Optical coherence tomography (OCT) is a revolutionary diagnostic technique that performs high-resolution cross-sectional imaging of the internal structures of the eye ( Fig. 40.1 ). It enables imaging in real time with resolutions of 1 to 15 μm. This high-resolution imaging is 1 to 2 orders finer than with the use of ultrasound, magnetic resonance imaging, or computed tomography. The OCT, which was developed in 1991, has become a major advance in the field of ophthalmology, allowing for the imaging of the anterior segment and retina at resolutions that allow for a proper diagnosis.
An anterior segment OCT can image the anterior structures of the cornea, measure the corneal thickness, and allow visualization of the anterior chamber angle and related internal structures. Imaging of the retina can evaluate the optic disc, nerve fiber layer, vitreoretinal relationship, and macula. The high-resolution images allow for a diagnosis of glaucoma, an epiretinal membrane, cystoid macular edema, central serous retinopathy, macular hole, macular degeneration, macular complications of diabetic retinopathy, and other conditions.
The OCT device is a noncontact method that allows for detailed cross-sectional imaging of the anterior eye and retina. The cross-sectional information obtained is complementary to the conventional testing of fundus photography and Florescein angiography. In addition to its diagnostic ability, the monitoring of diseases, such as macular edema and glaucoma can provide helpful information as to disease progression. A measurement of the thickness of the nerve fiber layer is a diagnostic indicator for early glaucoma and disease progression. OCT imaging is useful in determining the effectiveness or adequacy of treatment in disease conditions like glaucoma, macular edema, central serous retinopathy, macular holes, and exudative age-related macular degeneration (AMD).
OCT performs cross-sectional imaging by measuring the time delay and intensity of back-scattered or back-reflected light from structures inside tissue. Unlike with the use of light, ultrasound imaging depends on the reflection of sound waves from intraocular structures; it requires a direct contact of the ultrasound probe with the cornea or immersion of the eye in a liquid bath to transmit sound waves into the eye. It is the frequency or wavelength of the ultrasound waves that determines resolution of the images. Although typical ultrasound systems yield a resolution of 150 μm, high-resolution imaging devices using higher-frequency sound waves can achieve resolutions of 20 on the 20 μm scale. At these high-frequency sound waves, the ultrasound signal is greatly attenuated in tissue and is limited to a depth of only 4 to 5 mm, which means that high-resolution imaging of the retina is not possible.
OCT is an optical imaging technique that uses light instead of sound, so the optical imaging is limited to tissues that are optically assessable. In addition to its use in ophthalmology, it can be incorporated into devices, such as endoscopes or catheters. Patients are more comfortable with OCT imaging than ultrasound because the former is a noncontact testing device. OCT imaging devices have continued to improve in terms of resolution. The initial OCT units had resolutions of 10 μm. Current-generation units are in the 5 to 7 μm range, with a potential in the future for even finer resolution of 2 to 3 μm. The high-resolution devices today allow visualization of individual retinal layers, thus allowing for the diagnosis of a wide range of retinal conditions.
Histologic evaluation of the retina allows for the distinction of 10 layers, including four cell layers and two layers of neuronal interconnections. High-resolution OCT imaging allows for the detection of these internal retinal structures. These distinct layers, from the inner retina to the outer retina, are the inner limiting membrane, the nerve fiber layer, the ganglion cell layer, the inner plexiform layer, the inner nuclear layer, the outer plexiform layer, the outer nerve layer, the external limiting membrane, the photoreceptor inner segment and outer segment of the photoreceptor layer, and the retinal pigment epithelium. The choriocapillaris and choroid are immediately posterior to the retinal pigment epithelium. Ultrahigh resolution allows for excellent visualization of the retinal microstructure.
The technician’s role
Many OCT devices have come to market in recent years. The software and hardware of these devices can vary, but the basic principles are similar. If it is a new device for a practice, proper training should come from the distributor. If the device has been in the practice for some time, then training may come from a knowledgeable in-house technician. A grounded power supply is a requirement; in areas where the power supply is unstable, it is advisable to install an uninterrupted power supply. The OCT unit should be placed in a room in which there is a satisfactory area for both the operator and the patient to maneuver. The room should generally be void of sunlight and the lighting condition should be adjustable with a dimmer switch. The room should have a solid floor to minimize vibration. The unit must be kept in a fairly stable temperature environment because extreme fluctuations in temperatures can have an adverse effect on the internal settings. The practice should decide on which scan protocols are used for evaluation of the cornea, anterior chamber angle, optic nerve head, nerve fiber layer, and macula. In addition to onsite training sessions, some companies offer online sessions (called webinars).
The patient should be seated in front of the OCT device in a sturdy chair with a back, and locking or no wheels. Make sure that the patient is comfortable by adjusting the table height and chin rest. Basic information needs to be entered into the computer system, including the first and last names. The technician then activates the required testing to be performed. On the first evaluation, the patient’s data are compared with those of normal patients with similar characteristics. On a follow-up visit, in addition to the normative database one also has the patient’s previous records. Whereas the normative database can determine whether the patient is relatively normal, the progression reports allow the determination of any deterioration.
Macular thickness data can be used for comparison with a normative database. The thickness values are compared with a normative database and a color scale is used to indicate which percentile each given thickness value falls into compared with normal thicknesses: red indicates greater than 99% of normal, yellow indicates greater than 95%, green is between 5% and 95%, light blue is less than 5%, and purple is less than 1% of normal thickness.
When patients return for a repeat OCT, image thickness calculations can be compared from visit to visit using this software. These important values can be used to monitor disease progression or response to treatment.
Nonexudative age-related macular degeneration
AMD is a common cause of central vision loss among individuals ages 70 years and more. The nonexudative form of macular degeneration accounts for close to 90% of all diagnosed cases. AMD can be classified into early, intermediate, and advanced stages based on clinical findings of the amount of drusen and retinal pigment epithelial atrophy. OCT evaluation can recognize findings of AMD with hard and soft drusen, pigmentary abnormalities, and geographic atrophy with dropout of photoreceptors ( Fig. 40.2 ).
Exudative age-related macular degeneration
Although only 10% of AMD patients have the neovascular form of the disease, more than 80% of individuals that are legally blind (20/200 or worse) as a result of AMD have the exudative form. The exudative form of AMD, also called neovascular AMD or wet AMD, is characterized by neovascularization within the macula, detachment or tears of the retinal pigment epithelium (RPE), fibrovascular scarring, and vitreous hemorrhage. OCT is a complement to fluorescein angiography and more recently, OCT angiography in the diagnosis of wet AMD. Small changes in the structure of the retinal layers and subretinal space may signal progression or regression of the neovascular lesions. Choroidal neovascular lesions can appear with OCT imaging as an enlargement of the RPE-Bruch’s membrane-choriocapillaris. A pigment epithelial detachment can be precisely imaged using OCT ( Fig. 40.3 ), which appears as a localized prominence. OCT imaging can identify an increase in foveal thickness, cystoid macular edema, subretinal fluid, RPE tears, and subretinal fibrosis or disciform scarring. Wet AMD is typically treated in a “treat and extend” fashion. Patients have a minimum of three loading doses of monthly antivascular endothelial growth factor injections. The injection intervals are then extended as long as the macula remains dry.