Determination of concave body outlines from SPECT projection data

The determination of the body outline from filtered back-projection or maximum likelihood reconstructions of cardiac SPECT emission data which have not been corrected for photon scatter and attenuation can be inaccurate in female patients because of high uptake of activity in the heart. Reconstructions without these corrections can misplace the activity from the heart beyond the true outline and into the space between the breasts. An accurate determination of the body outline, used as a prior in determining the attenuation map, has been shown to be helpful in reducing breast artifacts in cardiac polar maps. We have developed a method of automatically determining the body outline directly from the projection data before reconstruction. By reconstructing the emission data from the outside inward in polar coordinates, we avoid using those projector bins which observed the hot heart. The outer most, ring of activity is determined by using only the data from the inner most detector bin which can observe that ring. The next ring inward is calculated by using the estimate from the outer most ring and subtracting its contribution from the next ring inward. Thus, we can then treat this ring as if it were the outer most ring. Comparisons of boundary detection algorithms which use the zero crossing of the second derivative of the reconstructed image with this algorithm demonstrate a significant increase in sensitivity to concave body outlines in cardiac SPECT using this algorithm.