Low coherence interferometry in turbid media: The effect of multiply scattered light detection on image quality

Depth-resolved, diffraction-limited resolution imaging in turbid media like biological tissue requires detection of ballistic and rejection of out-of-focus, multiply scattered light. In optical microscopy a number of photon gating techniques based on a variety of physical characteristics inherent to single and multiply scattered light have been devised to suppress multiply scattered light detection. Though photon-gating techniques provide significant rejection of background light, they cannot exclude completely detection of multiply scattered light, the contribution of which to the measured intensity increases with imaging depth. Subsequent distortion of the microscope point-spread function is observed, that results in degradation of image quality and consequently limits image penetration depth.; This dissertation focuses on a particular optical imaging technique: Low Coherence Interferometry (LCI), which utilizes a combination of a spatial filter and a coherence gate to provide significant rejection of multiply scattered light at imaging depths up to 1.5 mm inside turbid tissues. The current research work examines the effect multiply scattered light detection has on the instrument point-spread function and the resulting loss of image contrast and resolution. Experimental and computational methods are used to investigate the effect of sample scattering anisotropy and numerical aperture of the imaging objective on deterioration of image quality.; The sensitivity of LCI to various sources of optical contrast imbedded in turbid media is discussed in this dissertation. The ability of LCI to resolve local changes in the optical properties of scattering media is investigated as a function of the type and the strength of the source of contrast relative to the optical properties of the scattering background. Practical limits to imaging inhomogeneities within turbid media with LCI are discussed.; A novel imaging modality, Dynamic Low Coherence Interferometry, designed for characterization of structural and dynamical properties of dynamic turbid media has been developed. This thesis describes in detail the principle of operation of DLCI, the instrumentation and the measurement procedure. Evaluation of the suitability of DLCI for characterization of particle dynamics in scattering media is also presented. DLCI has been used to investigate the transition between the single scattering and light diffusion regimes in dynamic turbid media.