An iterative transmission algorithm incorporating cross-talk correction for SPECT

Simultaneous emission/transmission acquisitions in cardiac SPECT with a Tc/Gd source combination offer the capability for non-uniform attenuation correction. However, cross-talk of Tc-99m photons down-scattered into the Gd-153 energy window contaminates the reconstructed transmission map used for attenuation correction. The estimated cross-talk contribution can be removed prior to transmission reconstruction or incorporated in the reconstruction algorithm itself. Here, the authors propose an iterative transmission algorithm (MLTG-S) based on the maximum-likelihood gradient algorithm (MLTG) that explicitly accounts for this cross-talk estimate. Clinical images were acquired on a three-headed SPECT camera, acquiring Tc-99m emission and Gd-153 transmission images simultaneously. Subtracting the cross-talk estimate prior to transmission reconstruction with MLTG causes the number of zeros to increase in the corrected projections due to increased attenuator size or enhanced cross-talk. This results in inaccurate attenuation coefficients. MLTG-S reconstructions on the other hand, yield better estimates of attenuation maps, by avoiding the subtraction of the cross-talk estimate. Comparison of emission slices corrected for nonuniform attenuation also reveal that inaccuracies in the reconstructed attenuation map can artificially enhance the extracardiac activity, confounding one´s ability to visualize the left-ventricular walls