Understanding Today's State-of-the-art OCT Technology—and Anticipating Tomorrow's
This family of devices has changed the way ODs diagnose and monitor disease. This guide explains when to use which application.
By Lee Vien, OD, and David Yang, OD
Release Date: September 2017
Expiration Date: September 15, 2020
Goal Statement: With optical coherence tomography techniques expanding, optometrists need to know when to apply which modalities to which patients. This article explains how OCT—with an emphasis on enhanced depth imaging, swept source technology and OCT angiography—can be applied clinically to diagnose and monitor macular disease, diabetic retinopathy and glaucoma, among others.
Faculty/Editorial Board: Lee Vien, OD, David Yang, OD
Credit Statement: This course is COPE approved for 2 hours of CE credit. Course ID is 54707-PD. Check with your local state licensing board to see if this counts toward your CE requirement for relicensure.
Authors: The author has no relationships to disclose.
Editorial staff: Jack Persico, Rebecca Hepp, William Kekevian, Michael Riviello and Michael Iannucci all have no relationships to disclose.
40th Annual Technology Report
Follow the links below to read the other articles from our 40th annual Technology Report:
Here, we provide an overview of the many OCT modalities and how you can apply them to the patients walking into your clinic.
Using OCT, Cover to Cover Anterior segment OCT (AS-OCT) is now available on many instruments. Retinal OCTs can capture and analyze anterior segment scans with a license and software upgrade. Some OCTs require anterior lenses that attach to the instrument to provide wide-view scans to visualize the anterior chamber or angle-to-angle anatomy.
AS-OCT can analyze anterior chamber depth, angle measurements and provide pachymetry maps. Its clinical applications range from rigid contact lens fitting to corneal pathologies and complications from corneal surgeries as well as conjunctival lesions. It can also provide an assessment of the tear meniscus in dry eye patients.
|SD-OCT line scan with (at left) and without enhanced depth imaging, which allows for visualization of the choroidal layers and the choroid-scleral interface. Click image to enlarge. |
AS-OCT can be a useful technique for evaluating anterior chamber inflammation in patients with uveitis. This imaging is especially useful in patients with decreased corneal clarity from corneal edema.1
For glaucoma evaluation, AS-OCT can provide valuable images of filtering blebs and tube positioning, as well as the anterior chamber angle, trabecular meshwork and Schlemm’s canal. These images aid ODs in the assessment of structural abnormalities.2-4
Although AS-OCT does not replace gonioscopy in the detection of angle closure, it has increased our understanding of the disease and helped us identify new risk factors so that treatment can be targeted to those patients with higher risk.5,6
Posterior segment OCT provides two main types of macular scans: line scans and macular volume scans. Line scans have adjustable orientation and length and provide a higher definition image of a small area. Macular volume scans, also called cube scans, can provide a three-dimensional image of a larger area, relative to a line scan. Macular volume scans provide macular thickness measurements in the nine standard Early Treatment Diabetic Retinopathy Study macular grid subfields.7 The boundaries of the macular thickness scans are typically the internal limiting membrane (ILM), the inner boundary to the retinal pigment epithelium (RPE) and the outer boundary.8 Variations in the outer boundary, either above or below the RPE, are instrument dependent. Analysis software can help determine whether progression or change has occurred, allowing for the tracking of retinal thickening or thinning over time.
