Optical Coherence Tomography
Overview
Optical Coherence Tomography (OCT) is a non-invasive diagnostic technique that renders an in vivo cross-sectional view of the retina. OCT utilizes a concept known as interferometry to create a cross-sectional map of the retina that is accurate to within at least 10-15 microns. OCT was first introduced in 1991 by Huang and colleagues[1] and has found many uses outside of ophthalmology, where it has been used to image certain non-transparent tissues. Due to the transparency of the eye (i.e. the retina can be viewed through the pupil), OCT has gained wide popularity as an ophthalmic diagnostic tool.
Time Domain vs. Spectral Domain vs. Swept Source
From its inception, OCT images were acquired in a time-domain fashion. Time domain systems acquire approximately 400 A-scans per second using 6 radial slices oriented 30 degrees apart. Because the slices are 30 degrees apart, care must be taken to avoid missing pathology between the slices.
Spectral-domain technology,[2] on the other hand, scans approximately 20,000-40,000 A-scans per second. This increased scan rate and number diminishes the likelihood of motion artifacts, enhances the resolution, and decreases the chance of missing lesions. Spectral-domain systems increase the signal-noise ratio by image-averaging multiple B-scans at the same location. Whereas most time domain OCTs are accurate to 10-15 microns, newer spectral domain machines may approach 3-micron resolution. Whereas most time domain OCTs image 6 radial slices, spectral domain systems continuously image a 6mm area. This diminishes the chance of inadvertently missing pathology. Spectral-domain systems typically operate at 800-870 nm wavelengths, although longer wavelengths of 1050-1060 nm are being developed for deeper penetration in the tissue.
In the late 2000s, the advent of enhanced depth imaging (EDI)[3] allowed for better visualization of the choroid and choroidoscleral interface using the spectral domain system. EDI employed the use of image averaging and it set the zero-delay line to adjacent to the choroid.
Swept-source technology,[4] uses a wavelength-sweeping laser and dual-balanced photodetector, allowing for faster acquisition speeds of 100,000-400,000 A-scans per second. This technology uses longer wavelengths of 1050-1060 nm for deeper tissue penetration without the need for EDI. This wavelength provides an axial resolution of about 5.3 um in tissue compared to the approximately 5 um axial resolution of the standard 800 nm wavelength of commercial spectral domain devices. The enhanced axial resolution along with the faster scanning speeds, which allows for greater image averaging, improves image quality and the ability to visualize deeper structures in more detail.
Most recently, both spectral domain and swept source OCT have been used to generate non-invasive non-dye-based OCT angiography (OCTA)[1] images. In brief, OCT angiography uses motion contrast by comparing the decorrelation signal between multiple B-scans obtained at each retinal cross-section to detect blood flow, employing the principle that theoretically only circulating erythrocytes within the retinal capillaries should be moving in the retina.
International Nomenclature for OCT
Suggested the terms band, layer, and zone for the layers of the retina
•The term band refers to the three-dimensional structure of the retinal layers anatomically.
•The term zone describes those regions on OCT whose anatomical correlation is not clearly delineated.
•The RPE/Bruch’s complex is one of the layers ascribed as zone as they are inseparable owing to interdigitation of cellular structure or tissue.
ARTIFACTS on OCT
Mirror Artifacts : It occurs when the area of interest to be imaged crosses the zero delay line and results in an inverted image
VIGNETTING :
This occurs when a part of the OCT beam is blocked by the iris and is characterized by a loss of signal over one side of the image.
MISALIGNMENT:
This occurs when the fovea is not properly aligned during a volumetric scan. Typically it is due to the patient exhibiting poor or eccentric fixation or poor attention
Out of Range Error: Out of range error. Notice that the outer retina/choroidal image is cut off because of
improper positioning of the machine during image acquisition.
Blink artifact: blink artifacts result in partial loss of data due to the momentary blockage of OCT image acquisition during the blink. Blink artifacts are easily recognized as black horizontal bars across the OCT image and macular map.
Motion Artifact :This occurs when there is movement of the eye during OCT scanning leading to distortion or double scanning of the same area.
