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).
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.
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
- Kashani AH, Chen CL, Gahm JK, et al. Optical coherence tomography angiography: a comprehensive review of current methods and clinical applications. Prog Retin Eye Res. 2017;60:66-100.
- Fujimoto J, Swanson E. The development, commercialization, and impact of optical coherence tomography. Invest Ophthalmol Vis Sci. 2016;57:OCT1-OCT13.
- Told R, Ginner L, Hecht A, et al. Comparative study between a spectral domain and a high-speed single-beam swept source OCTA system for identifying choroidal neovascularization in AMD. Sci Rep. 2016;6:38132.
- Ploner SB, Moult EM, Choi W, et al. Toward quantitative OCT angiography: visualizing flow impairment using variable interscan time analysis (VISTA). Retina. 2016;36 Suppl 1:S118-S126.
- Choi W, Moult EM, Waheed NK, et al. Ultrahigh-speed, swept-source optical coherence tomography angiography in nonexudative age-related macular degeneration with geographic atrophy. Ophthalmology. 2015;122(12):2532-2544.
- Rebhun CB, Moult EM, Ploner SB, et al. Analyzing relative blood flow speeds in choroidal neovascularization using variable interscan time analysis oct angiography. Ophthalmology Retina. In Press.
- Choi W, Waheed NK, Moult EM, et al. Ultrahigh speed swept source optical coherence tomography angiography of retinal and choriocapillaris alterations in diabetic patients with and without retinopathy. Retina. 2017;37(1):11-21.
- 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):1272-1281.
- Choi W, Mohler KJ, Potsaid B, et al. Choriocapillaris and choroidal microvasculature imaging with ultrahigh speed OCT angiography. PLoS ONE. 2013;8:e81499.
- Hwang TS, Jia Y, Gao SS, et al. Optical coherence tomography angiography features of diabetic retinopathy. Retina. 2015;35(11):2371-2376.
- Coscas F, Querques G, Forte R, Terrada C, Coscas G, Souied EH. Combined fluorescein angiography and spectral-domain optical coherence tomography imaging of classic choroidal neovascularization secondary to age-related macular degeneration before and after intravitreal ranibizumab injections. Retina. 2012;32(6):1069-1076.
- Spaide RF, Fujimoto JG, Waheed NK. image artifacts in optical coherence tomography angiography. Retina. 2015;35(11):2163-2180.
- de Carlo TE, Romano A, Waheed N, Duker JS. A review of optical coherence tomography angiography (OCTA). Int J Retina Vitreous. 2015;1.
- Louzada RN, de Carlo TE, Adhi M, et al. Optical coherence tomography angiography artifacts in retinal pigment epithelial detachment. Can J Ophthalmol. 2017;52(4):419-424.
- Wei DW, Deegan AJ, Wang RK. Automatic motion correction for in vivo human skin optical coherence tomography angiography through combined rigid and nonrigid registration. J Biomed Opt. 2017;22(6):66013.
- Zhang A, Zhang Q, Wang RK. Minimizing projection artifacts for accurate presentation of choroidal neovascularization in OCT micro-angiography. Biomed Opt Express. 2015;6(10):4130-4143.
- Zhang Q, Zhang A, Lee CS, et al. Projection artifact removal improves visualization and quantitation of macular neovascularization imaged by optical coherence tomography angiography. Ophthalmol Retina. 2017;1(2):124-136.
- Arya M, Cole ED, Sabrosa AS, et al. Visualization of choroidal neovascularization using two commercially available spectral domain optical coherence tomography angiography devices. Retina. Manuscript under review.
- Campbell JP, Zhang M, Hwang TS, et al. Detailed vascular anatomy of the human retina by projection-resolved optical coherence tomography angiography. Sci Rep. 2017;7:42201.
- Chen CL, Zhang A, Bojikian KD, et al. Peripapillary retinal nerve fiber layer vascular microcirculation in glaucoma using optical coherence tomography-based microangiography. Invest Ophthalmol Vis Sci. 2016;57:OCT475-OCT485.
- Rezaei KA, Zhang Q, Kam J, Liu J, Wang RK. Optical coherence tomography based microangiography as a non-invasive imaging modality for early detection of choroido-neovascular membrane in choroidal rupture. SpringerPlus. 2016;5:1470.
- de Carlo TE, Bonini Filho MA, Chin AT, et al. Spectral-domain optical coherence tomography angiography of choroidal neovascularization. Ophthalmology. 2015;122(6):1228-1238.