Analysis of position-dependent Compton scatter in scintimammography with mild compression

During the past several years, we have been developing a breast scanning system that combines digital X-ray mammography with breast scintigraphy using a dedicated small field of view gamma camera. The relatively low uptake of ⁹⁹mTc-sestamibi in the breast compared to other organs such as the heart results in a large fraction of the detected events being Compton scattered gamma rays. In this study, our goal was to determine whether generalized conclusions regarding scatter-to-primary ratios at various locations within the breast image are possible, and if so, to use them to make explicit scatter corrections to the breast scintigrams. Energy spectra were obtained from patient scans for contiguous regions of interest (ROIs) centered left to right within the image of the breast, and extending from the chest wall edge of the image to the anterior edge. An anthropomorphic torso phantom with fillable internal organs and a compressed-shape breast containing water only was used to obtain realistic scatter-only spectra for each ROI. The measured patient energy spectrum was fitted with a linear combination of the scatter-only spectrum from the anthropomorphic phantom and the scatter-free spectrum from a point source. This procedure was repeated for each of the ROIs. We found that although there is a very strong dependence on location within the breast of the scatter-to-primary ratio, the spectra are well modeled by a linear combination of position-dependent scatter-only spectra and a position-independent scatter-free spectrum, resulting in a set of position-dependent correction factors. These correction factors can be used along with measured emission spectra from a given breast to correct for the Compton scatter in the scintigrams. However, the large variation among patients in the magnitude of the position-dependent scatter makes the success of universal correction approaches unlikely.