Optical Coherence Tomography
Optical coherence tomography (OCT) is a noninvasive, noncontact imaging modality that produces micrometer-resolution images of tissue. Low-coherence light is directed into tissue and into a reference arm. An interferometer combines the light returning from the tissue with the light from the reference arm, producing an interferogram. The benefit of using low-coherence light is that its spectral makeup changes rapidly with time; thus, light produced at a particular instant will not interfere well with light produced at other times. This means the exact position from which the interfering light came can be determined by the resolution dictated by the coherence length of the light source, which is typically 5–7 μm. In any given A-scan, time-domain OCT is used to interrogate each point in the tissue sequentially. In more modern techniques, for example either spectral-domain (SD) or swept-source (SS) OCT, a more efficient approach is taken. In SD-OCT, a broad-spectrum light source is used, and the resulting interferogram produced varies with the reflectivity of the tissue. SS-OCT uses a more complicated light source that sequentially scans through successive wavelengths of light across a spectral range (see Optical Coherence Tomography Technologies: A Comparison of SD-OCT and SS-OCT).
Both SD-OCT and SS-OCT create an A-scan through tissue. B-scans consist of a collection of many A-scans conducted through a plane of tissue. A volume scan consists of an assembly of numerous B-scans; this volume scan is stored in computer memory as a block of data in which each memory location stores a value that corresponds to a specific small volume of tissue. The voxels (a portmanteau of volume and pixels) in the volume of data may be represented in many ways; 1 simple way is to make planar slices producing an image called a C-scan. C-scans are difficult to interpret because in a curved structure, many planes of tissue can be crossed. Another, more advanced method is to segment the data according to tissue planes; a thickness of voxels presented this way is called an en face scan. The tissue thicknesses can be measured in an en face scan; the retinal nerve fiber layer is commonly measured. Maps of the thickness of the retina or a specific retinal layer can also be produced. Actual correlation between OCT scans and histology of the retina has not yet been thoroughly explored, and the correlation of the identity of structures seen in OCT has changed numerous times. The International Nomenclature of OCT Panel has proposed nomenclature terminology, but these assignments are likely to change over time (Activity 2-1).
Optical coherence tomography (OCT) nomenclature terminology, based on the International Nomenclature for OCT Panel for Normal OCT Terminology.
From Staurenghi G, Sadda S, Chakravarthy U, Spaide RF; International Nomenclature for Optical Coherence Tomography (IN•OCT) Panel. Proposed lexicon for anatomic landmarks in normal posterior segment spectral-domain optical coherence tomography: the IN•OCT consensus. Ophthalmology. 2014;121(8):1572–1578.
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Some OCT scanners offer eye movement tracking. With the addition of this feature, ocular motion can be detected and corrected in the final image, improving the quality of the resulting scan. Tracking methods rely on recognizing fundus features and registering the scan pattern with the fundus image. This capability expands the utility of OCT; with it, scans interrupted by patient blinks still produce usable images. In addition, it is possible to perform repeated scans of the same fundus location over time, enabling assessment of disease progression (Fig 2-2).
In addition to B-scans and en face imaging, volume rendering shows the 3-dimensional character of the tissue. Compared to ordinary B-scans, volume rendering is computationally intensive. It is used in radiology, but is not yet widely used in ophthalmology (Fig 2-3).
Figure 2-2 Evolution of a macular hole, visualized with optical coherence tomography (OCT). A, OCT image of a patient with a perifoveal posterior vitreous detachment and no obvious traction on the macula. B, After 1 year, the patient experienced visual distortion; the image shows obvious traction with foveal tractional cavitations. C, Image taken 2 months later; note the full-thickness macular hole. D, Image taken 1 month after macular hole surgery; the hole is closed. Note the subtle area of increased reflectivity in the center. E, Image taken 3 months later shows the fovea with a nearly normal contour and laminar structure.
(Courtesy of Richard F. Spaide, MD.)
Figure 2-3 Imaging of a lamellar macular hole. A, Fundus photograph of a patient with prominent drusen and distorted vision. B, A B-scan section of the OCT imaging shows a lamellar hole with an unusually thick epiretinal membrane, which is sometimes seen in association with lamellar macular holes. C and D, Two different views from volume-rendered imaging of the lamellar macular hole taken in sections, showing the thick epiretinal membrane, as well as the absence of induced distortion of the retina. Note the cavities within the undermined retina and the attachment of the proliferation to the central foveal tissue.
(Courtesy of Richard F. Spaide, MD.)
Excerpted from BCSC 2020-2021 series: Section 10 - Glaucoma. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.