Investigating the effects of energy resolution in dedicated emission mammotomography

This study probes the recent debate over the necessity for good energy resolution for uncompressed breast, 3D lesion imaging with dedicated single photon emission mammotomography. Here, the imaging system consists of a commercial, discretized CZT gamma camera having ~6% FWHM intrinsic energy resolution (at 140 keV) and intrinsic spatial resolution corresponding to the 2.5 mm square pixilation, and is used on a fully-3D positioning gantry. Wider energy windows are used on list mode acquired data as a surrogate for having otherwise identical detection systems with poorer energy resolution characteristics. Scans using simple circular trajectories are first obtained of an aqueous Tc-99m filled mini resolution cold-rod phantom at various radii-of-rotation, and also immersed in a larger uniform water bath. Multiple 3D orbits about Tc-99m filled anthropomorphic breast and torso phantoms are acquired, with the breast containing two large lesions. The list mode data files were multiply processed to obtain images of varying energy window widths (from symmetric 6% to an asymmetric 18% (-12+6)) but with the same projection image count density. Counts were randomly subsampled from the entire list mode data set in order to maintain equivalent levels of count density for several bootstrap realizations. All data was then reconstructed using OSEM for various iterations. Profiles were obtained from the cold rod images, and regions of interest were drawn in and about the spherical lesions to determine signal-to-noise ratios and contrasts for each iteration. Results clearly illustrate both visual and quantitative differences between the various energy windows, with smaller energy windows (corresponding to better energy resolution) having better image quality