Optical Coherence Tomography—Atlas and Text Samuel Boyd, Rosario Brancato, Bradley Straatsma
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Introduction to Optical Coherence TomographyChapter 1

Rosario Brancato, MD
L. Pierro, MD
 
Optical Coherence Tomography (OCT) is a modern diagnostic imaging technique that enables the visualization “in vivo” of the cross sectional structure of the retina, the vitreo-retina interface and the anterior segment of the eye with higher resolution than any other non invasive imaging technique.
It is based on a complex analysis of the reflections of low coherence radiation from the tissue under examination.
The resolution available with current instrumentation at present varies approximately from 5 to 10 microns, according to the instruments used. These imaging techniques, which can provide cross-sectional images of intraocular structures, give important diagnostic information complementary to conventional fundus photography, fluorescein angiography and indocyanine green angiography.
OCT is rapidly emerging as a basic imaging tool for the diagnosis and consequently for the control of the evolution of macular diseases: diabetic retinopathy, age-related-macular degeneration, macular holes, epiretinal membranes and other retinal diseases.
The discomfort of patients is minimized because the acquisition of the images is rapid and this permits us to acquire many images in different cross-sectional planes of the retina, the vitreo-retina interface and also of the anterior segment of the eye.
OCT images contain much information on the retina structure and have an important role in the evaluation of the disease progression and the response to therapy.
Recently the introduction of new OCT systems, using Spectral/Fourier domain, has allowed a higher resolution of the retinal images (4-5 micron) and a faster images acquisition.
In the past, fluorescein angiography (FA) and indocyanine green angiography (ICGA) have allowed visualization of the retinal vessels; today OCT allows the visualization of the structure of the retina, the retinal pigment epithelium and the choriocapillary inner spaces and highlights the vitreoretinal interspace. Moreover OCT can quantify thickness of the retina and the pigment epithelium. For all these aspects OCT is today a very important tool for the assessment of the macular diseases, in the choice of the treatment and in the follow-up of the evolution of chorioretinal diseases.
 
What is OCT?
OCT uses near infrared, low coherence light to achieve a resolution of approximately 5-10 microns, depending on the instrument used. Similar to an ultrasound which uses sound waves, a CT scan which uses X-rays, and an MRI which uses electron spin resonance, OCT uses light to obtain a cross-sectional image. It uses a non-contact transpupillary approach to obtain a tomograph of the retina which is displayed in real time through a computer. The scan length for each tomogram may be between 2.83 and 12 mm. Quantitative measurement of retinal thickness is possible because of the well-defined boundaries of optical reflectivity at the inner and outer margins of the neurosensory retina. Quantification of juxtapapillary Retinal Nerve Fiber Layer (RNFL) thickness in glaucomatous eyes is also available. Circular scans around the optic nerve, with a circle diameter of either 2.25 or 3.37 mm, without overlapping the disc itself, can be performed. These measurements are obtained by means of a computer algorithm that searches for the characteristic changes. A transverse sequence of optical ranging measurements is used to construct a false color tomographic image of tissue microstructure which appears incredibly similar to a histologic section. Spectral OCT today can function as a type of “Optical Biopsy” in an even more precise way.
Since OCT is based on near-infrared interferometry, it is not affected by axial length, refraction or by the degree of nuclear sclerosis; however large posterior subcapsular or cortical cataracts, as well as a poor compliance of the patient, do impair the ability to perform OCT. This technology is capable of reproducible measurement of retinal thickness in normal eyes (1,2,3).
 
