Restoration of combined conjugate images in SPECT: comparison of a new Wiener filter and the image-dependent Metz filter

Two image-dependent restoration filters were applied to projection image sets obtained with single-photon-emission computed tomography (SPECT). Wiener and Metz restorations of combined conjugate views are compared to each other, Wiener restoration of individual projection images, and one-dimensional Butterworth smoothing. The combined view restoration filters adapt to the average thickness of the object by estimating a modulation transfer function (MTF) for that thickness. Simulated Tc-99m liver-spleen studies with randomly placed cold spot tumors, a projector which accounts for the spatially variant blurring in SPECT, and a Poisson noise generator are used to compute simulated projection image sets. These sets are filtered and reconstructed using the method of intrinsic attenuation correction of Tanaka et al. (1984). Cold spot contrast, automated receiver operator characteristic (ROC) analysis of cold spot detectability, and the normalized mean squared error (NMSE) are used to compare the four processing methods. Little difference is noted between the three restoration methods. However, the restoration filters all yield noticeably better ROC and NMSE results than the one-dimensional smoothing. Inspection of the three-dimensional MTF derived from reconstructions of point sources indicates that the restoration filters substantially reduce the low-frequency degradation which is primarily due to scatter