Optical computed tomography in a turbid medium using early arriving photons

We employ photon migration to image absorbing objects embedded in a turbid medium. For improved resolution, we use early arriving photons (a few hundr ed picoseconds in excess of the time of flight), a regime in which the diff usion approximation breaks down. Our image reconstruction method is based o n extension of x-ray computed tomography (CT) to the optical regime. The CT algorithm must be generalized to take into account the distributions of ph oton paths. We express the point spread function (PSF) in terms of the Gree n's function for the transport equation. This PSF then provides weighting f unctions for use in a generalized series expansion method of x-ray CT. Expe riments were performed on a turbid medium with scattering and absorption pr operties similar to those of human breast tissue. Multiple absorbers were e mbedded into the medium to mimic tumors. Coaxial transmission scans were co llected in two projections, and the early-lime portions were analyzed. Thro ugh accurate modeling, we could remove the blurring associated with multipl e scattering and obtain high-resolution images. Our results show that the d iffusion approximation PSF is inadequate to describe the early arriving pho tons. A PSF incorporating causality is required to reconstruct accurate ima ges of turbid media. (C) 2000 society of Photo-Optical instrumentation Engi neers. [S1083-3668(00)01202-8].