D6 OCT: Optical Coherence Tomography

Topic

Optical coherence tomography splits a single low-coherence light beam into two distinct paths: a reference beam, which travels a fixed, known distance before reflecting back, and a sample beam, which is directed toward the eye and reflects off the various structures it encounters at different depths. When the reference beam and the sample beam recombine, they produce an interference pattern that appears clearly only when the distance traveled by the sample beam—before reflecting off a specific structure—matches, almost exactly, the fixed distance traveled by the reference beam.

Because optical coherence tomography knows the exact distance traveled by the reference beam beforehand, it can calculate—based on the moment the interference pattern appears—the precise depth of each eye structure that reflected the sample beam to produce that pattern. This method of calculating depth via interference, rather than by focusing a conventional optical image, enables optical coherence tomography to distinguish structures separated by an axial distance of just five to seven microns—an axial resolution far finer than that achievable by any conventional optical image of the same eye.

To produce this interference pattern and calculate the depth of each structure, spectral-domain optical coherence tomography captures the entire wavelength spectrum of the interference pattern at once using a spectrometer, whereas swept-source optical coherence tomography scans through those same wavelengths one by one, at high speed, using a laser that continuously changes its wavelength. By utilizing longer wavelengths than those used in spectral-domain systems, swept-source optical coherence tomography achieves greater penetration depth within the eye, clearly visualizing deeper structures that spectral-domain optical coherence tomography sometimes fails to resolve with the same sharpness.

Leveraging this same capability to measure depth via interference, optical coherence tomography can generate cross-sectional images of various parts of the eye depending on the direction of the sample beam: toward the retina, distinguishing individual retinal layers with the system's axial resolution of a few microns; toward the optic nerve, measuring fiber thickness layer by layer; or toward the anterior segment, measuring central corneal thickness and anterior chamber depth with that same axial resolution.

Because it can measure edema and fluid accumulation within the retinal layers of the macula with such precision, optical coherence tomography is the standard for monitoring age-related macular degeneration and diabetic macular edema, directly revealing whether exudation and edema persist between retinal layers. Its ability to measure optic nerve fiber thickness layer by layer also makes it the standard for glaucoma monitoring, complementing the sensitivity map provided by perimetry with a direct structural measurement of the optic nerve itself. Finally, because it measures central corneal thickness and anterior chamber depth with the same fine axial resolution, this type of optical coherence tomography is also useful for surgical planning—ranging from refractive surgery to lens replacement with an intraocular lens—providing precise measurements that previously required separate instruments for each ocular structure.

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