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.