Reducing the effect of non-stationary resolution on activity quantitation with the frequency distance relationship in SPECT

The determination of quantitative parameters 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 non-stationary spatial resolution inherent in SPECT imaging compared to 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 the FDR (Frequency Distance Relationship) restoration filtering can reduce the impact of distance dependent spatial resolution on the quantitation of activity. An analytical projector which incorporates attenuation and distance dependent blurring was used to simulate small hot spherical objects in an cylindrical attenuator as imaged with a LEUHR collimator. FDR restoration filtering regularized using different Gaussian and parametric Wiener filters, was employed after attenuation correction with Bellini´s method. Projections were also processed using Bellini´s attenuation correction method followed by filtered backprojection and 3D Butterworth filtering with different cut-off frequencies. CCR´s (Center Count Ratios) and TCR´s (Total Count Ratios) were determined as the observed counts over true counts. Results show that after FDR restoration the CCR and TCR become approximately position invariant. However, when regularizing the FDR inverse filter with a Gaussian function, CCR´s become highly unstable as the standard deviation (σ) decreased below that at the center of rotation. The use of the Gaussian and parametric Wiener filters to regularize FDR filtering introduce noise in the CCR´s but improve recovery of TCR´s over that of the center of rotation