Commercially available OCT systems measure the distribution of backscattered or reflected light intensity. As is well known from microscopy, intensity based imaging frequently suffers from a poor image contrast, making direct tissue differentiation difficult. On the other hand, some tissues can change other properties of the light, e.g., its polarization state, its spectrum, or its phase. Similar to microscopy, these phenomena can be used to enhance image contrast in OCT, and also to obtain quantitative measurements. In addition to these endogenous contrast mechanisms, exogenous contrast agents are being developed for OCT (in analogy to the familiar staining techniques used in microscopy).
It should be mentioned that there is an overlap of contrast enhanced imaging with functional imaging, e.g., the phase shift caused by moving blood cells, that is measured by Doppler OCT, can be used to measure the blood flow quantitatively, or to generate vessel contrast and display the microvasculature. Similarly, spectral changes measured by spectroscopic OCT can be used to generate absorption contrast, or to provide functional information on, e.g., blood oxygen saturation.
Since OCT is a rather new imaging modality, several of these techniques are still in their infancy, others, like PS-OCT, are further developed and have already been tested in several hundred patients. The following techniques of contrast enhanced imaging are currently under research and development at the Center for Medical Physics and Biomedical Engineering:
Polarization sensitive OCT
Doppler OCT (see "Functional Imaging")
Spectroscopic OCT (see "Functional Imaging")