OCT-A (Optical Coherence Tomography Angiography) is a non-invasive imaging method that uses near-infrared light interference signals to visualize blood flow in the retina and choroid. Since it does not require contrast agents, it has lower risk compared to fluorescein angiography (FA) and indocyanine green angiography (ICGA), and repeated imaging is easier.
OCT-A is not a replacement for FA but a complementary modality. While FA dynamically shows vascular permeability, leakage, and perfusion delay over time, OCT-A is characterized by capturing layered vascular structures in high-resolution static images5).
The central retinal artery (CRA) is an end artery with no collateral anastomoses1). This anatomical feature leads to rapid and irreversible retinal ischemia in CRA occlusion. OCT-A plays an important role in assessing the extent of such vascular occlusion and ischemic areas.
OCT-A detects the movement of red blood cells (blood flow) within vessels as changes in interference signals. While stationary tissue produces stable signals, the signal fluctuates in vessels where red blood cells pass, allowing vessel visualization without contrast agents by imaging these differences.
The retina is nourished by multiple capillary plexuses located at different depths. Each plexus functions as an independent unit, and different layers are affected depending on the disease.
The main vascular plexuses are summarized below.
| Vascular Plexus | Abbreviation | Location | Main Function |
|---|---|---|---|
| Superficial capillary plexus | SCP | Nerve fiber layer to ganglion cell layer | Main nutrition to inner retina |
| Intermediate capillary plexus | ICP | Upper inner nuclear layer | Nutrition of inner nuclear layer |
| Deep capillary plexus | DCP | Inner nuclear layer lower part to outer plexiform layer | Complementary oxygen supply (10–15%)3) |
| Radial peripapillary capillary plexus | RPCP | Peripapillary nerve fiber layer | Nourishment of optic nerve fibers |
| Choriocapillaris | CC | Innermost layer of the choroid | Nourishes the outer retina and photoreceptor layer |
The deep capillary plexus (DCP) accounts for 10–15% of the oxygen supply to the retina3), and in early diabetic retinopathy (DR), microaneurysms and non-perfusion areas are known to appear preferentially in the DCP.
The foveal avascular zone (FAZ) is the avascular area directly beneath the foveal photoreceptors, with a normal diameter of 400–500 μm. FAZ area and shape can be quantitatively assessed using OCT-A and are used as early indicators of ischemic changes.
OCT-A
Non-invasive, no contrast agent needed: Suitable for repeated imaging.
Layer-specific visualization: SCP/ICP/DCP/CC can be evaluated individually.
High resolution: Captures capillary-level vascular structures in still images.
Limitations: Cannot evaluate leakage or perfusion delay.
FA (Fluorescein Angiography)
Contrast agent use: There is a risk of anaphylaxis.
Temporal assessment: Captures vascular permeability, leakage, and perfusion delay.
Wide-field imaging: Excellent for evaluating peripheral non-perfusion areas.
Limitations: Cannot visualize layered vascular structures.
In retinal vein occlusion (RVO), OCT-A can quantitatively assess non-perfusion areas, FAZ enlargement, and capillary dropout 5). The extent of non-perfusion correlates with the risk of neovascularization and macular edema.
Early OCT-A findings in diabetic retinopathy include microaneurysms, irregular FAZ enlargement, and non-perfusion areas in the DCP 3). Since DCP ischemia affects visual prognosis, layer-specific evaluation has clinical significance.
For detecting macular neovascularization in age-related macular degeneration (AMD) and related diseases, OCT-A has high sensitivity: 87% compared to FA and 97% compared to ICGA 8). In pachychoroid-related diseases, OCT-A shows loss and thinning of the choriocapillaris (CC) 8).
In polypoidal choroidal vasculopathy (PCV), OCT-A can noninvasively visualize the branching neovascular network (BNN) and part of polypoidal lesions 7). Combined with ICGA, it helps assess lesion activity and treatment response.
Inner retinal system (CRA supply)
SCP/ICP/DCP/RPCP: Branches from the central retinal artery.
End artery: No anastomoses. Occlusion causes immediate ischemia1).
Representative diseases: Diabetic retinopathy, RVO, CRAO, BRAO.
Outer retinal system (choroidal supply)
CC, medium and large vessel layers: Branches from the posterior ciliary arteries.
Nourishes the outer retina and photoreceptors: Involved in macular neovascularization and age-related macular degeneration.
Representative diseases: Age-related macular degeneration, polypoidal choroidal vasculopathy, central serous chorioretinopathy, pachychoroid disease.
Enlargement of the FAZ indicates loss of capillaries around the fovea. It has been reported not only in retinal ischemia (such as diabetic retinopathy or retinal vein occlusion) but also in neurodegenerative diseases like Alzheimer’s disease, and can cause decreased visual acuity and central scotoma. For details, see Section 6: Details of Vascular Anatomy.
Since OCT-A detects blood flow indirectly, false signals (artifacts) that differ from true vascular structures are prone to occur. Pay attention to the following points when interpreting images.
