Optical Coherence Tomography Angiography (OCT-A) | Retina & Glaucoma

Optical Coherence Tomography Angiography (OCT-A) | Retina & Glaucoma

September 22, 2021

Optical coherence tomography angiography (OCTA) is a new non-invasive imaging technique that generates volumetric angiography images in a matter of seconds. This is a nascent technology with a potential wide applicability for retinal vascular disease.

In contrast to standard structural optical coherence tomography, OCTA analyzes not only the intensity of the reflected light but also the temporal changes of the OCT signal. Δ

Based on repeated OCT section images (B-Scans) from the same location of the retina, it is possible to separate the temporal signal changes caused by moving particles (such as erythrocytes flowing through vessels) from other sources of signal change (i.e. eye motion or noise in the OCT signal).

Thereby image contrast between perfused vessels and static surrounding tissues can be created. 

Optical coherence tomography angiography (OCT-A) is an emerging, non-invasive imaging technique that generates angiographic images of the retina and choroid within seconds. Δ

Optical coherence tomography angiography (OCTA) employs motion contrast imaging to high-resolution volumetric blood flow information generating angiographic images in a matter of seconds. Δ Δ

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A Breakthrough In Ophthalmology News

A Major Breakthrough In Ophthalmic News - Optical coherence tomography angiography (OCTA) is a recent advancement which is a non-invasive alternative to fundus fluorescein angiography (FFA) and indocyanine green angiography (ICGA) to assess the retinal and choroidal microvasculature.

OCTA compares the decorrelation signal (differences in the backscattered OCT signal intensity or amplitude) between sequential OCT b-scans taken at precisely the same cross-section in order to construct a map of blood flow. Δ

Axial bulk motion from patient movement is eliminated so sites of motion between repeated OCT b-scans represent strictly erythrocyte movement in retinal blood vessel.

OCT-A technology uses laser light reflectance of the surface of moving red blood cells to accurately depict vessels through different segmented areas of the eye, thus eliminating the need for intravascular dyes.

The OCT scan of a patient's retina consists of multiple individual A-scans, which when compiled into a B-scan provides cross-sectional structural information.

With OCT-A technology, the same tissue area is repeatedly imaged and differences are analyzed between scans (over time), thus allowing one to detect zones containing high flow rates (i.e. with marked changes between scans) and zones with slower, or no flow at all, which will be similar among scans. Δ Δ

In every sense of the word, this is a major development in Ophthalmology news.

Light is emitted through either a spectral domain OCT (SD-OCT), with a wavelength of near 800nm; or a swept-source OCT (SS-OCT), which utilizes a longer wavelength, close to 1050nm.

Longer wavelengths have a deeper tissue penetrance, but a slightly lower axial resolution. OCT-A employs two methods for motion detection: amplitude decorrelation or phase variance.

The former detects differences in amplitude between two different OCT B-scans. Phase variance is related to the emitted light wave properties, and the variation of phase when it intercepts moving objects. Δ

A Brillant Cornerstone In Ophthalmology News  - The New Non-Invasive Imaging Technique

First available commercially in 2014, OCT-A relies on motion contrast to produce an image of blood flow. The machine takes multiple sequential A-scans of the part of the retina and/or choroid of interest.

Because the retina itself is static, differences in the light reflected back are due to movement of blood through choroidal and retinal vessels. A map representing blood flow is then generated.

Two types of OCT-A machines are commercially available: spectral domain (SD-OCT-A) and swept source (SS-OCT-A). These types differ in the light source used. SD-OCT-A utilizes nearinfrared light (wavelength approximately 840 nm), whereas SS-OCT-A relies on a wavelength around 1050 nm.

The increased wavelength of SS-OCT-A permits faster scanning, larger scan areas, improved penetration through the retinal pigment epithelium and better visualization of the choroid.  

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Conventional angiography in the retina clinic relies on fluorescein angiography (FA) and indocyanine green angiography (ICG), two commonly used imaging modalities to image the vascular structures in the retina and choroid.

Generally, FA provides excellent visualization of the retinal vasculature, and ICG provides excellent visualization of the choroidal vasculature.

There are some key differences between FA/ ICG and OCT-A. FA/ICG typically requires placement of an intravenous cannula, and possible side effects include nausea, vomiting, skin rash and, in extremely rare circumstances, anaphylaxis.

OCT-A is non-invasive and can obtain images within seconds, whereas FA/ICG requires the use of a dye, most commonly through an intravenous injection, and the images are captured over approximately 10-15 minutes. Δ Δ

Dye that leaks out of the vasculature may obscure the images captured on FA/ICG, which is not a concern with OCT-A. Images of the deep capillary plexus and vascular networks in the choroid are generally of higher definition than those obtained using FA/ICG.

There are certain drawbacks to OCT-A. As OCT-A relies on motion contrast to generate images, the patient must remain as still as possible to capture the highest quality images and reduce artifact.

