A practical correction of scatter-related artifacts in SPECT reconstruction

We have observed that an expectation maximization (EM) algorithm applied to SPECT reconstruction may produce hotspot artifacts of varying intensity. Our hypothesis was that scatter caused these artifacts. To test this assumption, we studied the performance of forward- and back-projection procedures in the EM algorithm for simulated and experimental SPECT data. First, synthetic scatter-free projections and projections with only one scattered photon in each view were created for a simulated simple object, and reconstructed with a fully 3D ordered-subsets EM (OSEM) algorithm. Then, Monte Carlo simulated brain SPECT (with no scatter and with scatter present), a mini-Defrise phantom, and patient SPECT were reconstructed. We confirmed our hypothesis: hot-spot artifacts appeared only in the reconstruction from noisy projections but not in the reconstruction from scatter-free projections. We investigated a practical and simple method, critical path-length control (CPLC), for suppression of the hot-spot artifacts. To this end we performed reconstructions with or without CPLC and quantitatively evaluated the results including estimation of accuracy, bias, contrast-to-noise ratio, and uniformity. We found that the OSEM-with-CPLC method significantly reduced hot-spot artifacts, and yielded a similar or improved image quality. We conclude that the CPLC method provides a useful yet simple tool to reduce scatter-related hot-spot artifacts.