Broadband optical mammography: visualization, oximetry, and depth assessment of blood vasculature

We present diffuse optical mammography images that capitalize on the large optical absorption contrast (two orders of magnitude) between blood vessels and breast tissue, thus displaying breast vasculature. We have found a good correspondence between blood vessels displayed in the optical mammogram and those visible in the x-ray mammogram from the same subject in the same view (craniocaudal). By using broadband spectral information (wavelength range: 650-900 nm), we measured the hemoglobin saturation for the blood vessels displayed in the optical mammogram, for which we obtained an average value of 70%. In order to assess the z-axis depth of optical inhomogneities detected in this planar scanning approach, we have introduced pairs of detector optical fibers that are off-axis with respect to the illumination optical fiber. The spatial shift between the locations of the optical inhomogeneities in the two off-axis images can be translated into a depth measurement through a quasi-universal depth-shift reference curve. We report Monte Carlo simulations that show how this depth-shift reference curve is affected by the distance between the source and detector scanning planes (sample thickness) and to the specific arrangement of the two off-axis detectors. Liquid tissue-like phantoms were used to test this depth assessment approach for an absorbing rod placed at a depth of 33.6 mm. The depth measured with our method over the rod length ranged within 33-35 mm, in excellent agreement with the actual rod depth. The ability to identify blood vessels, measure their hemoglobin saturation, and assess their depth within breast tissue is a valuable feature that can advance optical mammography by providing additional structural and functional information about breast tissue.