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Optical coherence tomography
Last reviewed: 22.11.2021
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Optical coherence tomography (OCT) is a method of non-invasive research, which is increasingly used in medicine for diagnostic purposes. The optical coherent tomograph (Humphrey Systems, Dublin, CA) calculates the parameters of the SNV thickness for transverse high-resolution retinal scan scans.
[When is optical coherence tomography used?
Optical coherence tomography is important in detecting glaucoma and monitoring its progression.
Advantages of optical coherence tomography
Optical coherence tomography is of interest for clinical use for a number of reasons. The resolving power of OCT is 10-15 microns, which is almost an order of magnitude higher than the resolution of other diagnostic methods, including ultrasound. This high resolution allows you to study the architecture of the fabric. The information obtained with the help of OCT is intravital and reflects not only the structure, but also the features of the functional state of the tissues. Optical coherence tomography is not invasive, since it uses radiation in the near infrared range with a power of the order of 1 mW, which does not have a damaging effect on the body. The method excludes trauma and has no limitations inherent in the traditional biopsy.
How does optical coherence tomography work?
In optical coherence tomography, an interferometer with low coherence radiation is used to obtain high-resolution images. The procedure for performing optical coherence tomography is similar to obtaining an image with ultrasound B-scanning or on a radar, in addition to using light more as acoustic rather than radio waves. Measurements of distance and microstructure in optical coherence tomography are based on the measurement of the time of passage of light reflected from various microstructural elements of the eye. Sequential longitudinal measurements (A-scangrams) are used to construct the spectrozonal topographic image of tissue microsections, the form of which is very similar to the histological sections. The resolution of the longitudinal section of optical coherence tomography is about 10 μm, the resolution of the transverse cut is about 20 μm. In the clinical evaluation of glaucoma when scanning a circle with a diameter of 3.4 mm, where the center is the optic nerve disk, in the case of optical coherence tomography, cylindrical sections of the retina form. The cylinder is unfolded, presenting as an image of a flat cross-section. Optical coherence tomography is used to create a map of the thickness of the macula in a series of six radial images that pass along the meridians of the clock on the dial, centered on the fovea; the optic nerve disk is mapped in the same way, with the center of the radial images on the optic disc. An automated computer algorithm measures the thickness of START without user intervention. Unlike confocal scanning laser ophthalmoscopy, an optical coherence tomography does not require a basal plane. The thickness of the START is the absolute parameter of the transverse section. Refraction or axial length of the eye does not affect the measurements of optical coherence tomography. The parameters of optical coherence tomography of the SNB thickness do not depend on the birefringence of the tissue.
How is optical coherence tomography performed?
OCT uses near infrared light, which illuminates the tissue site being examined. Any biological tissue, including skin and mucosa, consists of structures of different density and therefore optically heterogeneous. Infrared light, falling on the boundary of two media with different densities, is partially reflected from it and dissipated. Analyzing the backscattering coefficient of light, one can obtain information about the structure of the tissue in this section.
Scanning the tissue with an optical beam, a series of axial measurements are carried out in different cross sections and directions - both axial (in depth) and lateral (lateral). A powerful computer built into the OCT system processes the obtained numerical data and draws a two-dimensional image (a kind of "morphological slice"), convenient for visual evaluation.
Restrictions
Optical coherence tomography requires a nominal pupil diameter of 5 mm, but in practice most optical coherence tomography can be performed without mydriasis. The possibilities of optical coherence tomography are limited in cortical and posterior subcapsular cataracts.