|This OCT-A image (6mm by 6mm) shows a patient with a previous branch retinal artery occlusion, as indicated by the well-demarcated area of flow interruption in both superficial and deep vascular capillary plexuses visible on the scan. Click image to enlarge.|
For optic disc scans, OCT captures the circumpapillary retinal nerve fiber layer (cpRNFL) thickness is evaluated. The scan circle diameter can vary with each instrument. The data from the cpRNFL is sampled and thickness values are generated. These can be divided into clock hours, sectors and quadrants that are then compared with a normative database. Optic nerve head (ONH) parameters, such as disc area, rim area, cup-to-disc ratio and cup volume, are also captured and research demonstrates they are sensitive in the detection of glaucomatous change.9,10 Thickness maps based on a thickness scale and deviation maps based on a normative database provide spatial and morphologic information of RNFL damage and can detect RNFL defects.9,10
Macular ganglion cell thickness is now available on SD-OCT instruments. The macular parameters are the ganglion cell complex (GCC)—which includes the ganglion cell layer (GCL), inner plexiform layer (IPL) and RNFL, and ganglion cell-inner plexiform layer (GCIPL)—which includes the GCL and IPL.6
For the diagnosis of early glaucoma, these macular parameters have a high discriminating power and reproducibility comparable to peripapillary retinal nerve fiber layer (pRNFL).11 Macular ganglion cell thickness analysis is also a more reliable diagnostic parameter in the detection of glaucoma for myopic optic disc tilt than ONH parameters.12,13 In advanced stages, research shows GCIPL glaucoma progression analysis (GPA) is more useful than cpRNFL GPA in detecting disease progression.14
OCT can track non-exudative age-related macular degeneration (AMD) progression and identify many features related to the
degenerative process.15-17 These findings include reticular drusen, subretinal drusen deposits, pseudocysts, outer retinal tubulations and drusen-associated acquired vitelliform lesions.15-17
OCT is now routinely used to evaluate diabetic macular edema (DME) and macular edema associated with other conditions, such as retinal vein occlusions. The instrument allows an objective quantification and localization of retinal thickening, providing the ability to track treatment response. Many large clinical trials incorporate OCT measurements into their definitions of macular edema and use those measures as a clinical endpoint.18-22 DME, for instance, was defined by reduced vision and an increase in central subfield thickness on OCT in the RISE and RIDE studies.18 The mean change of central foveal thickness based on OCT was also evaluated as a secondary outcome in the trials.18
New techniques, such as enhanced depth imaging (EDI), SS-OCT and OCT angiography (OCT-A), are providing further insight into ocular structures, such as the choroid, lamina cribrosa and ocular vasculature.
Enhanced Depth Imaging
This recently developed modality allows for improved choroidal evaluation by producing images of structures such as Bruch’s membrane, the choriocapillaris complex, Sattler’s layer, Haller’s layer and the suprachoroidal layer.13 Studies on choroidal thickness with EDI-OCT have identified differences with age, myopia, female gender and increased intraocular pressure (IOP), as well as in certain ocular pathologies such as central serous chorioretinopathy, polypoidal choroidal vasculopathy, vitelliform macular dystrophy and glaucoma.24-28
Researchers have extensively studied choroidal thickness in glaucoma with EDI-OCT.29-33 The pathogenesis of glaucoma has been linked to ischemia that occurs in the prelaminar area of the optic nerve.34 The blood supply to the optic nerve originates from the branches within the peripapillary choroid. OCT research supports an association between glaucoma and impaired choroidal circulation.35 Research using EDI-OCT does not appear to highly correlate choroidal thickness with either open-angle or normal-tension glaucoma, or the severity of glaucomatous damage.31-33
Investigators also studied the lamina cribrosa (LC) and its association with glaucoma.36-38 LC thickness has been associated with severity of glaucoma and found to be thinner in normotensive glaucoma eyes and pseudoexfoliative glaucoma eyes than in primary open-angle glaucoma eyes with the same severity of damage.36-38 In addition, a thinner LC and a larger LC displacement had a significant influence on the rate of progressive RNFL thinning.39
|This OCT-A image (3mm by 3mm) of both superficial and deep vascular capillary plexuses shows capillary nonperfusion from severe nonproliferative diabetic retinopathy. An abnormal vascular network is visible adjacent to the area of nonperfusion. Click image to enlarge.|
SS-OCT provides higher image acquisition speed (~100,000 A-scans/seconds), use of a longer wavelength (1050nm to 1060nm), and less variation in sensitivity with depth compared with SD-OCT.40,41 These differences offer advantages over SD-OCT when imaging the vitreous and choroid. In addition, modalities such as en face and OCT-A imaging are improved with SS-OCT as they aid in imaging further into the anterior, as well as deeper into the ocular tissue, than we could with SD-OCT.40,41 Structures in the vitreous that can be visualized with SS-OCT include the posterior precortical vitreous pockets—which play a role in various vitreomacular disorders—Cloquet’s canal, posterior cortical vitreous, posterior hyaloid and vitreous opacities.42,43
The longer wavelength used in SS-OCT provides better penetration of ocular tissues and results in less scattering at the photoreceptor and RPE layers, allowing simultaneous visualization of the choroid, sclera and vitreous.42,44 SS-OCT also offers automated segmentation of choroidal thickness and choroidal thickness maps to help identify abnormalities in the choroid.