Interpretation of OCT Maps
OCT images can be presented as either cross sectional images or as topographic maps. Cross-sectional or B-mode imaging is accomplished by acquiring a sequence of 100 interferometric A-scans across a section of retina. To facilitate interpretation a false color scheme is added in which bright colors such as red and white correspond to highly reflective 2areas and darker colors such as blue and black correspond to areas of lower reflectivity. Topographic maps obtained by OCT are displayed by a false-color scheme to facilitate interpretation. For cross-sectional images, bright colors correspond to areas of high reflectivity while darker colors correspond to areas of low reflectivity. For topographic maps, bright colors are assigned to areas with increased retinal thickening and darker colors are assigned to areas with less retinal thickness.
Retinal thickness is converted to a false color value for each of the 600 points measured within 3,000 microns from the center. Interpolation of polar coordinates is performed to estimate thickness in the wedge-shaped areas between each cross-sectional scan. To further facilitate interpretation, the macula is divided into 9 ETDRS regions with a central circle of 500 um radius. Two outer circles with radii of 1,500 um and 3,000 um complete the display.
In Figure 1 we show the normal retina, for comparison with abnormal cases. Figure 2 shows alterations in age-related macular degeneration with choroidal neovascularization. In Figure 3 we present central serous chorioretinopathy; in Figure 4 a severe non-proliferative retinopathy with macular edema. Figure 5 shows a central venous occlusion with cystoid macular edema.
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Figure 1: Optical Coherence Tomography (OCT) of the Normal eye.
The color photograph shows a normal macular reflex in a normal eye. OCT shows the depression appearance of the fovea in relation to the rest of the macula. We can observe the high reflectivity of the retinal pigment epithelium and the chorio capillaries (pink color structure). Opposite to this, there is a less precise appearance in the photoreceptor's area because of the poor return reflection effect (blue and black structure).3
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Figure 2: Choroidal Neovascularization in ARMD.
Fluorescein angiography shows the late hyperfluorescence corresponding to the macular lesion. Optical Coherence Tomography (OCT) shows how neovascularization is causing a thickening of retina layers with presence of intraretinal fluid and little neuroepithelial detachment., confirming the activity of the lesion.4
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Figure 3: Central Serous Chorioretinopathy.
Fluorescein Angiography. We can observe hyperfluorescence in the areas of leakage of the macular pathology. Optical Coherence Tomography (OCT). A neuroepitelial detachment is clearly seen.5
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Figure 4: Non-Proliferative Diabetic Retinopathy.
With the help of Optical Coherence Tomography (OCT) we can observe the presence of little pseudocysts near to hard hyperreflective exudates secondary to the alterations of the macula in this stage of the retinopathy.6
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Figure 5: Central Venous Occlusion with Cystoid Macular Edema.
Fluorescein Angiography shows the presence of numerous retinal hemorrhages in the macular area, secondary to the venous occlusion. In this Optical Coherence Tomography (OCT) we can observe the presence of numerous intraretinal areas of high reflectivity, characteristic of hemorrhage and thick retinal layer tissue.7
 
Current Clinical Application
 
Imaging of Anterior Segment Structures
This technology is also very helpful for the anterior segment surgeon, either the refractive or the cataract surgeon. The strongest reflected signals arise from epithelial surface of the cornea and the highly scattering sclera and the iris. Other clearly identifiable structure is anterior capsule of lens. Structures in the angle region like trabecular meshwork and canal of Schlemm are not clearly visualized in the tomogram since the incident and backscattered light is highly attenuated after traversing the overlying scleral tissue.
 
Glaucoma
Due to the OCT scan, the user can visualize the angle in multiple cross-sections of the anterior chamber. Because the OCT uses infrared light, the pupil does not constrict, providing a more natural view of the angle without changing its anatomy. A measuring tool can then be used to calculate a definitive angle depth in degrees. Now, patients at risk for angle-closure glaucoma may be monitored more closely as the crystalline lens matures.
 
Evaluation of RNFL in Glaucoma
Optical coherence tomography is one of the most reliable, reproducible and accurate methods of monitoring changes in the optic nerve and retinal nerve fiber layer (RNFL), which is imperative for diagnosis and management of early glaucoma (Figure 6). When used in conjunction with regular clinical examinations with IOP measurements and periodic visual field testing, retinal tomography offers accurate assessment of the retinal nerve fiber layer integrity. Quantification of the peripapillary retinal nerve fiber layer (RNFL) thickness can provide clinicians with objective information about the optic nerve in different pathologic conditions. Several imaging techniques can be used to obtain such a measurement; most recently, optical coherence tomography (OCT) has demonstrated several merits. This technology has been used extensively to quantify RNFL thickness in atrophic diseases such as glaucoma, Leber hereditary optic neuropathy, traumatic optic neuropathy, and band atrophy.
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Figure 6: Optic nerve head scan on OCT showing a normal nerve head.
 
