Optical coherence tomography (OCT) is a new technique that has recently been widely used in ophthalmology to visualize the different layers of the retina.
- In OCT, an interference signal is generated by interfering the reflection in the retina of a narrow beam of near-infrared, low-coherence light with the reflection of the same light beam in a mirror. The strength of the signal is a measure of the reflection from the retinal layer, which is the same distance from the light source as the mirror. By accurately moving the mirror, a transverse image of the retina is obtained with an axial resolution of 10 μm, approximating the resolution of a histological section.
- OCT is used for ophthalmic diseases such as macular edema, macular holes and other abnormalities at the retinal-vitreous transition, after photodynamic therapy and in glaucoma.
- With OCT one can measure the thickness of the retinal nerve fiber layer, so that glaucoma can be detected at an early stage.
- OCT and fluorescein angiography complement each other as diagnostic methods, but OCT is superior in the diagnosis of macular edema and macular holes.
- Both ophthalmologists and patients are enthusiastic about the ease with which retinal abnormalities can be visualized and interpreted using OCT.
Optical coherence tomography (OCT) was introduced to ophthalmology by Huang et al. in 1991 to visualize the structure of the transparent retina of patients. 1 Now that the technique has been further developed and a commercial device is available, OCT has been used since 2003 in a number of Dutch academic ophthalmology centres. The enthusiasm of ophthalmologists and patients is great. The technique not only complements current diagnostics, but has also given us new insights into the pathophysiology of retinal abnormalities. In this article we describe the technique and we illustrate its use on the basis of a number of clinical pictures.
the technique of optical coherence tomography
With OCT, two-dimensional cross-sections of the retina and underlying structures are made in a manner analogous to ultrasonography. A narrow beam of near-infrared, low-coherence light passes through the pupil into the retina and is reflected by the various retinal layers. Interfering with the reflection of the light beam hitting the retina with the reflection of the same light beam in a mirror produces an interference signal.
The strength of the signal is a measure of the reflection from only the retinal layer that is exactly the same distance from the light source as the mirror). By moving the mirror very accurately, the reflection of another retinal layer can be measured. The computer processes 1000 scans in depth and 500 in width to a cross-section of the retina.
2 Since it uses light, the technique is limited by media opacities such as cataracts. The latest generation of OCT devices (Stratus OCT; Carl Zeiss Meditec Inc., Dublin, CA, USA) can perform measurements with an axial resolution of 10 µm, approximating the resolution of a histological section. The images are so refined that the main histological layers of the retina can be clearly distinguished from each other.
3 The software offers possibilities to further analyze the images and to make three-dimensional reconstructions.
The OCT has added value in the diagnosis of macular edema, abnormalities at the retinal-vitreous transition and glaucoma; in this field, OCT can compete with fluorescein angiography. In fluorescein angiography, a fluorescent liquid is injected into the bloodstream, after which several pictures are taken with flash light during 5 minutes. An OCT examination is considerably less invasive.
The patient sits behind the device and fixates on a light object. The examination only takes a few minutes, the retina is not overexposed and there are no potential risks associated with intravenous fluorescein. In addition, the OCT images are clearly interpretable for patients, making them very useful for patient education.
Fluorescence angiography and OCT are largely complementary: whereas fluorescence angiography provides a dynamic image in the flat plane, OCT provides a static, digital, quantifiable image in the transverse plane. Applications of OCT outside ophthalmology are found in gastroenterology, cardiology and dermatology, where the technique is used for non-invasive biopsy. 4
applications of optical coherence tomography
When leakage from the retinal capillaries exceeds absorption, retinal edema develops. This mainly occurs in the macula and is then often related to diabetes, uveitis or an idiopathic reaction after intraocular procedures: Irvine-Gass syndrome. Macular edema can sometimes be seen with biomicroscopy and almost always with fluorescein angiography.
However, these methods have the disadvantage that the edema can only be quantified to a limited extent. Retinal edema is visible on OCT images as a thickening of the retina with possible cystoid spaces. This thickening is measurable and its course can be followed. In this way,
for example, the effect of intravitreal triamcinolone and of laser treatment in diabetic macular edema can be measured. The fact that OCT and fluorescein angiography complement each other becomes apparent when macular edema appears to be present on fluorescein angiography, but is not detectable with OCT, and vice versa.
