Optimization of a High-resolution Collimator for a CdTe Detector: Monte Carlo Simulation Studies

Photon counting detectors using cadmium zinc telluride (CZT) or cadmium telluride (CdTe) have benefits compared to conventional scintillation detectors, and CZT and CdTe have advantageous physical characteristics for nuclear medicine imaging. Recently, many studies have been conducted using these materials to improve the sensitivity and the spatial resolution of the photon counting detector. By using a pixelated parallel-hole collimator, we may be able to improve the sensitivity and the spatial resolution. The purpose of this study was to optimize the design of a collimator to achieve excellent resolution and high sensitivity for a gamma camera system based on the CdTe detector. In this study we simulated a gamma camera system with a photon counting detector based on CdTe and evaluated the system's performance. We performed a simulation study of the PID 350 (Ajat Oy Ltd., Finland) CdTe detector by using a Geant4 Application for Tomographic Emission (GATE) simulation. This detector consists of small pixels (0.35 x 0.35 mm(2)). We designed two parallel-hole collimators with different shapes and verified their usefulness. One was the proposed pixelated parallel-hole collimator in which the hole size and the pixel size are the same, and the other was the hexagonal parallel-hole collimator, which had a hole size similar to that of the pixelated parallel-hole collimator. We evaluated the sensitivity, spatial resolution, and contrast resolution to determine which parallel-hole collimator was best for the PID 350 CdTe detector. The average sensitivity was 22.65% higher for the pixelated parallel-hole collimator than for the hexagonal parallel-hole collimator. Also, the pixelated parallel-hole collimator provided 10.7% better spatial resolution than the hexagonal parallel-hole collimator, and the contrast resolution was improved by 8.93%. These results reflect an improvement in sensitivity and spatial resolution, and indicate that the imaging performance of the pixelated parallel-hole collimator is better than that of the hexagonal parallel-hole collimator. In conclusion, we successfully established a high resolution gamma camera system with a pixelated parallel-hole collimator, and based on our results, we recommend using the pixelated parallel-hole collimator to improve the sensitivity and the spatial resolution of gamma camera systems with semiconductor detectors such as CdTe.