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Upcoming Advances in Optical Coherence Tomography Angiography

Improvements could make OCTA a mainstay for retinal imaging.

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OCT angiography (OCTA) is a noninvasive imaging modality that allows for the three-dimensional visualization and analysis of retinal and choroidal vasculature. The introduction of spectral-domain optical coherence tomography (SD-OCT) allowed for acquisition speeds up to 80,000 A scans per second, while achieving an image resolution of 5-8 µm.1,2 SD-OCTA revolutionized our understanding of macular pathologies. However, upcoming advances in OCTA have the potential to further transform retinal imaging with both high-speed spectral-domain and ultrahigh-speed swept-source (SS) technology.

SD-OCT employs a broad-bandwidth light source coupled with a spectrometer and line-scan camera, while comparatively, SS-OCT uses a light source that sweeps through a range of frequencies. Since SS-OCT systems are not limited by camera reading rates, SS-OCT can achieve faster acquisition speeds, at 100,000-400,000 A-scans per second.1-3 Faster scan times allow for greater retinal coverage and higher pixel resolution. The most commonly employed macular scan pattern is 3 mm x 3 mm, due to its higher resolution with highly concentrated B-scan positions that spread out as scan pattern size increases. Therefore, for a single scan, an increase in scanning area is likely to increase imaging time or reduce scan density and compromise its resolution.

With faster imaging speeds, a greater number of A scan locations can be employed in larger scan patterns, expanding the OCTA field of view while maintaining high resolution, all without drastically increasing overall image acquisition time. On SS-OCTA devices, larger single acquisition scan patterns are available, such as 12 mm x 12 mm and 15 mm x 9 mm. During scan acquisition, the internal fixation point can be circled parafoveally to acquire 12 mm x 12 mm scans centered at the fovea and in parafoveal regions. These scans can then be montaged either automatically by device-manufacturer proprietary software or manually by a third-party software to generate a wide-field view of the retina comparable to that visualized by fluorescein angiography (Figure 1).

Figure 1. Montage of 12 mm x 12 mm scans from a normal eye obtained on the swept-source Zeiss Plex Elite 9000. Five 12 mm x 12 mm scans were obtained, one centered at the fovea, and the other 4 centered at the 4 quadrants around the fovea. The montaging of these 5 scans, by the Zeiss proprietary software, allows for visualization of a wider retinal field of view than that allowed by individual OCTA scan protocols.

One of the current limitations of commercially available OCTA technology is that it provides binary images. The images only show whether there is flow or not and cannot delineate the speed of flow. An exciting development, with improved scanning speed, is the ability not only to differentiate between flow and no flow, but also to assess flow speeds. A proof of concept of this is a technique called variable interscan time analysis (VISTA), which has been applied to high-speed prototypes to compare relative flow speeds in the retinal vasculature (Figure 2).4,5 VISTA has been invaluable in our understanding of the changes in flow speed in diabetic patients as well as in the vessels of choroidal neovascularization.6,7 Future advancements in VISTA technology may even allow for quantification of retinal and choroidal blood flow velocities.

Figure 2. 3 mm x 3 mm scan full retina segmentation of a normal eye imaged on an SS-OCTA prototype device with subsequent processing by the variable interscan time analysis (VISTA) algorithm. VISTA processing was applied to a 3 mm x 3mm SS-OCTA scan to obtain a color-coded map of relative flow speeds. Red indicates faster relative flow, while blue indicates slower relative flow.

In addition to advancements of speed in both SD and SS systems, SS-OCT systems also have the potential to allow for deeper light penetration into the choroid. This is because SS-OCT utilizes a longer wavelength of 1,050 nm, compared to the 850 nm employed on SD-OCT that is highly scattered by media opacities, causing less light to penetrate deeper layers. Longer wavelengths, such as in SS-OCT, experience less scattering and interference, allowing for greater light penetration into deeper tissue, and thus improved imaging of the choroid and choriocapillaris.8 SS-OCT is able to provide improved resolution of choroidal vasculature below the RPE.5,9

Although SS-OCT offers ultrahigh speeds and increased imaging sensitivity, the axial image resolution is less than that of SD-OCT (5 µm to 8 µm). However, due to its enhanced resolution compared to other retinal imaging techniques, OCTA in general has a unique capability to identify various chorioretinal vascular abnormalities in their various stages and to guide their treatment.10,11

Nevertheless, OCTA is an evolving technique that is not without its limitations. OCTA images are susceptible to degradation by artifacts, particularly motion and projection artifacts.12-14 With the development of faster computing methods, better artifact removal techniques are being developed. Examples include pixel and volume registration of OCTA scan volumes to remove artifacts, as well as development of the ability to view vasculature in 3 dimensions.15,16 Additionally, new projection removal algorithms are being developed, such as one that identifies the depth of specific vessels prior to resolving projection artifacts on a voxel-to-voxel basis.17

Further limitations occur in the analysis of generated OCTA images. The volumetric cube scan produced by OCTA imaging can be manually scrolled through, which is time-consuming, or automatically segmented to visualize predefined en face images at various retinal layers. While most OCTA devices have designated retinal slabs corresponding to the superficial and deep layers, these sections are not standardized among devices. Additionally, studies have shown automated segmentation algorithms to be less accurate than manually adjusted segmentation, in which the thickness and axial position of each retinal segment is manually optimized in a laborious process.18 Improved segmentation schemes based on vascular plexi have been described, which may provide a basis for standardization across devices and thus improve automated visualization of vascular pathology.19 Additionally, semiautomatic algorithms are being developed to improve segmentation of the retinal nerve fiber layer or retinal layer in diseased eyes.1,20 The development of such future segmentation algorithms has the potential to minimize inaccuracies in automated segmentation, improving visualization of retinal pathology as well as improving clinical efficiency.

OCTA has greatly enhanced our knowledge of retinal and choroidal pathology. The benefit of OCTA in visualization of certain diseases, such as CNV, has been well documented.21,22 However, overall, OCTA is still defining its role in a clinical setting. With upcoming advancements of faster scan times, improved image resolution, longer-wavelength light sources, artifact correction algorithms, and variable interscan time analyses, OCTA has the potential to become a mainstay in retinal imaging. RP

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