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

Describes a method to estimate unattenuated projection images directly from Ga-67 SPECT data acquired over 360° at three photopeak energies. The algorithm compensates to third order for the effects of nonuniform attenuation, thereby yielding unattenuated projections with <3% accuracy, provided that scattered photons have been successfully subtracted from each energy window. Previously, the authors demonstrated an approach for using prior information to control noise in the estimated projections. They describe here an accelerated, robust algorithm for implementing these constraints. They have also evaluated the accuracy of the method for a region-of-interest (ROT) activity estimation task as a function of the residual error following scatter subtraction, and under conditions of nonstationary spatial resolution. When the maximum residual scatter subtraction errors at 93, 185, and 300 keV, respectively, were assumed to be 8%, 4%, and 2%, all ROI estimates were biased less than ±7% after attenuation compensation. Projection inconsistencies arising from distance-dependent geometric response and energy-dependent collimator penetration increased the standard deviation of ROI estimates (over location) to ~10%; however, methods of correcting for distance-dependent collimator response can probably be adapted well to Ga-67 imaging. If scatter can be adequately removed from the three Ga-67 photopeak windows, this technique is expected to provide reliable compensation for nonuniform attenuation with no need for an independently measured attenuation map