Optical Coherence Tomography
Oct is a non invasive, non contact imaging modality that generates micrometer resolution cross sectional images of ocular tissue . oct is analogus to ultrasonic b scan and radar imaging and produces a 2 dimensional image of backscattered light from different layers of retina .
Optical coherence tomography (OCT), first described by Huang et al. in 1991, allows high-resolution crosssectional (tomographic) images of the neurosensory retina to be obtained in a noninvasive manner. Commercially available OCT systems are now capable of obtaining retinal images with an axial resolution of approximately 5 to 8 μm, and a transverse resolution of approximately 15 to 20 μm. As a result, OCT has been characterized as in vivo “clinical biopsy” and has transformed the diagnosis and management of vitreoretinal disorders.
BASIC PRINCIPLES
OCT works by measuring the properties of light waves reflected from, and scattered by, tissue (analogous to ultrasonography). The use of light waves in OCT results in images with much greater resolution than ultrasonography as the wavelength of light is many times less than that of sound.. OCT instruments employ the principle of interference.
Interference is a phenomenon that occurs when two light waves meet each other and are superimposed. The resulting waveform depends on the wavelength, amplitude, and relative phase of the two waves. Interferometry uses the principle of interference to deduce the original state of waves by analyzing the result of their combination. In interferometers, a single beam of light is split into two identical beams—each beam travels a separate path before being recombined together at a single detector. The combination of light reflected from the tissue of interest, and light reflected from a reference mirror, produces characteristic patterns of interference that are diagnostic of the mismatch between the reflected waves..
TIME DOMAIN OCT
In the earliest OCT systems, these interference patterns were assessed as a function of time—“time domain” OCT . In 1993, the first time domain OCT device capable of in vivo use in humans was developed termed StratusOCT. Although time domain OCT systems are capable of generating highresolution images (8-10 μm for StratusOCT), they require the use of a mobile reference mirror for the assessment of interference patterns—a requirement that limits their image acquisition speed (400 A-scans per second for StratusOCT). This is a major restriction for clinical practice as only sparse coverage of the macular area is possible when acquiring any given image set.
SPECTRAL DOMAIN OCT
Fortunately, this technological hurdle has been overcome in recent years with the introduction of “spectral domain” OCT—a technology that removes the requirement for a mobile reference mirror by the assessment of interference patterns as a function of frequency rather than of time.
Spectral domain OCT systems use spectral interferometry and a mathematical function (Fourier transformation) to assess interference patterns as a function of frequency. Thus, light scattered from different depths within the tissue can be measured simultaneously, and images can be acquired 50 to 100 times more quickly than in time domain systems (typically over 20,000 A-scans per second).
The first in vivo imaging of the human retina using spectral domain OCT was reported in 2002. Each of the recently introduced commercial spectral domain OCT systems is capable of acquiring sizable image sets over short time periods. As a result, new methods of utilizing these image sets have evolved, significantly enhancing the evaluation of vitreoretinal disorders. Fundus photography and fluorescein angiography yield diagnostic information about retinal topography . oct provides information that is like cross sectional biopsy and complimentary to topography .
RETINAL IMAGING WITH SPECTRAL DOMAIN TECHNOLOGY
Raster Scanning
The high speed of spectral domain OCT facilitates significantly greater sampling of the macular area via the use of raster scanning protocols. and Raster scans consist of a rectangular pattern of horizontal line scans that run in parallel across the macula (e.g., 128 B-scans, with each B-scan consisting of 512 A-scans). The greater retinal sampling density of spectral domain raster scans may facilitate early detection of morphologic changes in disease states, as well as allowing these changes to be more accurately followed over time.
When dense raster scanning is performed, spectral domain OCT systems have the ability to perform volumetric rendering of the OCT images.
