Abstract
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Objectives The aim of this study is to exploit artifact-free overlaps in multi-pinhole (MPH) collimator design for a stationary small animal (SA) SPECT system to maximize the detection efficiency with minimal reconstructed image artifacts.
Methods We recently developed a theory that shows several categories of projection overlaps do not lead to artifacts in the reconstructed SPECT images. In this study, the type-II artifact-free overlaps were employed. The MPH collimator is a tube with 3 circular rows of pinholes and can be rotated in precise steps within the detector ring. The distribution pattern of the multiple pinholes on the collimator tube was designed such that with 4 projections from 4 rotational, type-II artifact-free overlaps were formed in the transaxial plan. A simple shutter, that could be opened or closed to block the entire middle row of pinholes or the entire 2 side rows of pinholes, is employed to vary the axial overlaps. This way,type-II artifact-free overlaps were also produced for the axial overlaps. Noise-free and noisy projections from a hot rod phantom and the MOBY phantom using the MPH collimator were simulated in an evaluation study. They were reconstructed using a MPH ML-EM algorithm and the normalized-mean-square-errors (NMSEs) between the reconstructed and the phantom image were calculated. The results were compared to that using a previously designed MPH collimator with 0.5 times the detection efficiency and minimum projection overlaps.
Results Results from the noise-free simulations show that the proposed MPH collimator achieved NMSEs comparable to that of the non-multiplexing MPH collimator in most situations. In one situation, the proposed design achieved significantly smaller NMSE. Simulations with noisy projections are ongoing.
Conclusions In conclusion, artifact-free overlaps can achieve NMSEs comparable to or even smaller than the non-multiplexing designs for noise-free projections. The increased detection efficiency could be used to improve SNR in noisy reconstructed images.