Just as different stains provide additional contrast in traditional histology, various functional extensions of optical coherence tomography (OCT) can enhance visualization of properties and features that are not immediately apparent in traditional OCT. By detecting not only the intensity, but also the polarization state of light back-reflected from a sample, polarization-sensitive OCT (PS-OCT) yields depth-resolved information on any light polarization changing properties of the sample. Birefringence, the main polarization property of interest for biological tissues, is caused by a difference in index of refraction that results in a phase retardation between orthogonal polarization states, and is often exhibited by fibrous structures such as collagen or the retinal nerve fiber layer (RNFL). The effects of diattenuation, which is marked by a differential absorption for orthogonal polarization states, are generally negligible compared to those due to birefringence.
In-vivo application of this technique often necessitates optical fiber-based implementation of a PS-OCT system. The inherent and unknown light-polarization changing properties of these components complicates determination of similar properties from a sample; any polarization state changes must be decomposed into contributions due to the system effects and those that arise due to the sample. We have developed a number of algorithms to solve this problem for fiber-based PS-OCT, including a computationally efficient method based on the evolution of Stokes vectors in a Poincaré sphere representation that determines the amount and optic axis for sample birefringence that has been implemented in clinical systems, and a more comprehensive Jones matrix-based solution that determines the amounts and common optic axis for both birefringence and diattenuation simultaneously for systems with the unrestricted use of optical fiber and fiber components.
Figure 1: Information from pairs of depth profiles acquired with two different incident polarization states is used to calculate the polarization properties of a sample. The polarization effects of the system can be calibrated out to determine the amount and optic axis for birefringence by comparing the states reflected from the surface of the tissue ( I 1 and I 2) to those reflected from some depth in the sample ( I 1' and I 2') in a Poincaré sphere representation. The amount of rotation in the Poincaré sphere can be denoted on a 256-color grayscale.
Figure 2: Intensity (left) and phase retardation (right) images of skin overlying the metacarpal phalangeal joint of the index finger (5 mm wide x 1.2 mm deep). The epidermal and dermal layers are visible, covering the extensor tendon. The birefringence of the tendon can be quantified from the banded pattern in the phase retardation image.
This technique has been applied to a wide variety of clinical problems, including ophthalmology for possible early diagnosis of glaucoma.