OCT Fundus Images
Another important feature of spectral domain OCT is the ability to generate “OCT fundus images” that mimic the images obtained from standard fundus photography
RETINAL IMAGE INTERPRETATION IN NORMAL EYES
Normal retina layers
- internal limiting membrane ( hyper reflective )
- nerve fiber layer ( hyper reflective )
- ganglion cell layer ( hypo reflective )
- inner plexiform layer ( hyper reflective )
- inner nuclear layer ( hypo reflective )
- outer plexiform layer ( hyper reflective )
- outer nuclear layer ( hypo reflective )
- external limiting membrane ( hyper reflective )
- photoreceptors ( hyper reflective )
- pigment epithelium ( hyper reflective )
vitreous
internal limiting membrane
nerve fiber layer
ganglion cell layer & inner plexiform layer
inner nuclear layer & outer plexiform layer
outer nuclear layer & external limiting membrane
photoreceptors
pigment epithelium
bruch’s membrane & choriocapilaris
choroid
sclera
Oct normal macula
oct showing different retinal layers
The high axial resolution offered by OCT is particularly well suited to assessment of the multilayered retinal structure On OCT false-color B-scans, highly reflective tissue is reddish-white in color, while hyporeflective tissue is blue-black in color.
| In most scans, the first hyperreflective layer detected is the internal limiting membrane (ILM) at the vitreoretinal interface. In a subset of the population, the posterior hyaloid may be seen as a thin hyperreflective layer above the ILM. Next to ILM is hyperreflective NFL . outer to NFL is hyporeflective ganglion cell layer . outer to ganglion cell layer is hyper reflective inner plexiform layer . next is hyporeflective inner nuclear layer of bipolar cells . next is hyper reflective outer plexiform layer . next if hyper reflective ELM .Within the retina, the ILM , RNFL and both the inner and outer plexiform layers & ELM are seen as hyperreflective layers while the ganglion cell layer , inner and outer nuclear layers are hyporeflective. |
Correlation of OCT images with the microstructure of the outer retina is less well defined.
Four bands are resolved in outer retina
Inner most band ELM
Second band boundary between IS / OS of photoreceptors
Third band OS tip
The fourth hyper reflective outer retinal band is RETINAL PIGMENT EPITHELIUM
The high axial resolution offered by OCT is also well suited to the objective, accurate measurement of retinal thickness. OCT uses image processing techniques to automatically detect the inner and outer retinal boundaries on OCT B-scans (segmentation) and thus provide measurements of retinal thickness. Using these techniques, it is possible to measure retinal thickness at multiple locations and to construct retinal thickness maps corresponding to the Early Treatment of Diabetic Retinopathy Study (ETDRS) subfields.
OCT IN DIFFERENT RETINAL DISEASES
OCT is useful for differentiating lamellar from pseudo- and full-thickness macular holes, diagnosing vitreomacular traction syndrome, differentiating various presentations of traction-related diabetic macular edema, monitoring the course of central serous chorioretinopathy, making treatment decisions in the management of age-related maculardegeneration (AMD), and evaluating eyes for subtle subretinal fluid that is not visible with FA. A benefit of higher-resolution systems is the ability to better delineate retinal layers, including the internal, middle, and external limiting membranes and the junction between the inner and outer photoreceptor segments.
OCT can also produce a retinal thickness map. The OCT software automatically determines the inner and outer retinal boundaries and produces a false-color topographic map showing areas of increased thickening in brighter colors and areas of lesser thicken -ing in darker colors. An assessment of macular volume can also be obtained from the retinal thickness map. By evaluating differences in retinal volume over time, the clinician can judge the efficacy of therapy. Time-domain OCT produces retinal thickness maps from 6 x 6-mm radial scans centered on the fovea, with interpolation between the scan lines, to produce a map of the macula. In contrast, Fourier-domain OCT can image the entire macula through its increased scanning speed and improved accuracy of thickness and volume measurements; it also improves registration, allowing for repeated imaging of the same area during follow-up visits.
Diabetic retinopathy
Fundus imaging is required to document various morphological features of diabetic retinopathy . color photography is gold standard to detect retinal haemorrhages , microaneurysms and vascular abnormalities . oct is the principal method to measure retinal thickness . it is the principal modality to document and detect macular odema even when biomicroscopy and fundus photography fails .
