Anterior Segment Optical Coherence Tomography
Optical coherence tomography (OCT) is a noninvasive technology that produces 2-dimensional, high-resolution, and high-definition cross-sectional images of ocular tissue. These images are similar to ultrasonographic images, but they are based on the emission and reflection of light (low-coherence interferometry). The extremely fine resolution of the images (5–10 μm) allows exquisite delineation of the layers of the cornea, anterior chamber, and iris. There are 2 types of anterior segment OCT: time-domain (TD-OCT) and frequency-domain (FD-OCT). Spectral-domain OCT (SD-OCT), also called Fourier-domain OCT, is a subtype of the latter. A new, faster OCT technique that features components of both TD- and SD-OCT, called swept-source OCT, has been developed; it is being used both in the clinical setting and in real time in the operating room as an attachment to the operating microscope.
OCT scans can measure the depth, width, and angle of the anterior chamber (Fig 2-7). The corneal pachymetry feature of these devices is useful in the preoperative evaluation of patients with Fuchs endothelial corneal dystrophy. In postoperative follow-up of endothelial keratoplasty cases, the shape, thickness, and attachment of the donor lenticule can be visualized and quantified. In patients who have undergone laser in situ keratomileusis (LASIK), clinicians can use OCT to measure the thickness of the corneal flap and the residual stromal bed to determine the safety of an enhancement (re-treatment). Recent software provides information on corneal curvature and epithelial thickness that is helpful in screening refractive surgery patients. Software is also available to calculate the true corneal power, which can be used in IOL power calculation after LASIK or photorefractive keratectomy. Intraoperative OCT has also recently been developed as an adjunct in Descemet membrane endothelial keratoplasty (DMEK) to ensure correct orientation of endothelial grafts.
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Figure 2-7 Anterior segment optical coherence tomography image of a phakic eye. The central anterior chamber depth is 2.73 mm, and there is moderate narrowing of the anterior chamber angle.
(Reproduced from Goins KM, Wagoner MD. Imaging the anterior segment. Focal Points: Clinical Modules for Ophthalmologists. San Francisco: American Academy of Ophthalmology; 2009, module 11.)
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.