Uses
Retina
OCT is useful in the diagnosis of many retinal conditions, especially when the media is clear. In general, lesions in the macula are easier to image than lesions in the mid and far periphery. OCT can be particularly helpful in diagnosing:
- Macular hole
- Macular pucker/epiretinal membrane
- Vitreomacular traction
- Macular edema and exudates
- Detachments of the neurosensory retina
- Detachments of the retinal pigment epithelium (e.g. central serous retinopathy or age-related macular degeneration)
- Retinoschisis
- Pachychoroid
- Choroidal tumors
In some cases, OCT alone may yield the diagnosis (e.g. macular hole). Yet, in other disorders, especially retinal vascular disorders, it may be helpful to order additional tests (e.g. fluorescein angiography or indocyanine green angiography).
Pseudohole:
The OCT scan of a pseudohole will reveal an epiretinal membrane with contraction of the retina and sometimes compression of the retinal layers, but no loss of retinal layers.
The term “pseudohole” reflects the fact that although this looks like a macular hole, it is not a hole in the retina.
Ectopic inner foveal layer (EIFL)
EIFL is characterized by the presence of continuous hyporeflective and hyperreflective bands extending from the inner nuclear layer and inner plexiform layer across the foveal region.
Inner retinal hyperreflectivity ( Diffuse)
In CRAO ,inner retinal layers appear as a hyperreflective demarcated and mostly edematous band.
Focal hyperreflectivity in Hyperreflective Foci (HRF) :
These are typically dot like or round regular lesions seen in all the retinal layers and choroid,less than 30 microns in size . They typically lack back-shadowing and do not have a representative visible fundus lesion.
DRIL ( Disorganized retinal inner layers ) : DRIL is identified when the boundaries of the ganglion cell layer , inner plexiform layer , inner nuclear layer , and outer plexiform layer could not be identified and demarcated .It is a potential biomarker for DME.
Hyporeflectivity :
Cystoid macular edema :
Cystoid macular edema can be seen on OCT scans as multiple circular cystic spaces in the retina, indicating intraretinal edema.
Retinoschisis:
There is splitting of retinal layers at outer plexiform layer.
Pearl necklace sign :
Hyperreflective dots in a contiguous ring around the inner wall of cystoid spaces in the outer plexiform layer of the retina ; usually seen in exudative macular diseases.
Paracentral Acute middle maculopathy PAMM :
It is an OCT finding characterized by parafoveal hyperreflective band at the level of inner nuclear layer.
ISOS Loss
Disruption of the ISOS line has been demonstrated to correlate with retinal function loss in several retinal disorders and is considered a useful indicator of photoreceptor integrity and predictor of visual function.
ILM Drape Sign in IPFT
The ILM drape sign occurs when a thin membrane overhangs this central cystoid lesion at the base of the fovea of normal contour and thickness. It is seen in macular telangiectasia 2.
Outer Retinal Tubulations ( ORT) :
It is a hyporeflective area surrounded by hypereflective band in the outer nuclear layer .It comprises interconnecting tubes containing degenerating photoreceptors, almost exclusively cones and muller cells.
Subretinal hyperreflectivity in CNVM :
This SD-OCT feature is identified as hyperreflective material located between neurosensory retina and retinal pigment epithelium (RPE).
Subretinal hyporeflectivity in SMD (Serous macular detachment)
Serous macular detachment (SMD) with cystoid diabetic macular edema (DME), shows retinal elevation with an optically clear space between the sensory retina and the retinal pigment epithelium.
Pigment epithelial detachment PED
Serous PEDs appear as well-demarcated, abrupt elevations of the RPE with a homogenously hyporeflective sub-RPE space.
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Idiopathic polypoidal choroidal vasculopathy IPCV
OCT features of IPCV are multiple PEDs, sharp PED peak, PED notch, and rounded sub-RPE hyporeflective area.
Choroidal NEVUS :
It is seen as Hyporeflectivity in anterior surface of choroid.
CONTOUR abnormalities :
Dome shaped maculopathy : It is an anterior convex protrusion of the macula towards the vitreous cavity seen on OCT. It is associated with high myopia and posterior staphyloma.
Posterior Staphyloma
It is seen as an outward globe bulge, resulting in a deep concave B-scan OCT and distorted retinal structures.
Bacillary layer detachment : a photoreceptor splitting detachment
Note the hyperreflective material along the outer retinal surface. A thin band at the base of cystic detachment is continuous with the adjacent ellipsoid band and the external limiting membrane.
Focal Choroidal Excavation
It is defined as an area of concavity in choroid detected on OCT . These are mostly present in macular region without evidence of accompanying scleral ectasia or posterior staphyloma.
Foveal Pseudocyst
The presence of subfoveal perfluorocarbon liquid (PFCL) after vitreoretinal surgery can cause the appearance of cystoid foveal edema.
