Segmentation of the body and lungs from Compton scatter and photopeak window data in SPECT: a Monte Carlo investigation

In SPECT imaging of the chest, non-uniform attenuation correction requires use of a patient specific attenuation map. Such a map can be obtained by estimating the regions occupied by (1) the lungs and (2) the soft tissue and bones, and then assigning values of the attenuation coefficient to each region. The authors propose a method to segment such regions from the Compton scatter and photopeak window SPECT slices of Tc-99m Sestamibi studies. The Compton scatter slices are used to segment the body outline, and to estimate the region of the lungs with the anatomic information on the back bone and sternum locations from the photopeak window slices. To investigate the accuracy of using Compton scatter slices in estimating the regions of the body and the lungs, a Monte Carlo SPECT simulation of an anthropomorphic phantom with an activity distribution and noise characteristics similar to patient data was performed. Different activities were simulated in the lungs to study the influence of lung uptake. Energy windows of various widths were simulated for use in locating a suitable Compton scatter window for imaging. In a separate simulation, the map of the probability of scatter interactions (up to third order) from photons originating at a point within the heart was recorded to allow investigation of the contrast provided by the difference in density between the lungs and surrounding bones and soft tissue. The results demonstrated that (1) sufficient contrast can be derived from Compton scatter data for segmentation of the lungs; (2) accuracy of determination of body and lung regions of about 99% and 89%, respectively, can be achieved and (3) a wide energy window away from the photopeak window for recording the scattered events is preferred for the segmentation of lungs