Reducing the effect of nonstationary resolution on activity quantitation with the frequency distance relationship in SPECT

The determination of quantitative estimates of activity such as the maximum and total counts in a volume are influenced by the partial volume effect. The magnitude of this effect varies with the nonstationary spatial resolution inherent in SPECT imaging, the size and shape of the object, and the relative concentration of the object to its background. The objective of this investigation was to determine if frequency distance relationship (FDR) restoration filtering can reduce the impact of distance-dependent spatial resolution on the quantitation of activity. An analytical projector was used to incorporate attenuation and distance-dependent blurring when simulating a set of small hot spherical objects in a cylindrical attenuator. Reconstruction was performed by filtered backprojection (FBP) with Bellini's method of attenuation correction. The simulations were filtered by: (1) three-dimensional (3-D) Butterworth filtering; (2) stationary parametric Wiener filtering; and (3) FDR restoration filtering regularized using Gaussian and parametric Wiener filters. The criteria used to compare the accuracy of quantitation were the center count ratios (CCRs) and total count ratios (TCRs). Both were determined as the ratio of observed counts over true counts. Results showed that after FDR restoration the TCRs varied less with location, and for both CCRs and TCRs the bias decreased compared to Butterworth filtering.