Near infrared optical imaging and light propagation in highly scattering random media

This thesis presents a study of near-infrared optical Imaging and photon migration through highly scattering random media. Near-infrared (NIR) time-sliced and continuous-wave (CW) spectroscopic imaging techniques were developed and used to obtain in vitro images of excised human breast tissue specimens and to characterize inhomogeneities and locate objects inside highly scattering biological and model random media. Investigation of photon migration involved time-resolved studies of light propagating through various types of highly scattering biological and non-biological media.; NIR CW transillumination images of an object hidden inside biological media of different thickness were improved by preferentially selecting the image-bearing photons through the use of polarization gating and spatial filtering techniques. Through the use of a time-gated imaging approach to sort out the early photons, e.g. ballistic and snake components, the transillumination images of objects embedded in highly scattering diffusive media were improved. Higher image contrast was obtained using time-resolved as compared to CW imaging techniques.; A tunable chromium-doped forsterite laser system was used to explore the wavelengths that enhance image contrast of excised human breast tissue samples. The contrast of the images was improved by selecting appropriate wavelengths of light for better penetration and enhancement of the intrinsic properties of tissue constituents. In particular, the image contrast of human breast tissue samples showed strong wavelength dependence in the absorption band of fat, one of its main constituents. In the two-dimensional time-sliced transillumination imaging approach, an ultrafast electronic gated imaging system was used to select and record the image-bearing photons. Images recorded with different temporal slices of the transmitted light are found to selectively highlight different types of tissues, such as, adipose and fibrous or normal and cancerous in excised breast tissue specimens.; Time-resolved and CW light propagation in tissues with tubular structures were studied. Results show that transmission of linearly polarized light through tissues with tubular structures depends on the orientation of the structure of the sample with respect to the incident light, proving the existence of different paths for photon propagation.; Intensity temporal profiles at a fixed point in space as a function of angle of detection (arrival angle of light) were measured for ultrashort pulses of light propagating through highly scattering random media. It was found that there is a strong angular dependence of light traveling in a highly scattering media even for distances as large as 20 transport mean free path. To describe the high degree of anisotropy in the medium, two new anisotropy parameters are introduced.; The accuracy of the Non-Euclidean Diffusion (NED) equation to predict photon transport in a highly forward scattering medium was tested against experimentally measured temporal intensity profiles and compared to theoretical predictions given by the Diffusion Approximation (DA) in the pre-diffusive and diffusive regimes. It was found that the NED is more accurate than the DA, specifically in the pre-diffusive regime.