Spillover compensation in the presence of respiratory motion embedded in SPECT perfusion data

The objective of the study was to determine what impact uncorrected respiratory motion has on spillover compensation of extra-cardiac activity in the right coronary artery (RCA) territory and if it is possible to use segmentation to define the extra-cardiac activity template. Two separate MCAT phantoms (128³ matrices) were simulated to represent patients with and without respiratory motion. Perfusion defects were simulated in a similar fashion. The SIMIND Monte Carlo package was used to generate 120 projections through 360 degrees into 64×64 matrices. Near noise free projections were simulated for the liver, heart and the lesion separately. For each phantom the heart was modeled 1) with a normal perfusion pattern, and 2) with an RCA defect equal to 50% of the normal myocardium count level. Poisson noise was added. All data were reconstructed using the rescaled block iterative (RBI) algorithm with 30 subsets and 5 iterations. The triple energy window (TEW) method was used during iterative reconstruction to correct for Compton scatter. A 3D Gaussian post-filter with a sigma of 0.75 pixels was used to suppress noise. Filtered emission slices were used to manually segment the liver to form 3D binary templates. These templates were projected using a ray-driven projector to simulate the imaging system with the exclusion of Compton scatter. In addition to these segmented templates, the true liver distribution (with and without respiratory motion included) was also used to generate sets of template projections. It was evident from the polar maps that the segmented templates were unable to remove all the spillover originating in the liver from the inferior wall. This is especially noticeable when the perfusion defect is present. The emerging combined SPECT/CT technology can play a vital role in identifying and segmenting extra-cardiac structures more reliably thereby facilitating spillover correction. This study also indicates that compensation for respiratory motion might play an important role in spillover compensation.