A Simulation Study Comparing Different Pixel Sizes of CZT Detectors Combined with Pitch-Matched Collimators for SPECT Imaging

Pixelated direct-conversion detectors are poised to replace scintillators in single photon emission computed tomography (SPECT) cameras with pixel-matching parallel-hole collimators, to compensate for lost resolution while yielding higher sensitivity. However, smaller pixel sizes to improve resolution leads to increased inter-crystal signal sharing, resulting in a drop in sensitivity. It is, therefore, evident that the pixel pitch of the detector plays a prime role in tuning the trade-off between sensitivity and resolution. Our objective in this simulation study is to determine an acceptable combination of detector pixel and collimator hole pitch for improved performance of the SPECT gamma camera in detecting small lesions. Using the Geant4 Monte Carlo toolkit, a family of CZT detectors, with varying pixel size, mounted with corresponding parallel-hole pitch-matched collimators were simulated. For the entire study, the septal thickness of the collimators was fixed at 0.16 mm keeping in line with the Siemens LEHR collimator specifications, and the ratio of hole length to hole pitch was maintained at 16 to obtain approximately 7 mm planar collimator resolution when the source-to-collimator distance is 10 cm. From the simulations, the figures of merit such as sensitivity, resolution, signal sharing fraction (SSF) have been compared for pixel size ranging from 1 to 2 mm in steps of 0.2 mm. Recovery ratio calculated from a hot rod phantom filled with Tc-99m helped ascertain the optimum pixel design. Based on observations from the Monte Carlo simulations, we concluded that smaller pixels do not necessarily imply improved performance since they are more susceptible to lateral signal sharing. Pitch-matched collimators with larger holes exhibited higher detection efficiency but with deteriorated resolution, supporting our conclusion. The best recovery ratio was attained at 1.4 mm pixel pitch with matching collimator pitch and 19.84 mm hole length.