Frequency-domain methods in optical tomography: detection of localized absorbers and a backscattering reconstruction scheme

We studied the ability to detect small absorbing objects embedded in a highly scattering medium. Absorbing spheres of varying size, from 0.8 mm to 6.8 mm radius, submerged in a solution of highly scattering, low absorbing liquid: skim milk, were studied in a trans-illumination geometry. Groups of more than one sphere and a single circular disk, with radius identical to that of one of the spheres, were also studied. Single linear raster scans in the plane of the sphere, with the spheres centered between the source and detector, were made. Data was taken in the frequency-domain, yielding profiles of the objects in each of the three measurable quantities: dc intensity, phase, and modulation. The diffraction pattern form the sphere differed from that of the disk, demonstrating a volume effect associated with photon diffusion. The diffraction pattern of multiple spheres differed from that of single spheres