Other OCT Signs :
Dipping sign
Dipping (tenting down) sign may be observed in some acute CSCR patients.
It is characterized by dipping or tenting at the outer surface of detached neurosensory retina due to hyperreflective material accumulation such as subretinal fibrin or fibrinous exudate connecting the detached neurosensory retina and RPE at its opposite.
OMEGA Sign :
OMEGA sign is an omega-shaped disorganization of inner retinal layers bounded posteriorly by the outer plexiform layer.
Omega sign is a characteristic feature of macular CHRRPE and may help to distinguish macular CHRRPEs from ERMs.
ONION Sign :
The "onion sign" refers to layered hyper-reflective bands in the sub-RPE space usually associated with chronic exudation from type 1 neovascularization in patients with AMD.
Cotton Ball sign :
It is a roundish or diffuse highly reflective region observed between the photoreceptor inner segment/outer segment junction line and the cone outer segment tip line at the center of the fovea.
This highly reflective region is a characteristic sign observed in the OCT images of eyes with VMT and ERM.
BRUSH BORDER PATTERN :
This is an irregular, serrated, and thicker appearance of the detached neurosensory retina seen in CSCR.
It is due to accumulation of waste products in the photoreceptor outer segment on the outer surface of the detached neurosensory retina over subretinal fluid.
Various Classifications based on OCT
Macular Hole classification:
Diabetic macular edema classification:
| OCT CLASSIFICATION OF DME | OCT features |
| 1 | Diffuse retinal thickening |
| 2 | Cystoid macular edema |
| 3 | Posterior hyaloid traction |
| 4 | Subretinal fluid/serous retinal detachment |
| 5 | Tractional retinal detachment |
Ectopic inner foveal layer Classification:
| Ectopic Inner Foveal Layer classification | |
| Stage 1 ERM | Epiretinal membrane with foveal preservation |
| Stage 2 ERM | ERM
Loss of foveal depression Thickening of ONL |
| Stage 3 ERM | ERM
Loss of foveal depression Continuous & clearly identified EIFL |
| Stage 4 ERM | ERM with loss of foveal depression
EIFL with anatomy & identification completely lost |
Optic nerve
OCT is gaining increasing popularity when evaluating optic nerve disorders by accurately and reproducibly evaluating the retinal nerve fiber layer and ganglion cell layer thickness:
- Glaucoma
- Optic neuritis
- Non-glaucomatous optic neuropathies
- Alzheimer's disease
Anterior segment
Anterior segment OCT utilizes higher wavelength light than traditional posterior segment OCT. This higher wavelength light results in greater absorption and less penetration. In this fashion, images of the anterior segment (cornea, anterior chamber, iris, and angle) can be visualized.
Limitations
Because OCT utilizes light waves (unlike ultrasound which uses sound waves) media opacities can interfere with optimal imaging. As a result, the OCT will be limited the setting of vitreous hemorrhage, dense cataracts, or corneal opacities.
As with most diagnostic tests, patient cooperation is a necessity. Patient movement can diminish the quality of the image. With newer machines, acquisition time is shorter which may result in fewer motion-related artifacts.
The quality of the image is also dependent on the operator of the machine. Early models of OCT relied on the operator to accurately place the image over the desired pathology. When serial images were acquired over time (e.g. during treatment for AMD with anti-VEGF therapy), later images could be taken that were off-axis compared to earlier images. Newer technologies, such as eye-tracking equipment, limit the likelihood of acquisition error.
Additional Resources
- Turbert D, Janigian RH. Optical Coherence Tomography. American Academy of Ophthalmology. EyeSmart/Eye health. https://www.aao.org/eye-health/treatments/optical-coherence-tomography-list. Accessed March 21, 2019.
References
- ↑ Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science. 1991;254(5035):1178-1181.
- ↑ Wojtkowski M et al. Ophthalmic imaging by spectral optical coherence tomography. Am J Ophthalmol. 2004 Sep;138(3):412-9.
- ↑ R.F. Spaide, H. Koizumi, M.C. Pozzoni. Enhanced Depth Imaging Spectral-Domain Optical Coherence Tomography. Am J Ophthalmol. 2008;146(4):496-500.
- ↑ Adhi M, Liu JJ, Qavi AH, et al. Choroidal Analysis in Healthy Eyes using Swept-Source Optical Coherence Tomography Compared to Spectral Domain Optical Coherence Tomography. Am J Ophthalmol. 2014;157(6):1272-81.e1