Studies comparing EDI-OCT and SS-OCT on penetration depth in choroidal imaging have found both imaging modalities to be comparable in normal eyes.45 However, research shows in conditions such as PCV where the choroid is abnormally thicker, SS-OCT may better detect the choroid-sclera interface.46 SS-OCT also has an advantage over EDI-OCT in imaging the posterior sclera in myopic glaucoma eyes.47 Compared with SD-OCT, SS-OCT has similar intradevice repeatability on the assessment of cpRNFL and macular GC-IPL thickness.48 Imaging of the LC has also been studied with SS-OCT, which can provide a three-dimensional image to better evaluate LC morphology, compared with SD-OCT.49
Although the literature has reported on SS-OCT technology for the last few years, the instruments were mainly prototypes for research. Recently, the FDA approved the first commercially available SS-OCT for assessment of the retina, the Plex Elite 9000 SS-OCT (Carl Zeiss) which has an axial resolution of 5.5µm and a scan speed of 100,000 A-scan points per second. This instrument—as well as the not-yet-FDA-approved Triton SS-OCT (Topcon)—offers widefield scans and OCT-A, but differ in their software and analysis. The Triton’s normative database is not FDA approved in the United States, and the current Plex Elite 9000 does not have a reported normative database.
|This SD-OCT PanoMap shows a glaucoma patient with thinning of the retinal nerve fiber layer as well as ganglion cell analysis. Click image to enlarge. |
This technique employs sequential high-speed OCT scans taken at the exact same location to detect motion. Movement is determined when the camera detects a difference in signal intensity between the two scans. Assuming steady patient fixation, the successive OCT scans should be identical, except for moving elements such as erythrocytes. The movement of red blood cells can be seen due to variation from one image to another, thus allowing for examination of blood flow in the retinal vasculature at various depths. This makes OCT-A a beneficial adjunctive test to fluorescein angiography (FA) and indocyanine green angiography (ICG).
OCT-A can delineate the foveal avascular zone and areas of capillary dropout better than FA and ICG.50-54 However, there are limitations to OCT-A, such as a small field of view, making it a poor technique for imaging the peripheral retina. Additionally, it is not able to show leakage, staining or pooling, which FA and ICG can.50-52,55 Moreover, current OCT-A instruments are prone to artifacts from eye movement, eyelid blinks, signal attenuation, shadows and segmentation errors. These movements can result in vessel doubling, stretch artifacts, displacement artifacts, checkerboard defects, gap defects and false-positive flow defects. Blockage and attenuation of the OCT-A signal can occur due to blinking and ocular media opacities such as cataracts, vitreal floaters and corneal scars. If the signal is sufficiently blocked, it can result in vascular areas misidentified as vasculature loss or areas of no flow. Other artifacts can result from errors in the segmentation of the various retinal layers. If layers are misidentified, data can be combined from multiple layers, resulting in inappropriate mapping.51,53,56,57
Vascular layers deep to the RPE can be imaged better with swept-source technology vs. SD-OCT, owing to the deeper penetration of tissue with its longer wavelength. In a study comparing the two techniques for imaging the choriocapillaris beneath drusen, SS-OCT-A was found to be less prone to false-positive low flow areas.58
OCT-A is currently being used to study various retinal and choroidal pathologies, as well as ONH disorders.50-53 Increasingly, the evidence in the literature describes its use in AMD, diabetic retinopathy and vascular occlusive disease, as well as other retinal conditions.50-53 In some cases, it is even detecting pathology that was not seen on FA.59,60 Doctors can use OCT-A’s ability to detect movement to identify nonexudative type 1 neovascular (NV) lesions, even in asymptomatic patients with intermediate AMD.59,60 These lesions were not appreciated on FA and there was no fluid seen on structural OCT imaging, although these areas of neovascularization were associated with plaques seen on ICG.59,60 A recent study showed the combination of OCT-A and structural OCT can detect type 1 NV lesions in 85.7% of eyes, which was superior to structural OCT or FA alone (66.7%).61
In patients with diabetic retinopathy, OCT-A can better delineate the foveal avascular zone and areas of capillary dropout compared with FA, while also providing a higher percentage of gradable images.54