Refractive Surgery Application
When applied to refractive evaluation the anterior segment OCT maps corneal thickness in 25 spots across the cornea and has great repeatability. It can also create a differential map to compare past readings and detect subtle changes involved in corneal-thinning conditions. This can be very useful in following a patient with keratoconus or pellucid marginal degeneration. Even if the corneal topography is symmetric and central-ultrasound pachymetry is normal, the OCT pachymetry map can reveal an abnormal pattern of corneal thickness, raising suspicion for forme fruste keratoconus.
 
High-Resolution OCT Corneal Scan
Post LASIK procedures, high-resolution corneal scans detail the actual thickness of the flap and the residual stroma. This is useful in ensuring that enough residual stroma will remain after an enhancement.
 
Imaging of Abnormal Retinal Structures
OCT can effectively distinguish lesions that ophthalmoscopically are difficult to observe and resemble various stages of macular hole development like lamellar macular hole, macular cysts, macular edema, sub-retinal hemorrhages, retinal and/or foveal detachments of neurosensory retina or pigment epithelium, and epiretinal membranes with macular pseudoholes.
 
Enhanced Visualization of Macular Holes
Comparing to biomicroscopical observations OCT gives additional information about idiopathic macular holes, especially in their early stage. According to the literature, the foveal cystoid space or pseudocyst is considered the first step of full-thickness macular hole formation, instead of foveolar detachment as proposed by Gass (4,5). A foveal pseudocyst appears in the tomographic imaging as a large intraretinal cystoid formation that occupies the inner part of the foveola and disrupts the outer retinal layers. A foveal pseudocyst is considered a specific entity that may be the result of the incomplete separation of the vitreous cortex at the foveal center.8
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Figure 7: The OCT image shows a full thickness macular hole with a well evident vitreous operculum at its top.
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Figure 8: A reflective band is present just anterior to the neurosensory retina. A thickening of the neurosensory retina and diffuse macular edema is visible under the tractional epiretinal membrane.
The role of the vitreous cortex in development of macular holes using OCT has also been clarified in various studies(6,7)(Figure 7).
But the greatest advances obtained by OCT are in the field of vitreo-retinal surgery.
Indeed the ability of OCT to more accurately identify macular holes allows clinicians to better predict the surgical outcome. OCT reveals anatomic configuration of surgically closed macular holes within 24 hours after successful surgery(8).
Interesting results have been obtained by using OCT also in the study of retinoschisis, that is represented in the OCT as retinal splitting of the outer retinal layers in the macula, with inner retinal columnar structures that bridge the inner and outer retinal layers(9).
OCT has demonstrated that foveal retinal detachment and retinoschisis are common in severely myopic patients, with posterior staphyloma, while biomicroscopic observation revealed only a retinal detachment. Retinal detachment may precede the formation of a macular hole in severely myopic eyes(10).
Also, idiopathic posterior pole retinoschisis in highly myopic eyes is easily diagnosed by OCT and it is possible to establish the true extent of these macular changes(11).
In the presence of idiopathic macular membranes, OCT can give complementary information in the evaluation of anatomical features of the macula before and after surgical removal of the membrane.
The epiretinal membranes are identified by OCT when they are separated from the inner margin of the retina, and appear as a hyper-reflective thin band anterior to the 9retina. When they are tightly adherent to the retinal surface they are identifiable by an increased reflective image on the retina (Figure 8). OCT, in distinction from other diagnostic methods such as ultrasound, can detect the presence of hidden retinal alterations, such as a cystoid macular edema or a sub-foveal retina detachment, or a tractional macular hole.
OCT examination has demonstrated that the thickness of the macula decreases after epiretinal membrane surgery.
 
