Optical Coherence Tomography
Optical coherence tomography (OCT) is an optical analogue to ultrasound imaging, using infrared light instead of sound. The much higher speed of light compared with sound allows for finer resolution, but direct electronic measurement of the shorter “echo” times it takes light to travel from different structures at axial distances within the eye is not feasible. Interferometry enables us to overcome this difficulty in the following manner. Light is split into 2 beams, and the beam backscattered from the ocular tissue is then compared (interfered) with the beam that has traveled a known time from the reference mirror. Interference patterns are observable when the optical distances traveled by the 2 beams match to within the coherence length of the light. In OCT, broadband (ie, low coherent) light sources are used (eg, a superluminescent diode emitting a beam of light with long [red] wavelengths—reds being chosen because they are scattered in tissue less than is blue light), rather than narrow-band (ie, high coherent) light sources, such as a laser, because it gives the instrument greater sensitivity to the differences in time the 2 beams have traveled (see Video 2-2).
In time-domain OCT (TD-OCT; Fig 8-24), the position of the mirror is adjusted so that interference patterns show up, as a function of time, whenever the 2 beams have traveled almost the same amount of time. Results similar to the ultrasound’s A-scan are generated, as light is reflected at interfaces between layers of tissue. Cross-sectional images are generated by performing successive A-scans at different transverse positions on the retina or cornea, yielding 2-dimensional results like an ultrasound’s B-scan (see BCSC Section 12, Retina and Vitreous and BCSC Section 8, External Disease and Cornea). By acquiring sequential cross-sectional images, 3-dimensional, volumetric results are obtained.
Figure 8-24 Optical coherence tomography based on the principle of low-coherence interferometry.
(Courtesy of Neal H. Atebara, MD. Redrawn by C. H. Wooley.)
In Fourier-domain OCT (FD-OCT), also called spectral-domain OCT or frequency-domain OCT, the reference beam mirror is fixed at one position. Interference fringe patterns, all mixed together, arise from the various tissue interfaces, but Fourier analysis enables them to be dissected apart. When the pattern arises from closer tissue interfaces, the fringe patterns’ undulations are spaced farther apart than those arising from deeper tissue planes, which yield fringes spaced more closely together. The more highly reflective tissue plane interfaces yield higher-amplitude fringe patterns. Thus, the spacing of the fringe pattern tells us how deep in the tissue it comes from, and its amplitude tells us how much the light is reflected by that tissue plane interface. In this manner, the A-scan of all the depths is obtained instantly without moving the reference mirror. Scanning across the retina (or cornea) yields 2-and 3-dimensional images. Thus, FD-OCT is much more efficient than TD-OCT, resulting in both greater speed and higher signal-to-noise ratio, and therefore higher-resolution images.
There are 2 implementations of FD-OCT: spectrometer-based or swept-source implementation. A spectrometer-based version of FD-OCT uses a prism or grating in front of the detector to separate light into its spectral components, whereas a swept-source version of the FD-OCT replaces the superluminescent diode’s band of frequencies with a tunable laser. The laser sequentially sweeps through different frequencies, one at a time, and the A-scan is performed for each frequency at each location. Unlike spectrometer-based FD-OCT systems, swept-source-based systems are not limited by spectrometer resolutions and thereby support larger axial depth measurement ranges. Also, commonly available swept sources have a wavelength centered at about 1 μm, thereby enabling imaging deeper into the tissue. This allows us to see the vitreous, retina, and choroid more easily in a single image.
Excerpted from BCSC 2020-2021 series : Section 3 - Clinical Optics. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.