Energy-based compensation for nonuniform attenuation in Ga-67SPECT imaging

We describe a method to estimate unattenuated projection images directly fr om Ga-67 SPECT data acquired over 360 degrees at three photopeak energies. The algorithm compensates to third order for the effects of nonuniform atte nuation, thereby yielding unattenuated projections with <3% accuracy, provi ded that scattered photons have been successfully subtracted from each ener gy window. Previously, we demonstrated an approach for using prior informat ion to control noise in the estimated projections. We describe here an acce lerated, robust algorithm for implementing these constraints. We have also evaluated the accuracy of the method for a region-of-interest (ROI) activit y estimation task as a function of the residual error following scatter sub traction, and under conditions of nonstationary spatial resolution. When the maximum residual scatter subtraction errors at 93, 185, and 300 ke V, respectively, were assumed to be 8%, 4%, and 2%, all ROI estimates were biased less than +/-7% after attenuation compensation. Projection inconsist encies arising from distance-dependent geometric response and energy-depend ent collimator penetration increased the standard deviation of ROI estimate s (over location) to similar to 10%; however, methods of correcting for dis tance-dependent collimator response can probably be adapted well to Ga-67 i maging. If scatter can be adequately removed from the three Ga-67 photopeak windows, this technique is expected to provide reliable compensation for n onuniform attenuation with no need for an independently measured attenuatio n map.