Application of OCT in Diabetic Retinopathy
Accurate longitudinal comparisons of serial OCT scans depends upon reproducibly locating the central fovea. In patients with central fixation each OCT scan is centered on the patient's fixation such that the OCT scan passes through the central fovea. In patients with eccentric or imperfect fixation, the location of the fovea can be estimated from each OCT scan using a computer algorithm that searches for a focal minimum in total intraretinal reflectivity which typically coincides with the central foveal depression.
OCT can be considered a sensitive technique in the study of diabetic retinopathy for the early detection of retinal abnormalities and in quantifying macular thickness after laser treatment.(12,13)
By OCT it is possible to differentiate between cystoid and diffuse edema. In cystoid edema, low reflective spaces, divided by thin hyperreflective membranes, correspond to cystic spaces in the outer plexiform and inner nuclear layers. A large central cyst was occasionally noted to extend beneath the inner limiting membrane. Intraretinal fluid accumulation causes reduced optical reflectivity. In diffuse edema, an area of low reflectivity was present within the retina.
Significant differences in retinal thickness comparing patients with diabetic retinopathy and normals have been detected by OCT, even in absence of clinically significant macular edema. Also an increase in macular thickness in diabetics either without retinopathy and/or edema compared to controls has been demonstrated(14,15). OCT is also useful for evaluating and documenting macular edema and mapping it(16) (Figures 9 and 10).
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Figure 9: The tomography demonstrates a cystoid macular edema in a patient with diabetic retinopathy. The strict adherence of the surface of the inner retina layer with the vitreous band is also evident.
10
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Figure 10: The map shows areas of thickening of the macula.
After vitreous surgery for diabetic macular edema, best-corrected visual acuity improvement is greater in eyes with less preoperative increase in thickness of the neurosensory retina(17). Recently an OCT retinal thickness map has been developed to provide more precise measurements of macular edema(18).
 
Age-related Macular Degeneration
Severe vision loss in this disease is the result of choroidal neovascular membrane formation. Choroidal neovascularization typically appears as either classic choroidal neovascularization (well delineated) or occult neovascularization (less well delineated).
A number of new pharmacologic approaches are being applied to macular degeneration. Vision loss in age-related macular degeneration typically results when choroidal neovascular tissue with or without concomitant hemorrhage and exudation into the fovea occur. The presence of hemorrhage, subretinal fluid or hard exudate under the fovea are usually detrimental to vision. The presence of choroidal neovascularization itself underneath the central fovea may likewise be detrimental to vision.
Optical coherence tomography, because of its high resolution capability, is able to image subretinal fluid, intraretinal thickening and sometimes choroidal neovascularization. As a result these capabilities, OCT may have utility in the assessment of new treatment modalities for age-related macular degeneration.
 
Cystoid Macular Edema (CME)
Ophthalmoscopically, CME appears as elevation or thickening of the central macula. Intraretinal cyst formation is often present. The area of retinal elevation often has ill defined borders both on ophthalmoscopy and clinical examination. The presence of media opacity and/or a small pupil, as is common in uveitic patients, may make determination of the presence and area of CME difficult.
The use of optical coherence tomography for the measurement of cystoid macular edema may be useful. Longitudinal measurement of either axial scans and/or topographic images as described for diabetic macular edema can be utilized. Additionally, the amount of media opacity and pupillary miosis in patients with uveitis will not likely interfere significantly with the images obtained by optical coherence tomography.11
 
Pathological Macular Disorders
In the presence of macular diseases, OCT has demonstrated several new findings that may help the interpretation of the pathophysiologic changes in various disorders.
In idiopathic juxtafoveolar retinal telengiectasis, OCT shows a hyperreflective band within the inner retina, and has demonstrated the presence of a small plaque, consistent with the hypothesis of Gass and Blodi, of an epithelial proliferation into the inner retina in some cases of retinal teleangiectasis(19).
In idiopathic polypoidal choroidal vasculopathy, OCT has demonstrated a serosanguineous detachment of the retinal pigment epithelium, suggesting that these lesions are situated beneath Bruch's membrane and are covered anteriorly by both retinal pigment epithelium and Bruch's membrane(20).
OCT is useful to establish the presence of cystic degeneration of the macula, when macular modifications are not noted clearly on biomicroscopic examination or fluorescein angiography, in patients with initial central serous chorioretinopathy, in patients with no specific serous retinal detachment, and in inflammatory diseases.
With recent advances in technology, a third generation of OCT devices is now been developed. This new OCT technology may achieve “in vivo” retinal imaging with less than 3 µm axial resolution (only experimental). A higher longitudinal resolution may contribute to a better visualization of intraretinal structures and pathology and could increase the reproducibility, sensitivity and specificity for diagnosis of retinal and macular diseases.
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