OCT Angiography: Non-Invasive Retinal Vascular Imaging

Retinal disease is responsible for the majority of irreversible vision loss in high-income countries, yet the blood vessels at the center of that pathology have historically been difficult to image without injecting dye into a patient's arm and hoping the photographs captured what clinicians needed. OCT angiography (OCTA) changed the calculus. It images retinal and choroidal vasculature in three dimensions, without contrast agents, in under a minute — a shift significant enough that the American Academy of Ophthalmology recognized it as one of the most consequential diagnostic advances in retinal imaging of the past two decades.

How OCTA Works

Standard optical coherence tomography uses near-infrared light to generate cross-sectional images of retinal tissue — essentially an optical biopsy of the layers beneath the fundus. OCTA builds on that foundation by comparing sequential B-scans taken at the same location. Red blood cells moving through vessels create signal decorrelation between those repeated scans; static tissue does not. The algorithm isolates that decorrelation and maps it as a flow signal, producing depth-resolved angiographic images without any exogenous contrast.

The technical term for the underlying principle is "split-spectrum amplitude-decorrelation angiography" (SSADA), developed and described by researchers at Oregon Health & Science University, including David Huang, MD, PhD, who was also instrumental in the original development of OCT itself. SSADA improves signal-to-noise ratio by splitting the OCT spectrum into sub-bands before calculating decorrelation, which allows clearer vessel delineation at lower image acquisition speeds (Jia et al., 2012, Optics Letters).

What OCTA Reveals That Fluorescein Angiography Cannot

Fluorescein angiography (FA) — the previous gold standard — floods the entire retina with dye-enhanced signal. That makes it excellent for detecting leakage, but the hyperfluorescence obscures the precise boundaries of avascular zones and masks fine capillary detail. OCTA, lacking dye, produces no leakage artifact. The foveal avascular zone (FAZ) — the capillary-free region at the center of the macula, roughly 0.3 mm² in area in healthy eyes — can be measured with precision that FA cannot match.

OCTA also segments vascular plexuses independently: the superficial capillary plexus, the deep capillary plexus, the outer retina, and the choriocapillaris each appear as distinct en face images. That depth resolution is structurally impossible with FA, which collapses all layers into a single projection. For conditions like diabetic macular ischemia, where deep capillary dropout predicts vision loss before the superficial layer shows obvious change, this layered view is diagnostically meaningful rather than merely elegant.

Clinical Applications

Diabetic Retinopathy. OCTA detects microaneurysms, FAZ enlargement, and non-perfusion areas across all vascular layers. A 2016 study in JAMA Ophthalmology found that OCTA identified 90% of microaneurysms confirmed by FA in a head-to-head comparison — with the additional advantage of revealing deep capillary plexus lesions that FA missed entirely (Ishibazawa et al., 2015, Investigative Ophthalmology & Visual Science).

Age-Related Macular Degeneration (AMD). OCTA can detect choroidal neovascularization (CNV) — the abnormal vessel growth beneath the retina that drives wet AMD — without the dye injection that can occasionally cause adverse reactions in older patients. The National Eye Institute notes that AMD affects approximately 11 million people in the United States (NEI), making a non-invasive surveillance tool for CNV activity clinically significant at a population level.

Glaucoma. The optic nerve head and peripapillary capillary network are both visualized with OCTA, and reduced vessel density in the radial peripapillary capillary plexus correlates with structural loss on OCT and functional loss on visual field testing. Research published through the Glaucoma Research Foundation suggests that vascular changes may precede detectable nerve fiber layer thinning in some cases (Glaucoma Research Foundation).

Retinal Vascular Occlusions. Branch and central retinal vein occlusions produce patterns of capillary non-perfusion that OCTA maps without the late-phase leakage that can obscure FA reads in acute occlusions.

Limitations Worth Knowing

OCTA is not without constraints. Motion artifact degrades image quality; a patient who cannot maintain steady fixation for the 2–6 second acquisition window will produce scans that are difficult to interpret. The technology also cannot detect leakage — which is itself a clinical finding — so fluorescein or indocyanine green angiography remains necessary when active leakage characterization is the diagnostic question.

Field of view is another honest limitation. Most commercial OCTA systems image a 3×3 mm or 6×6 mm area centered on the fovea, while ultra-widefield FA can capture the peripheral retina to identify far temporal neovascularization in proliferative diabetic retinopathy. Widefield OCTA systems are under active development and clinical evaluation at academic centers including Moorfields Eye Hospital in London and the Doheny Eye Institute at UCLA.

Equipment and Availability

Commercial OCTA platforms include the Zeiss Cirrus HD-OCT 5000 with AngioPlex, Optovue Avanti with AngioVue, Heidelberg Spectralis with OCTA module, and Topcon Triton. Acquisition speeds range from approximately 70,000 to 100,000 A-scans per second on current swept-source and spectral-domain instruments, with faster systems producing better motion suppression. Reimbursement for OCTA in the United States is handled through CPT codes 92134 (scanning computerized ophthalmic diagnostic imaging), though coverage policies vary by payer.

References

The law belongs to the people. Georgia v. Public.Resource.Org, 590 U.S. (2020)