1.1 Diagnostic/Technology Overview
Optical coherence tomography (OCT) was first introduced in 1991 as a noninvasive technology that uses low-coherence interferometry to produce cross-sections of biological structures. OCT employs a light beam that travels to a beam splitter, which splits the beam into a sample and a reference beam (directed toward a mirror at a reference distance). The backscattered light from the sample then interferes with the reflected light from the reference mirror and is used to produce a reflectivity profile of the retina. Analogous to ultrasound, but using light instead of sound waves, OCT is able to obtain high-resolution images (ranging from 3 to 10 µm). The primary system currently used is spectral domain OCT, which uses Fourier transformation to obtain images faster and with higher resolution than time-domain OCT. Current OCT technology enables visualization of the various retinal layers (▶ Fig. 1.1). Next-generation swept source OCT technology utilizes a longer wave length and a faster engine that enables greater tissue penetration (e.g., choroidal visualization) and scan sizes (▶ Fig. 1.2).
Fig. 1.1 Normal retinal anatomy. Optical coherence tomography demonstrating normal retinal anatomy. The layers of the retina (from inner to outer): nerve fiber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer nuclear layer, external limiting membrane, ellipsoid zone, interdigitation zone, retina pigment epithelium, choroid.
Fig. 1.2 Swept source optical coherence tomography (SS-OCT). Longer wavelength SS-OCT demonstrating outstanding visualization of the vitreous, retina, and choroid. Examples include (a) an eye without pathology and (b) an eye with neovascular age-related macular degeneration with extensive subfoveal hyperreflective material and subfoveal choroidal neovascularization.
Since its introduction, OCT has become the primary diagnostic test for multiple diseases including macular holes, diabetic macular edema, age-related macular degeneration (AMD), vascular occlusion, and central serous chorioretinopathy (CSR).
1.2 Key Applications
1.2.1 Vitreoretinal Interface Disorders
OCT is the gold standard in diagnosis and evaluation of vitreoretinal interface disorders, including epiretinal membranes, vitreomacular traction, and macular holes (▶ Fig. 1.3).
Fig. 1.3 Macular hole. Optical coherence tomography demonstrating a full thickness macular hole with overlying vitreous separation. Note the anvil-shaped deformity of the macula, the loss of photoreceptors, and the intraretinal fluid at the edges of the hole.
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