Photon migration theory applied to 3D optical imaging of tissue

Noninvasive optical spectroscopic imaging of tissue has considerable potential for clinical screening and monitoring metabolic status, if accurate tomographic reconstruction techniques are developed. The rationale for specific diagnosis is based on different spectroscopic signatures of intrinsic tissue or specific exogenous labels which can distinguish between normal and abnormal tissue types. However, several critical technological elements are required for clinically useful imaging. These include obtaining a balance between attenuation of the detected light and resolution, and achieving specificity which is crucial for a purely noninvasive, definitive diagnosis. The authors use random walk theory to address these interrelated elements by calculating a time-dependent contrast function to describe photon paths in time-resolved transillumination detection of abnormally absorbing and/or scattering regions in tissue. The contrast function depends on the optical properties of the normal tissue as well as the optical properties, size and location of the abnormal target. Results of the theory are applied to estimate the absorption and the scattering coefficients of a cylinder embedded in a tissue-like phantom. Although the authors were able to detect the presence of the abnormal target and quantify its optical properties, the specificity of such quantitation is yet to be demonstrated. In fact, differences in scattering properties between normal and abnormal tissue can be rather nonspecific. Moreover, at near infra-red wavelengths in which the optical attenuation in tissue is rather small differences in absorptivity between normal and diseased tissue can be quite small. This implies poor contrast and therefore poor resolvability of a target from other tissue components.