Optical Coherence Tomography: Technical Aspects - Biomedical Optical Imaging Technologies: Design and Applications - page 194

to provide information about the morphological changes by altering the amplitude,
phase, or polarization states of the probing beam. Exploiting these properties, thus
providing novel imaging contrast mechanisms, enhances the clinical and biomedical
applications for OCT to achieve functional imaging that can reveal more details
about the tissue dynamics and physiology. Some of the important functional modes
of OCT are polarization-sensitive OCT, Doppler OCT, spectrometric OCT, and
differential absorption OCT.

5.6.1 Polarization-Sensitive OCT

Most of the body tissues such as muscle, tendons, and nerve fibers contain collagen
and elastin, which exhibit birefringence when they form a specific structure.
Birefringence depicts a change in polarization state of light due to the refractive
index difference for light polarized in two orthogonal planes. The propagation of
light through such birefringence sample may alter the optical polarization state
of reflected light. Therefore, polarization-sensitive measurements at the output of
the interferometer can provide depth-correlated information about the birefringence
nature of the material or tissue specimen. As well as providing added contrast,
changes in birefringence may indicate changes in functionality or structure of
the tissue. Since the first report of functional polarization-sensitive OCT (PS-
OCT) system [ 89 ], a variety of PS-OCT configurations have been investigated
[ 90 - 95 ]. They all differ in complexity, capabilities, and signal processing schemes.
The most complete information about the polarization properties of a biological
specimen is given by a system capable of producing depth-resolved Muller matrix
elements. These configurations can account for the depolarization as well as the
changes in total, linear, and circular degree of polarization of the probe beam during
propagation in tissue.
The TD-OCT and FD-OCT configurations do not account for birefringence
within a sample, treating the electromagnetic wave as a scalar quantity. However,
light waves are transverse and, therefore, have extra degrees of freedom described
by the polarization state. The PS-OCT measurement apparatus is similar to that
of TD-OCT or FD-OCT, with the addition of a light polarizer after the source
and a polarizing beam splitter (PBS) with an extra detector in the output arm
(Fig. 5.3 ). Propagation of light through a sample may alter the optical polarization
state of the reflected light. This can occur due to optical scattering and birefringence
within the sample. Birefringence describes a change in the polarization state of
light due to the refractive index difference for light polarized in two orthogonal
planes. Therefore, polarization-sensitive measurement of the output interferogram
can resolve depth-correlated information about the birefringence of the sample
material. Mathematically, the two orthogonal polarization states can be treated
separately as two electromagnetic waves propagating in separate interferometers.