RESOLUTION LIMITS FOR OPTICAL TRANSILLUMINATION OF ABNORMALITIES DEEPLY EMBEDDED IN TISSUES

Random walk theory is used to calculate the line spread function (LSF) of photons as they cross the midplane of a slab of finite thickness. The relationship between the LSF and the photon transit time in transi llumination time-resolved experiments is investigated. It is found tha t the LSF is approximately Gaussian distributed, with a standard devia tion, sigma, which can be used as a criterion of the spatial resolutio n of the imaging system. Results are substantiated by comparison with actual data in the literature. Any given resolution can be improved by reducing the excess transit time Delta t, but heterogeneity of the sc attering medium and low levels of detected light enormously complicate the achievement of subcentimeter spatial resolution. The latter point is discussed by using optical parameters of breast tissues for visibl e and near-infrared radiation (NIR) light.