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Chapter 9:
Optical Coherence Tomography and Heterodyning Imaging
9.1 OCT 9.1.1 Introduction A description of the fundamentals and basic principles of optical coherence tomography (OCT) can be found in Section 4.5. In Section 2.6, OCT is discussed as a method for tissue optical properties' measurements. Methods and data of measured absorption and scattering coefficients and refractive index are presented (see Table 2.1). The enhancement of OCT penetration depth and image contrast owing to the action of hyperosmotic optical clearing agents is given in Section 5.5.5 for skin, in Section 5.6.2 for gastric tissues, and in Section 5.8.2 for blood samples; and in Section 5.9.1, OCT glucose sensing is discussed. In this section, we will briefly give an overview of some typical OCT schemes and illustrate their biomedical applications. 9.1.2 Conventional (time-domain) OCT A large number of schemes of coherent optical tomography for the investigation of tissues have been described in the literature, with overviews given. Figure 9.1 presents one of the typical time-domain tomographic schemes based on a superluminescent diode (SLD) (λ = 830 nm, Δλ = 30 nm) and a single-mode fiber-optic Michelson interferometer. The power of IR radiation on tissue surface is about 30 μW. The interference signal at the Doppler frequency, which is determined by the scanning rate of a mirror in the reference arm [see Eq. (4.53)], is proportional to the coefficient of reflection of the nonscattered component from an optical inhomogeneity inside the tissue. One can localize an inhomogeneity in the longitudinal direction by equalizing the lengths of the signal and reference arms of the interferometer within the limits of the coherence length of the light source (~10 μm) [see Eq. (4.54)].
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