Out-of-plane photons in SPECT

This paper reports on photons originating from planes adjacent to the slice of interest, introduce considerable qualitative and quantitative error to Single Photon Emission Computerized Tomography (SPECT) images. Typically, images are reconstructed on a slice-by-slice basis and do not take into account out-of-plane radiation that may contribute to the projections. Scatter is a three dimensional (3-D) phenomenon, and full quantitative compensation requires a 3-D reconstruction. The extent of out-of-plane contributions and a method to compensate for these photons has been examined. Simulations using accurate Monte Carlo models were performed to acquire projection data. Analysis of these projections showed that as much as 12% of the total counts corresponding to a 1.8 cm thick slice of interest were due to out-of-plane non-scatter and that out-of-plane scatter could comprise up to 16% of a projection pixel's total count. Reconstructions were obtained by maximum-likelihood expectation maximization (MLEM) methods that accounted for the out-of-plane phenomena. These reconstructions gave an improvement in count quantitation, noise and contrast as compared to when only in-plane compensation was performed. These findings require 3-D reconstructions with out-of-plane compensation in order to obtain reliable quantitative SPECT images