Optical Properties Of Tissue

OCT is analogous to conventional clinical ultrasound, except that the optical rather than the acoustic properties of tissues are measured. Like ultrasound, cross-sectional images of reflectivity in tissue are obtained that can differentiate internal tissue structure. Since light incident on and reflected from deeper tissue layers must pass through more superficial layers before detection, the reflections from internal tissue structures depend critically on the effects of overlying tissue. An understanding of the general concepts of light propagation through tissue is essential to develop a basis for interpretation of OCT images.

Light incident onto a scattering or turbid medium such as tissue is either transmitted, absorbed, or scattered. Absorbed light is converted into heat in the tissue and is effectively removed from the incident beam, Absorption occurs because tissue chromophores, such as hemoglobin, have internal energy transitions which match the energy of the incident light. Transmitted light remains unaffected, and is free to interact with deeper tissue layers. Scattering is a fundamental property of a heterogenous medium, and occurs because of microscopic spatial variations in the refractive index within tissue. A scattering event causes light to experience a random directional change. Light that completely reverses direction when scattering is known as backscattered, or reflected light.

Three parameters are typically used to statistically summarize the optical properties of a scattering medium, and to define the ensemble effects of absorption and scattering on the incident light beam [3-5]. The absorption coefficient, with units of inverse distance,

Figure 2-1 ♦ OCT image of a normal anterior eye chamber. Optical backscatter is visible through nominally opaque structures, such as sclera and iris, and from within transparent structures such as cornea and lens.

Figure 2-1 ♦ OCT image of a normal anterior eye chamber. Optical backscatter is visible through nominally opaque structures, such as sclera and iris, and from within transparent structures such as cornea and lens.

defines the fraction of incident light absorbed bv the tissue after the light propagates an infinitesimal distance through the tissue. In a purely absorptive medium, the intensity of the incident beam would decrease exponentially with distance, with a rate determined by the absorption coefficient. The scattering coefficient, also with units of inverse distance, similarly defines the fraction of incident light scattered by the tissue in an infinitely small distance. A purely scattering medium would correspondingly attenuate the incident light exponentially with increasing depth, with a rate dependent on the scattering coefficient.

The effects of absorption and scattering on the incident light beam are indistinguishable, so that transmitted light will be attenuated exponentially with distance into the tissue by a combination of both processes. In most tissues,, however, light scattering predominates over absorption, and at a given time, a scattering event is about ten times more likely than an absorption event [41, Therefore, most major differences in the attenuation of light propagating through different tissues derive from differences in tissue scattering, rather than absorption properties. Absorption only becomes an appreciable factor if a high concentration of a chromophore is present with a specificity that matches the wavelength of the incoming beam.

The scattering uni sot ropy factor describes in a statistical manner the probable direction of light scattering events. A medium with isotropic scattering, for example, is equally iikelv to scatter incident light into all possible directions. In contrast, the scattering events in most tissues are predominantly forward directed

The OCT system is in principle sensitive to two types of reflected light. The dominant contribution to the OCT signal is derived from light which propagates to a particular laver in the tissue without scattering or absorption, experiences a single scattering event in the backwards direction, and returns to the detector without further scattering or absorption- In contrast to this singly backscattered light, light that has undergone multiple scattering events, but no absorption, may also be detected if it happens to take a path which returns to the detector The optics of the OCT system make it improbable that multiple scattered light will travel on such a path, because the con focal and interferometric nature of the detection process places additional requirements on the temporal and spatial coherence of the detected light. Thus, for all practical purposes, the OCT signal may be considered to be exclusively comprised of light that has undergone just a single back-scattering event at the tissue layer of interest.

The strength of the OCT signal at a particular tissue layer is defined bv the amount of incident light which is transmitted without absorption or scattering to that laver, bv the proportion of this light which is directly backscattered, and bv the fraction of the di-

j j rectly backscattered light which returns to the detector without furtlier attenuation. The proportion of the incident light which is directly backscattered by a tissue structure defines the reflectivity of that structure. Thus, the OCT signal from a particular tissue layer is a combination of its reflectivity and the absorption and scattering properties of the overlying layers. The internal reflectivity of a homogenous tissue layer depends on both the Liver s scattering coefficient and its scattering anisotropy. A highly reflective tissue will have both a high scattering coefficient and a strong

Cbrneat Epithelium

Stroma

Corneal Endothelium

Log Reflection

I igure 2-2. Narrow field-of-view OCT image of a healthy cornea showing the contrast in optical reflectivity between the corneal epithelium, stroma, and endothelium,

Corneoscleral Limbus

Aqueous

Iris Pigment Epithelium

Log Reflection

Figure 2-3- Narrow field-of-view OCT image of a healthy anterior chamber angle.

disposition to scatter light in the perfectly backwards direction. Strong reflections a 1st) occur at the boundaries between two medias, for example, between two materials of different refractive indices.

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