In this chapter, polarization-sensitive techniques for imaging and functional diagnosis of biological tissue are considered. Methods based on the polarization discrimination of a probe-polarized light that is scattered by, or transmitted through, a tissue or a cell structure are described. The advantages of polarization methods for tissue imaging and functional diagnostics are discussed. It is shown that polarization-spectral selection of scattered radiation used with the polarization-fluorescence method significantly improves the diagnostic potential of the method.
7.1 Polarization imaging
7.1.1 Transillumination polarization technique
The polarization discrimination of light transmitted through a multiply scattering medium may provide high-quality images of inhomogeneities embedded in the scattering medium. Principles of transillumination polarization diaphanography of a heterogeneous scattering object are described in the literature. This technique makes it possible to locate and to image absorbing objects hidden in a strongly scattering medium. The method uses modulation of the polarization azimuth of a linearly polarized laser beam and lock-in detection of polarization properties of light transmitted through the object. The scattering sample was probed by an Arion laser beam. The orientation of the polarization plane of the probe beam was modulated by a Pockel's cell as follows: during the first half-period of the modulating signal it was not changed, and during the second half-period it was rotated by 90 deg. Transmitted (depolarized) and forward-scattered (polarized) components of the probe light were collimated by two diaphragms and divided into two channels by a polarizing beamsplitter. It was found that in comparison with conventional diaphanography, polarization diaphanography allows one to get shadow images of a hidden object in a scattering medium that is characterized by up to approximately 30 scattering events on average.
A comparison of polarization and conventional transillumination imaging was carried out in Refs. 353â355 and 1139. The absorbing inhomogeneity, such as an absorbing plate placed in a scattering slab, was probed by a linearly polarized laser beam (Fig. 7.1). The shadow images were reconstructed from the profiles of the intensity and the degree of polarization P of the transmitted light (Fig. 7.2). Note that the dependencies of the degree of linear polarization on the edge position exhibit an increase in P in the vicinity of the edge.
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