Activity quantitation in SPECT: a study of prereconstruction Metz filtering and use of the scatter degradation factor

A study of activity quantitation with prereconstruction Metz filtering and use of the scatter degradation factor (SDF) to numerically correct for scatter was conducted. The ratio of the count rate per unit activity for source locations within a 30 x 23-cm water-filled tub phantom to the count rate per unit activity for Tc-99m point sources of known activity imaged in air was used to judge the accuracy of activity determination. The investigation was conducted for certain locations within the tub when it was uniformly filled with Tc-99m activity, and for the same locations at the center of 5, 4, 3, and 2-cm diam, hot spheres imaged in a cold background. The source locations were the center, and one-fourth, one-half, and three-fourths the major axis. Various methods of combining the conjugate views for use with prereconstruction attenuation correction (arithmetic and geometric mean), and extent to which the Metz filter followed the inverse filter before rolling off to suppress noise were investigated. Without Metz filtering, attenuation correction was performed using a transmission curve that included buildup. With Metz filtering, the good-geometry attenuation coefficient was used and the combined views were scaled by the SDF calculated for the average body thickness. Depending on the size of the sphere and the extent to which the inverse filter was followed, Metz filtering combined with use of the SDF improved the accuracy of activity quantitation.