The main artifacts are summarized below.
| Artifact | Cause | Effect |
|---|---|---|
| Low signal artifact | Cataract, vitreous opacity | Decreased signal in entire vascular plexus |
| Motion artifact | Eye movement during imaging | Vessel breakage/white linear noise |
| Segmentation error | Misrecognition of layer boundaries due to edema/atrophy | Misclassification of vascular plexus |
| Projection artifact | Blood flow signal from superficial vessels projected onto deeper layers | False signal of deep vessels |
OCT-A uses algorithms such as split-spectrum amplitude-decorrelation angiography (SSADA) or optical microangiography (OMAG) to detect signal changes between consecutive scans of the same location and generate blood flow maps. Areas with blood flow appear bright (white), while static tissue appears dark (black).
Current major commercial devices often have an imaging range of 6×6 mm or 3×3 mm, with a lateral resolution of 10 μm or less achievable at 3×3 mm. Wide-field OCT-A (12×12 mm or larger) devices are also becoming more common.
The central retinal artery (CRA) is a branch of the ophthalmic artery, dividing into inner and outer layers before and after the lamina cribrosa 4). The CRA is an end artery, and since there are no functional collateral pathways when occluded, inner retinal ischemia progresses rapidly and irreversibly 1).
The intraretinal arteries radiate from the optic disc as four main branches, further forming the aforementioned vascular plexuses. Veins run parallel to the arteries and eventually drain through the central retinal vein (CRV) at the center of the optic disc.
The cilioretinal artery is a variant vessel that branches from the posterior ciliary artery and supplies blood to the macular region of the retina via a route independent of the CRA. Its prevalence is reported to be approximately 22.75% 2).
Bhatt et al. (2023) reported that the presence of the cilioretinal artery contributes to the preservation of central vision in cases of CRAO (central retinal artery occlusion)2). The incidence of the cilioretinal artery is 22.75%, and if blood flow to the fovea is maintained even when the CRA is occluded, the visual prognosis is relatively favorable.
The CRA branches into nasal and temporal sides before the lamina cribrosa, and further divides into superior and inferior branches, distributing to four quadrants4). This branching pattern is related to the pathogenesis of hemi-central retinal artery occlusion (hemi-CRAO).
The DCP is located from the lower part of the inner nuclear layer to the upper part of the outer plexiform layer, and supplies 10–15% of the total oxygen demand of the retina3). Ischemia and non-perfusion of the DCP appear as early changes in diabetic retinopathy and have been shown to be associated with visual prognosis.
Pillai et al. (2023) investigated the relationship between the resistive index (RI) of the CRA and diabetic retinopathy, and reported that the DCP supplies 10–15% of the oxygen to the retina3). They stated that progression of diabetic retinopathy leads to enlargement of the DCP non-perfusion area, and macular ischemia causes visual impairment.
FAZ enlargement has also been observed in neurodegenerative diseases such as Alzheimer’s disease (AD).
Yoon et al. (2019) reported decreased macular vessel density and perfusion density on OCT-A and thinning of the ganglion cell complex in Alzheimer’s disease and mild cognitive impairment 6). It has been suggested that evaluation combining retinal vessels and neural retinal thickness, rather than FAZ alone, may serve as a non-invasive biomarker for neurodegenerative diseases.
The ICP has remained ambiguously positioned as a boundary layer between the SCP and DCP, but recent advances in segmentation technology have led to its recognition as an independent capillary plexus. The possibility that ICP ischemia plays a unique role in the pathogenesis of macular edema is being investigated.
Conventional 6×6 mm imaging ranges have made it difficult to evaluate peripheral retinal nonperfusion areas. Wide-field OCT-A of 12×12 mm or larger is increasingly enabling quantitative assessment of peripheral nonperfusion areas in diabetic retinopathy and RVO. This is expected to allow nonperfusion area evaluation comparable to FA with less invasiveness5).
Video OCT-A, which analyzes consecutive frames as a movie, is being used to evaluate hemodynamics (pulsatility, perfusion pressure) rather than just static vascular structure. Its application to early diagnosis of glaucoma is being studied.
Yoon et al. (2019) reported that decreased macular vessel density and perfusion density, along with neuroretinal thinning detected by OCT-A, are candidate biomarkers for Alzheimer’s disease and mild cognitive impairment 6). Since the retina shares the neurovascular unit as an extension of the brain, retinal vascular changes may reflect systemic neurodegenerative processes.
In the pachychoroid disease spectrum, OCT-A assessment of choriocapillaris blood flow is considered useful for elucidating pathology and guiding treatment, and further research is ongoing8).
For detection and evaluation of macular neovascularization (choroidal neovascularization), it shows a sensitivity of 97% 8), and is useful for assessing disease activity in age-related macular degeneration and polypoidal choroidal vasculopathy, as well as evaluating response to anti-VEGF therapy. It is also used for early capillary changes in diabetic retinopathy, evaluation of non-perfusion areas in RVO, and assessment of nerve fiber layer blood flow in glaucoma 5).
Pachychoroid is a group of diseases characterized by dilation and thickening of the choroid and thinning of the choriocapillaris. It includes central serous chorioretinopathy, PCV, and others. OCT-A is considered useful for evaluating choroidal vascular abnormalities in this group 8).