While absence of dye leakage may permit improved visualization of the vasculature, the presence of dye leakage is important to note as a diagnostic and prognostic feature in certain conditions, such as uveitis.

Last, in its current form, OCT-A does not have the ability to generate ultra-widefield images that include the far periphery of the retina and choroid. Thus, areas of interest that exist outside of the areas that OCT-A can capture will not be imaged. Δ

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Original research on OCT-A in retina has seen explosive growth since it became available in the last decade — a search on the NIH’s PubMed Central of the terms “optical coherence tomography angiography” and “retina” yields more than 1,400 results.

OCT-A has provided useful diagnostic information for a plethora of diseases seen in the retina clinic. Pathology in the macula is most accessible to imaging by OCT-A. Take the example of a 39-year-old woman with multifocal choroiditis who presented with metamorphopsia in the left eye.

SD-OCT showed disruption of the outer retinal structures and accumulation of abnormal material. Meanwhile, OCT-A revealed an abnormal network of vessels in the outer retina and choriocapillaris, consistent with neovascularization.

OCT-A in this case localizes the neovascularization to the outer retina and choriocapillaris. Multiple reports have described the imaging characteristics of CNV in AMD using OCT-A.

In diabetic retinopathy, OCT-A can identify microaneurysms, an enlarged foveal avascular zone, neovascularization and capillary non-perfusion.

In retinal artery occlusions, OCT-A shows decreased perfusion in retinal capillary plexuses, and in retinal vein occlusions OCT-A shows decreased perfusion as well as collaterals and enlargement in the foveal avascular zone.

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There is considerable discussion in the retina community regarding the utility of OCT-A technology in modern retina practice and whether OCT-A can serve as a replacement for conventional dye-based angiography.

Dye-based angiography remains the gold standard for evaluating retinal pathology. In its current form, OCT-A is likely not an all-encompassing replacement for FA and ICG due to the limitations discussed above. Δ

However, over the coming years, it is likely that OCT-A will continue to improve, allowing greater visualization of the retina and choroid in more detail.

Rather than seeing OCT-A as a potential replacement for dye-based angiography, it is best to view OCT-A as an adjunct imaging modality that can provide additional information regarding the pathophysiology of diseases affecting the retina and choroid.

For example, OCT-A provides higher definition images of the deeper layers in the retina than FA. This helps us better understand pathology that affects these layers such as acute macular neuroretinopathy, paracentral acute middle maculopathy and disorganization of the retinal inner layers in diabetic retinopathy.

Recently, OCT-A has allowed us to identify nonexudative macular neovascularization (MNV), an asymptomatic finding that may play a protective role against geographic atrophy.

Further, eyes with nonexudative MNV may be at higher risk of converting to exudative AMD. An awareness of these findings may influence how the retina specialist chooses to monitor a patient at risk for conversion to exudative AMD.

Our knowledge of disease processes has evolved considerably in recent years and will continue to evolve as OCT-A is more widely adopted and as the technology improves. Δ

OCT-A is an exciting technology, and many studies have demonstrated how it enhances our understanding of the pathophysiology of many diseases affecting the retina and choroid.

Compared to conventional angiography, it is fast and non-invasive, which may help improve clinic flow and enhance patient safety.

While it is not a replacement for conventional angiography in its current form, we will likely see expanding indications for its use in the coming years as the technology improves and as it becomes more widely adopted in modern retina practices. Δ Δ Δ

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OCT-A – Glaucoma

Glaucoma is the leading cause of irreversible blindness both worldwide and in the Western World. Since glaucomatous damage is preventable but irreversible, an early diagnosis and close follow-up of glaucoma patients are primordial.

Optical coherence tomography (OCT) angiography (OCTA) is a recently developed, noninvasive imaging modality that detects blood flow through the motion contrast generated by red blood cells.

It can be used to provide a quantitative assessment of the microcirculation of the retina and choroid in various layers.

Since glaucoma development and progression are both linked to the loss of retinal vessel density (as either a primary or a secondary effect), this technology has the potential to bring forward new information about the pathophysiology of glaucoma, as well as to help clinicians with glaucoma diagnosis and management.

With OCTA, the movement of red blood cells is used as a contrast to delineate blood vessels from static tissues.

The features seen with OCTA in eyes with glaucoma are reduction in the superficial vessel density in the peripapillary and macular areas, and complete loss of choriocapillaris in localized regions of parapapillary atrophy (called deep-layer microvascular dropout). Δ

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These OCTA changes correlate well topographically with the functional changes seen on visual field examination and structural changes seen on optical coherence tomography (OCT) (ie, parapapillary retinal nerve fiber layer changes and inner retinal layer thickness changes at macula). Δ

The OCTA measurements also have acceptable test-retest variability and well differentiate glaucomatous from normal eyes. OCTA measurements can be affected by various subject-related, eye-related, and disease-related factors. 

