In the iterative reconstruction method, image noise tends to increase in proportion to falling available photon count and increasing update number. Image filtering is an important factor in single photon emission computed tomography (SPECT) image reconstruction, but it is frequently treated in a subjective way. The aim of this study was to evaluate the effects of pre-reconstruction filtering and post-reconstruction filtering on the iterative reconstruction process. Using simulation phantoms, projection data were reconstructed using ordered subsets expectation maximization (OSEM) with or without compensation for resolution recovery. Pre- and post-reconstruction filtering was performed using a Butterworth filter (BW) (range: 0.3-1.3 cycles/cm) and a Gaussian filter (GA) (range: 0.3-1.3 mm) with various parameters. We evaluated the variances of full width at half maximum (FWHM), coefficients of variation (CV), image contrast and normalized mean squared error (NMSE) values. The FWHM values for pre-reconstruction filtering tended to be lower than those observed for post-reconstruction filtering. These values were 5.1 mm (pre-reconstruction) and 6.7 mm (post-reconstruction). The CV on pre- and post-reconstruction filtering was 7.5% and 11.6%. Pre-reconstruction filtering reduced image noise more effectively than post-reconstruction filtering. The contrast for pre-reconstruction filtering was similar to that observed after post-reconstruction filtering. However, contrast after filtering with a GA slowly decreased as compared to the BW. NMSE values obtained by pre-reconstruction filtering tended to be lower than those observed for post-reconstruction filtering. Pre-reconstruction filtering provided SPECT image quality comparable to that from post-reconstruction filtering, especially when using the BW. Our results suggest that pre-reconstruction filtering is a beneficial method when applied to the iterative reconstruction method with or without compensation for resolution recovery.
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