that of pinhole systems, the pixelated parallel-hole collimator may have still resulted in a partial loss of sensitivity due to its small collimator hole size. (b) The pixelated parallel-hole collimator with an adequate septal height was difficult to manufacture due to its small holes. Here, we present a new design for a parallel-hole collimator, which uses the stack-up method and a CZT pixelated semiconductor gamma camera system. The purpose of this study was to compare the performances of various geometric designs of our newly-designed parallel-hole collimator with those of conventional parallel-hole collimators [low-energy high-resolution (LEHR) and low-energy high-sensitivity (LEHS)]. The detector was modeled as an eValuator-2500 (eV Microelectronics Inc., Saxonburg, PA, USA) (3-mm thick, 0.5-mm pixel size) by using a Geant4 Application for Tomographic Emission (GATE) simulation. The proposed parallel-hole collimator consisted of two overlapping parallel-hole collimators. The size of each hole in the proposed parallel-hole collimator was four times that of the hole in the pixelated parallel-hole collimator. The overlap ratios of these collimators were 1 : 1, 1 : 2, 2 : 1, 1 : 5, and 5 : 1. To evaluate and compare the performances of these systems, we evaluated the sensitivity and the spatial resolution of each system. The average sensitivities of the proposed parallel-hole collimators with overlap ratios of 1 : 1, 1 : 2 or 2 : 1, and 1 : 5 or 5 : 1 and of the LEHS conventional parallel-hole collimator were 2.17, 3.45, 13.90, and 3.68 times higher than the average sensitivities of the LEHR conventional parallel-hole collimator, respectively. The average spatial resolution also varied depending on the distance from the collimator. In conclusion, we successfully designed a novel parallel-hole collimator for pre-clinical imaging; this novel collimator employs the stack-up method with a CZT pixelated semiconductor gamma camera.