NATURE OF RINGING ARTIFACTS IN SPECT RECONSTRUCTION AND THEIR REDUCTION BY THE USE OF SIDE INFORMATION
ML-EM iterative reconstruction algorithms tend towards ringing artifacts, and high noise, after a large number of iterations just as filtered backprojection algorithms tend toward streak artifacts. OS-EM is a type of ML-EM algorithm and an edge overshoot has been observed experimentally for uniform-activity spheres of large size but not smaller size. Herein, we shed light on such artifacts by showing that they are a function of the object size relative to the FWHM of the collimator-detector response function. That is, for a given number of iterations, an over- or undershoot can occur at a given spatial location or not depending on the value of the ratio. We further show that a new, regularized, penalized-likelihood algorithm based on ordered subsets and employing side information stabilizes the iterative procedure against the development of artifacts