Out-of-plane photons in SPECT

Photons originating from planes adjacent to the slice of interest introduce considerable qualitative and quantitative error in single photon emission computerized tomography (SPECT) images. The extent of out-of-plane contributions was studied, and a method to compensate for these photons was 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 nonscatter and that out-of-plane scatter could compromise up to 16% of a projection pixel´s total count. Reconstructions were obtained by maximum-likelihood expectation-maximization methods, that accounted for the out-of-plane phenomena. These reconstructions gave an improvement in count quantification, noise, and contrast as compared to performing in-plane compensation alone. These findings required 3-D reconstructions with out-of-plane compensation in order to obtain reliable quantitative SPECT images