Glaucoma is a multifactorial optic neuropathy characterized by retinal ganglion cell loss and damage to the optic nerve head (ONH). Although the pathophysiology of glaucoma remains unclear, it is hypothesized that vascular factors have a critical role in its pathogenesis.

Optical coherence tomography angiography (OCT-A) is a non-invasive imaging modality that enables retinal microvasculature visualization using the dynamic motion of red blood cells. OCT-A has been used to assess vessel density (VD) measurements specified as the area (%) occupied by blood flowing vessels. Δ

Several studies illustrated that retinal VD loss is associated with glaucoma development and progression. Furthermore, OCT-A was demonstrated to have good intra-visit repeatability and inter-visit reproducibility and can distinguish between glaucomatous and healthy eyes.

Thus, OCT-A has the potential to provide new information about the pathophysiology of glaucoma. As a result, OCT-A may supplement clinicians’ skills and help their decisions in diagnosing, monitoring and treating glaucoma. Δ

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In glaucoma, VD in the ONH, peripapillary retinal nerve fiber layer (RNFL) and macula are reduced.  VD is significantly correlated with visual field (VF) damage severity defined by VF mean deviation.

In general, VD has similar diagnostic performance and is strongly correlated with OCT-measured RNFL thickness. Further, OCT-A can identify dropout of the retinal microvasculature in some eyes that do not yet have detectable VF loss.

OCT-A can be helpful in detecting early stages of glaucoma. In early OCT-A studies, peripapillary and macular VD in glaucoma patients with a single hemifield was reduced in the intact hemiretinae of these eyes. Δ

Likewise, VD loss in eyes with unilateral glaucoma was detected prior to VF damage in the unaffected eyes. Although ganglion cell complex (GCC) loss might be on average greater than macular VD loss in early glaucoma, one-third of the eyes had greater VD loss.

Similarly, in longitudinal studies some glaucoma eyes showed a significant decrease in VD over time without any decrease in GCC thickness. For instance, Hou et al evaluated OCT-A measured macular VD and OCT measured GCC thickness longitudinally. Δ Δ

In this study, they showed significant macular VD loss and GCC thinning in healthy, pre-perimetric glaucoma as well as glaucoma eyes. However, more than 60% of glaucoma eyes showed a faster rate of macular VD loss than GCC thinning, which was associated with the severity of the disease.

In this study, IOP significantly affected GCC thinning rate but had no association with the rate of macular VD loss. Δ

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OCT-A VD measurement is particularly useful in monitoring the progression of moderate and advanced disease, because it lacks a detectable measurement floor when compared to OCT structural measurements.

With OCT measurements, structural change is detectable to a certain extent; it reaches a floor value in moderate glaucoma that no longer decreases in advanced glaucoma.

This may partially explain why OCT-A VD measures correlate better to VF parameters than RNFL and GCC thickness parameters measured by OCT.

This also implies that OCT-A is capable of detecting change that is not detectable with OCT in moderate and late-stage glaucoma. However, OCT-A has less steps within the dynamic range than with OCT.

This limitation is particularly important, because more steps can detect a significant glaucomatous change before reaching the measurement floor.

Assessment of ONH and macular VD using OCT-A adds significant information to the evaluation of the risk of glaucoma progression and prediction of rates of disease worsening.

In a prospective study, lower baseline macula and ONH VD was associated with the rate of RNFL thinning. Similarly, deep microvascular dropout — defined as sectorial loss of choriocapillaris around the ONH — was associated with VF progression. Δ

Therefore, OCT-A measured macular VD might be useful not only in early diagnosis but also in prediction of risk of glaucoma progression and monitoring of the disease, especially in severe glaucoma.

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A great advantage of OCT-A is the ease of performing a noninvasive and dye-free assessment of the ONH and macula microvasculature to screen for glaucoma patients and follow their progression.

OCT-A can also provide a supplementary information to OCT devices. Besides assessing the VD, OCT-A can be helpful in identifying focal RNFL microvasculature defects that can support the diagnosis of glaucoma, which may not be necessarily evident on OCT or during a clinical examination.

Further, high quality OCT scans can be obtained from OCT-A scans. Thus, using the same piece of equipment, RNFL thickness and perfusion measurements can be provided. Δ

The main limitation of OCT-A is the presence of artifacts, such as motion (10.6%), defocus (9.6%), segmentation error (7.6%), shadow (5.4%) and decentration (4.1%) in up to onefourth of images.

These artifacts are more present in OCT-A than OCT images. Therefore, elder people with mobility issues and patients with media opacities, such as corneal opacity, dense cataracts or central floaters, may be more challenging to image with OCT-A.

OCT-A is a rapidly evolving, non-invasive imaging technology that detects microvascular changes in glaucoma. It has the potential for improving early diagnosis, identifying and predicting progression and evaluating late-stages of the disease.

Moreover, improving the OCT-A analytical software and building a normative database will continuously enhance the efficiency of imaging time, accuracy of available data and quality of captured scans. Δ

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