OCT role in DME
Confirm presence of macular edema
Know type of macular edema
Assess macular thickness
Vitero macular interface abnormalities
Intra retinal exudates
Sub retinal fluid
Photoreceptor IS / OS junction abnormalities
Know response to laser , IV pharmacotherapy & vitreo retinal surgery
For follow up and documentation.
Here OCT is very helpful in measuring central foveal thickness. OCT has become the gold standard in monitoring the progression and treatment response in DME patients . It give micrometer sensitive measurements in central retinal thickness. Retinal thickness is the most commonly used quantitative parameter derived from oct measurements . cirrhus oct measures the retinal thickness between internal limiting memebrane and anterior edge of rpe layer . normal subjects central retinal thickness is 265 µm with cirrhus oct .
DME can be classified as
| Diffuse retinal thickness | Sponge like generalized mild hypo reflective swelling of retina |
| Cystoid macular odema | presence of intra retinal cystoid areas of low reflectivity & separated by higher reflectivity septa |
| Serous retinal detachment | focal elevation of neurosensory retina overlying a hyporeflective dome shaped space . |
| Viteromacular interface abnormalilities | may involve epiretinal membrane or vitreo macular traction or both |
AGE-RELATED MACULAR DEGENERATION
AMD is the leading cause of irreversible visual loss in people aged 50 years or older in the developed world. The clinical hallmark of AMD is the deposition of acellular, polymorphous material, termed drusen, between the RPE and Bruch membrane. In early AMD, drusen are often accompanied by focal retinal pigmentary abnormalities. As AMD progresses, alterations in the RPE often accumulate, resulting in the loss of large areas of RPE and outer retina, a phenomenon termed geographic atrophy (GA). In some patients with AMD, the abnormalities in the outer retina, Bruch membrane, and choroid may also result in the development of choroidal neovascularization (CNV), the characteristic feature of neovascular AMD.
In neovascular AMD, abnormal blood vessels develop from the choroidal circulation, pass anteriorly through breaks in Bruch membrane, and then proliferate in the sub-RPE or subretinal space. CNV lesions may thus result in pigment epithelium detachment (PED), fluid exudation, lipid deposition, subretinal hemorrhage, and ultimately fibrotic scar formation with irreversible visual loss. FA remains the gold standard for initial diagnosis of CNV, the defining characteristic of neovascular AMD. Correlation between OCT findings and leakage and staining on FA has been . with the recent introduction of antiangiogenic agents, e.g., ranibizumab (Lucentis; Genentech, South San Francisco, CA) and the need for frequent monitoring of patients receiving these treatments, OCT has emerged as an essential tool in the follow-up of patients with AMD. FA is useful for determining presence of leakage in neovascular AMD, this technique does not provide any three-dimensional anatomic information about retinal layers, the retinal pigment epithelium (RPE), or the choroid. The development of optical coherence tomography (OCT) makes it possible to have cross-sectional images of the macula or optic nerve head analogous to ultrasonography. SD-OCT has the advantage of detecting small changes in the morphology of the retinal layers and subretinal space, allowing for precise anatomic detection of structural changes that may correspond to progression or regression of the neovascular lesions Monitoring response to therapy is one of the most important clinical uses of OCT. OCT can be used to quantify changes in central retinal thickness, macular volume, and subretinal fluid. These parameters, in combination with visual acuity, are used to analyze the response to therapy in several diseases including neovascular AMD . oct is also helpful in other retinal conditions diagnosis . oct is also helpful in management of glaucoma by doing optic nerve head ( ONH ) and retinal nerve fiber layer ( RNFL ) analysis . oct also helps in finding out thickness of cornea and anterior chamber angle . hence oct is a versatile instrument which aids in diagnosis and management of different conditions ranging from retina to optic nerve head to retinal nerve fiber layer to cornea .
DRISHTI EYE HOSPITAL 