The 'ramp filter' artifact in filtered back projection is due to attenuation not ramp filtering

Artifactually decreased counts in regions of the left ventricle adjacent to intense splanchnic activity have been described in single photon emission computed tomography imaging and attributed to the side lobes of ramp filtering, a crucial step in filtered back projection. However, filtered back projection is based on a mathematical proof that makes no assumptions about the distribution of activity implying there is another cause for this artifact.

Computer simulations of a left ventricle with varying intensity, proximity and size of adjacent splanchnic activity was created with and without attenuation. Both filtered back projection and an iterative expectation maximization algorithm were performed. Volume weighted bullseyes were generated then the ratio of average activity in regions in the anterior and inferior walls of the bullseye were calculated.

In the absence of attenuation, there was no effect on uptake despite intense adjacent splanchnic activity. The effect of attenuation alone was mild but was greatly amplified by adjacent splanchnic uptake. This effect varied with the intensity, proximity and size of adjacent activity. Moving the attenuation to the apex of the left ventricle produced a paradoxical increase in counts to the inferior wall. The effect on iterative reconstruction was less pronounced than with filtered back projection and the paradoxical increase with attenuation at the apex did not occur.

The ramp filter artifact in single photon emission computed tomography reconstruction is due to attenuation, not ramp filtering per se. Adjacent splanchnic activity can result in either an increase or decrease in counts. These effects are less pronounced and more consistent with iterative reconstruction.