OPTIMUM TOMOGRAPHIC RECONSTRUCTION PARAMETERS FOR HMPAO BRAIN SPET IMAGING - A PRACTICAL APPROACH BASED ON SUBJECTIVE AND OBJECTIVE INDEXES

The purpose of this study was to define an optimal strategy for the to mographic reconstruction procedure in routine brain single-photon emis sion tomography (SPET) studies, including the number of projections, f ilter function and matrix size. A set of projection data with differen t count densities was obtained from a technetium-99m hexamethylpropyle ne amine oxime (Tc-99m-HMPAO) brain SPET acquisition from one voluntee r. The projections were reconstructed with different filters and the q uality of the reconstructed images was determined using both a subject ive observer rating score and the Gilbert index. For each count densit y, the observers' choice corresponded to images with the lowest Gilber t index. The noise level in brain SPET sections was estimated and corr elated with the fractal dimension. The results of this study indicate that although noise represents a fundamental component of brain SPET i maging, image quality also depends on the reconstructed spatial resolu tion. Image quality is satisfactorily described by fractal dimension. In addition the optimal filter function depends on the available count density. For high count levels, optimal reconstruction may be obtaine d by using a high-resolution matrix and a slightly smoother reconstruc tion filter. When count densities are low, best results are obtained b y using a low-resolution matrix and a sharper filter. Finally, this st udy suggests that image quality is not influenced by the number of pro jections for equivalent count densities. These results were confirmed by 30 HMPAO brain SPET studies acquired in a routine clinical setting.