SENSITIVITY STUDIES FOR IMAGING A SPHERICAL OBJECT EMBEDDED IN A SPHERICALLY SYMMETRICAL, 2-LAYER TURBID MEDIUM WITH PHOTON-DENSITY WAVES

We present analytic expressions for the amplitude and phase of photon- density waves in strongly scattering, spherically symmetric, two-layer media containing a spherical object. This layered structure is a crud e model of multilayered tissues whose absorption and scattering coeffi cients Lie within a range reported in the literature for most tissue t ypes. The embedded object simulates a pathology, such as a tumor. The normal-mode-series method is employed to solve the inhomogeneous Helmh oltz equation in spherical coordinates, with suitable boundary conditi ons. By comparing the total field at points in the outer layer at a fi xed distance from the origin when the object is present and when it is absent, we evaluate the potential sensitivity of an optical imaging s ystem to inhomogeneities in absorption and scattering. For four types of background media with different absorption and scattering propertie s, we determine the modulation frequency that achieves an optimal comp romise between signal-detection reliability and sensitivity to the pre sence of an object, the minimum detectable object radius, and the smal lest detectable change in the absorption and scattering coefficients f or a fixed object size. Our results indicate that (1) enhanced sensiti vity to the object is achieved when the outer layer is more absorbing or scattering than the inner-layer; (2) sensitivity to the object incr eases with the modulation frequency, except when the outer layer is th e more absorbing; (3) amplitude measurements are proportionally more s ensitive to a change in absorption, phase measurements are proportiona lly more sensitive to a change in scattering, and phase measurements e xhibit a much greater capacity for distinguishing an absorption pertur bation from a scattering perturbation. (C) 1996 Optical Society of Ame rica