Abnormalities at the retinal-vitreous transition
Over the course of life, the vitreous degenerates and the posterior vitreous membrane detaches from the retina. A number of things can go wrong in this process. It is known that too strong adhesion to the retina results in a defect in the neurosensory retina with the risk of retinal detachment.
When the vitreous shows too strong adhesion to the macula, vitreomacular traction syndrome or a macular hole can develop. In vitreomacular traction syndrome, traction of the vitreous at the macula causes a buildup of fluid under or in the neurosensory retina. OCT often very clearly shows the traction of the posterior vitreous membranes as well as the intra- and subretinal fluid
With a macular hole, the vitreous at the fovea probably pulls on the retina, eventually creating an opening in the neurosensory retina.. OCT has not only contributed to the understanding of the pathogenesis of this disease, but can also distinguish between the precancerous stage of a macular hole, in which spontaneous improvement is still possible due to detachment of the posterior vitreous membrane, and a candidate macular hole for surgical treatment.
We speak of an epiretinal membrane (‘macular pucker’) when glial tissue proliferates on the inside of the retina and the retina folds through contraction. There is an idiopathic form of this syndrome, but it also occurs after treated retinal detachment and after uveitis. On the OCT images, the membrane is visible as a highly reflective preretinal membrane with frequent retinal edema .
OCT after photodynamic therapy
In the exudative form of age-related macular degeneration, a choroidal neovascular membrane grows beneath the neurosensory retina. The blood vessels in this membrane leak and eventually destroy the photoreceptors. Photodynamic therapy is increasingly used in this form of macular degeneration. After intravenous administration of the photodynamic substance verteporfin, the neovascular membrane takes up this substance.
Photodynamic activation is achieved by selectively illuminating the neovascular membrane with laser light, causing thrombosis of blood vessels and destruction of tissue. After photodynamic therapy, OCT is used to monitor response to treatment. Due to photodynamic damage to the neovascular membrane, after an initial increase, a decrease in sub- and intra-retinal fluid will be visible.
In a later stage, subretinal fibrosis appears as a highly reflective band. With OCT, the effect can already be observed quickly, while the images from fluorescein angiography are less easy to interpret in the first few months after treatment.
Glaucoma
Glaucoma damages the retinal nerve fibers that carry information from the photoreceptors to the brain via the papilla. This damage leads to specific loss of the visual field. Damage to the retinal nerve fiber layer is believed to have existed for a long time before fiber bundle defects become visible on visual field examination.
By detecting this damage, glaucoma could be recognized at an earlier stage. Since the nerve fiber layer can be distinguished as a separate layer within the retina on the OCT image, its impairment can be measured with this technique. The OCT measures the thickness of the nerve fiber layer at a fixed distance around the papilla.
By comparing the measured thickness with normal nerve fiber layer thicknesses in a database, one can determine whether the nerve fiber layer is locally or diffusely thinned, which may be consistent with glaucoma . The examination can be repeated to see if there is any progression of the nerve fiber layer thinning.
consideration
In recent years, OCT has made great strides in the ophthalmological diagnosis of macular edema, abnormalities at the retinal-vitreous junction, and glaucoma. Its success is due to the rapid, non-invasive way in which cross-sectional images of the retina are obtained. OCT makes it possible to diagnostically examine different layers of the retina in a simple, quantifiable way.
The patient can also read the effect of a treatment from the insightful pictures. After further refinement of the technique, new applications can be expected in retinal disorders with fine morphological abnormalities, such as Leber’s hereditary optic atrophy and juvenile retinoschisis.
IIn addition, OCT is being made suitable for diagnostics of the anterior segment of the eye. Since applications are mainly found in the highly specialized ophthalmic subfields and because the current commercially available equipment is rather expensive, the OCT will probably only be available in the coming years in academic centers and the larger general hospitals as an addition to the current diagnostics.
Conflict of Interest: None reported. Financial support: The purchase of the OCT device was financially supported by the FPFischer Foundation.
LITERATURE
