Optical Coherence Tomography, or OCT, is a noncontact, noninvasive imaging technique used to obtain high resolution cross-sectional images of the retina. OCT was developed through a collaborative effort between the New England Eye Center, Tufts University School of Medicine, the Department of Electrical Engineering and Computer Science at MIT, and Lincoln Laboratory, MIT.
OCT is analogous to ultrasound B-scan imaging except that light rather than sound waves are used in order to obtain a much higher longitudinal resolution of approximately 10µm in the retina. OCT has been shown to be clinically useful for imaging selected macular diseases including macular holes, macular edema, age-related macular degeneration, central serous chorioretinopathy, epiretinal membranes, schisis cavities associated with optic disc pits, and retinal inflammatory diseases. In addition, OCT has the capability of measuring the retinal nerve fiber layer thickness in glaucoma and other diseases of the optic nerve.
Ultra-high resolution OCT also is available at New England Eye Center – one of the few centers in the world offering this new technology. With this system, images can be obtained in the 2-3 micron range. While this is an investigational technology, it has proven highly useful in evaluating disorders of the photoreceptors and vitreoretinal interface on an everyday clincial basis.
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The topographic mapping protocol is useful for longitudinally monitoring patients for the development of macular edema and for following the resolution of edema after laser treatment. The false-color map of retinal thickness provides an intuitive and efficient method of comparing retinal thickness over several visits which could be directly compared with slit-lamp observation.
Development and resolution of diabetic macular edema. A 50-year-old man with NPDR and CSME in the right eye was examined (fundus below). His visual acuity OD was 20/25. OCT at the initial visit showed foveal and juxtafoveal macular thickening which was most significant temporally and corresponded with areas of hard exudate. Eight months later, the patient's visual acuity OD had decreased to 20/40. OCT at the second visit displayed a substantial increase macular thickness centrally. Focal laser photocoagulation was performed and the patient returned five months later. On follow-up examination, the visual acuity OD had returned to 20/20. OCT at the final visit showed resolution of the edema with a normal foveal and central macular thickness. A remaining area of minimal retinal thickening was evident nasal to the fovea.
Lamellar or partial thickness macular holes, most often due to rupture of macular cysts, may also ophthalmoscopically resemble full-thickness holes. The visual acuity, which is often worse in eyes with a true macular hole due to the complete loss of photoreceptors, may not accurately indicate the diagnosis.
Macular hole and lamellar macular hole. A 69-year-old woman had a full-thickness macular hole in her right eye and a lamellar macular hole in the left. The visual acuity (20/100 OD and 20/200 OS) was worse in the eye with the partial thickness hole compared to the eye with a full-thickness hole.