On the Iterative Image Space Reconstruction Algorthm for ECT

The ISRA of [1] is shown to be an iterative algorithm that aims to converge to the least-squares estimates of emission densities. Convergence is established in the case where a unique least-squares estimate exists that is, elementwise, strictly positive. It is pointed out that, in terms of asymptotic theory, the resulting estimators are inferior to the maximum likelihood estimators, for which the EM algorithm is a computational procedure. Potential difficulties with the behavior of the ISRA are illustrated using very simple examples.