OCT-A is also being used to examine various optic neuropathies, including glaucoma, papilledema, anterior ischemic optic neuropathy and other acute and chronic optic nerve disorders.62-64 Studies show OCT-A can provide noninvasive imaging of the microvasculature of the ONH and adjacent peripapillary tissue.62-64 Research demonstrates decreased peripapillary perfusion—as imaged by OCT-A—has excellent correlation with areas of RNFL atrophy—as measured by structural OCT—in glaucoma patients.63
Additionally, other studies have revealed a decrease in vessel density of the peripapillary capillary network in eyes with various optic neuropathies. This supports the relationship of decreased retinal vasculature associated with glaucomatous and nonglaucomatous RNFL atrophy.62-64
|This OCT-A (6mm by 6mm) image shows the same patient as the image on page 73. Here, you can see a decrease in peripapillary capillary perfusion corresponding to the areas of RNFL loss. Click image to enlarge.|
Limitations of Today’s OCT
Artifacts can be a source of error when obtaining OCT images, and they may affect the measurement of retinal thickness. These artifacts include motion artifacts and media opacities that can block the signal. Some pathologies can also contribute directly to segmentation errors, such as epiretinal membranes, vitreopapillary traction or vitreomacular traction. Interpretation of the OCT color-coding can be falsely positive or negative given the limited normative database for each instrument. These databases have a narrow age range, refractive error range and limited ethnic groups.65,66 Patients with displaced RNFL bundles, such as high myopia and tilted discs, are examples of patients who may not fit the normative database. For these patients that fall outside the normative database, longitudinal evaluation of the cpRNFL is important to detect disease progression.
Recent innovations in OCT imaging have improved the diagnostic ability of the instrument, especially in the detection of glaucomatous damage. These new OCT parameters include Bruch’s membrane opening-minimum rim width (BMO-MRW) and BMO-minimum rim area (MRA). BMO-MRW quantifies the rim from its true anatomic outer border and accounts for variable orientation. Its width has a higher diagnostic accuracy for glaucoma and a stronger correlation with visual field compared with conventional rim parameters including cpRNFL, according to researchers.67,68 The new parameter is also more specific than GCIPL in tilted disc with moderate myopia.69 BMO-MRA is not influenced by disc size, unlike BMO-MRW, and offers superior diagnostic power to detect glaucoma compared with other parameters in a full range of very large to small disc sizes.70 Additional studies using these new parameters are still needed to establish a normative database. However, current research shows a greater specificity of these new parameters when considering the highly variable anatomy of the ONH both within and between individuals.67-70
The advent of OCT technology has improved optometrists’ ability to detect, understand, monitor and track progression of various ocular pathologies, as well as response to treatment modalities. Advances in imaging techniques with EDI-OCT and SS-OCT have provided the ability to visualize the choroid and gain new insights on the role of the choroid in chorioretinal conditions. SS-OCT technology has also improved imaging of the vitreous and enhanced techniques such as en face and OCT-A. With the introduction of OCT-A, we now have the ability to visualize abnormal blood flow in the choroid and retina.
New software and hardware upgrades have improved the repeatability, reliability and diagnostic capability of OCT scans. Recently introduced ONH parameters, such as BMO-MRW and BMO-MRA, have shown potential on its specificity, especially in glaucoma detection. OCT has many applications in clinic today, but future studies will determine the full extent to which these new technologies can improve patient care.
Dr. Vien practices at the Veterans Affairs Palo Alto Healthcare System and is an assistant clinical professor at University of California Berkeley School of Optometry and Southern California College of Optometry at Marshall B. Ketchum University.
Dr. Yang practices at the Veterans Affairs Palo Alto Healthcare System and is a clinical professor at the University of California Berkeley School of Optometry.
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