Quantification of a silver contrast agent in dual-energy breast x-ray imaging

Dual-energy (DE) breast x-ray imaging involves acquiring images using a lowand high-energy x-ray spectral pair. These images are then subtracted with a weighting factor that eliminates the soft-tissue signal variation present in the breast leaving only contrast that is attributed to an exogenous imaging agent. We have previously demonstrated the potential for silver (Ag) as a contrast material for DE breast imaging. Theoretical analysis shows that silver can provide better contrast to clinically-used iodine. Here, we present the subtraction method developed to eliminate the contrast between adipose and glandular tissue; the two major component materials in the breast. The weighting factor is calculated from the attenuation coefficients of the two tissue types and varies between values of 0 and 1 for the energy combinations studied. A spectral search was performed to identify the set of clinically-feasible imaging parameters that will optimize the contrast of silver using our subtraction technique. The subtraction methodology was tested experimentally using step-phantoms and demonstrated that we are able to a) nullify the soft-tissue contrast that arises from differences in glandularity, and b) preserve an image contrast for silver that is independent of the underlying soft-tissue composition. By applying the DE subtraction proposed, a silver-based agent will outperform an iodinated contrast agent on a commercially-available CEDE breast x-ray imaging system.