Optical Coherence Tomography Angiography (OCTA) is a non-invasive fundus angiography technique that adds blood flow detection to an OCT device using near-infrared light. First clinically applied in 2014, it has rapidly spread as a contrast-free vascular visualization technology.
The basic principle of OCTA is motion contrast. By repeatedly scanning the same area, it separates time-varying signal components (i.e., moving blood cells) from stationary tissue signals to extract blood flow information. A representative algorithm is SSADA (Split-Spectrum Amplitude-Decorrelation Angiography).
There are two types depending on the light source: SD-OCT (spectral domain) and SS-OCT (swept source). SS-OCT has a longer wavelength and is superior for visualizing deep choroidal structures.
FA involves intravenous injection of a contrast agent and two-dimensionally records fluorescence patterns including leakage. OCTA does not require contrast agents, three-dimensionally visualizes only blood flow, and allows layer-specific analysis and quantification. However, it cannot evaluate leakage, staining, or pooling, so it is used complementarily with FA. For details, see the section “Main Features and Comparison with FA”.
Non-invasive and rapid
No contrast agent needed: No risk of contrast agent side effects such as anaphylaxis.
Short examination time: Each scan is completed in seconds to tens of seconds.
Repeatable: Frequent imaging for follow-up can be performed without burdening the patient.
3D and quantitative
Layer-specific analysis: The retina can be divided into 4 layers, and the vascular network of each layer can be visualized individually.
Quantitative assessment: Vessel density (VD) and capillary perfusion density (MPD) can be quantified.
Simultaneous evaluation of structure and blood flow: OCT structural images and vascular images can be overlaid for review.
Advantages over FA
Flow void visualization: Non-perfused areas and capillary dropout can be visualized in detail.
Separation of capillary plexuses: Superficial and deep capillary plexuses can be evaluated individually.
Limitations compared to FA
Cannot detect leakage: Increased vascular permeability and leakage from neovascularization cannot be detected.
Narrow field of view: Standard scan sizes are about 3×3 to 12×12 mm, not matching wide-field FA.
Device-dependent quantitative values: Values such as vessel density cannot be directly compared between different devices.
The main differences between FA and OCTA are shown below.
| Feature | FA | OCTA |
|---|---|---|
| Contrast agent | Required | Not required |
| Leakage assessment | Possible | Not possible |
| Layer-specific analysis | Not possible | Possible |
At present, it cannot. FA remains essential for evaluating leakage, staining, and neovascular activity. The two are best used complementarily.
Proper preparation and imaging procedures are necessary to perform OCTA accurately.
Standard imaging ranges from 3×3 mm (high resolution) to 12×12 mm (wide field). For macular evaluation, 3×3 mm or 6×6 mm is often used. For optic disc evaluation, 4.5×4.5 mm is common.
OCTA automatically sets the boundaries (segmentation) of each layer based on OCT tomograms, but automatic segmentation often fails in diseased eyes. After imaging, always check the segmentation lines and manually correct any misalignment.
OCTA visualizes the retinal vascular plexuses divided into the following four layers.
| Layer name | Abbreviation | Main location |
|---|---|---|
| Superficial capillary plexus | SCP | Nerve fiber layer to ganglion cell layer |
| Deep capillary plexus | DCP | Inner to outer inner nuclear layer |
| Outer retina | — | Avascular layer (no blood flow normally) |
| Choriocapillaris | CC | Just beneath Bruch’s membrane |
Some devices also adopt a classification that includes the radial peripapillary capillary plexus (RPCP).
OCTA has specific artifacts that can affect clinical judgment, so understanding them is essential.
The main artifacts are summarized below.
| Artifact | Cause | Effect |
|---|---|---|
| Signal loss | Media opacity, pigment | False flow void |
| Projection | Shadow of superficial vessels | False blood flow in deep layers |
| Segmentation error | Pathological morphological changes | Interlayer signal contamination |
| Eye movement | Poor fixation | Linear white bands/duplication |
Before imaging, perform pupillary dilation, confirm fixation, and evaluate media, and check the image quality score. Segmentation must be visually confirmed after imaging. Enable projection removal function if available on the device.
OCTA is used for the diagnosis and management of various retinal and optic nerve diseases.
OCTA can finely depict capillary abnormalities in DR. It enables detection of FAZ enlargement and irregularity, capillary dropout (flow void), and neovascularization. The AAO Diabetic Retinopathy Preferred Practice Pattern (2024) states that OCTA is useful as a complementary test to FA, especially for evaluating the macular capillary network5).
Vessel density (VD) correlates with DR stage and is being studied as an objective indicator of retinal ischemia.
Srinivasan et al. (2023) reported in a longitudinal study of DR patients that lower baseline SCP-VD was associated with a higher risk of DR severity progression over one year2). The median SCP-VD in the progression group was 12.90%, compared to 14.90% in the non-progression group, with a significant difference (p=0.032), and a hazard ratio of 0.825 (AUC=0.643).
Detection of choroidal neovascularization (MNV) is one of the main indications for OCTA. The AAO AMD Preferred Practice Pattern (2024) reports that OCTA has a sensitivity of 0.87 and specificity of 0.97 for detecting macular neovascularization, with diagnostic accuracy comparable to FA6).
Additionally, OCTA may detect asymptomatic subclinical macular neovascularization (type 1 MNV, MNV under drusen) that is not detectable by FA, which is of interest for early intervention6).
In RVO, capillary dropout and flow voids at the occlusion site are clearly visualized on OCTA. The AAO RVO Preferred Practice Pattern (2024) states that OCTA is useful for evaluating the ischemic area of the macular capillary network7).
In RAO, flow voids in the superficial capillaries corresponding to the occluded vessel territory are observed from the acute phase. The AAO RAO Preferred Practice Pattern (2024) states that early blood flow assessment by OCTA is useful for management8).
In glaucoma, thinning of the nerve fiber layer and decreased peripapillary vessel density may occur before visual field abnormalities, and research on early detection using OCTA is progressing. Zuberi et al. (2022) reported decreased OCTA vessel density in an NTG case 4). However, at present, OCT structural imaging and visual field testing remain the mainstays for diagnosis and management, with OCTA playing a complementary role.
Research is advancing on using OCTA quantitative indicators as biomarkers for predicting DR progression.
Srinivasan et al. (2023) longitudinally showed that baseline SCP-VD (vessel density) is significantly associated with the risk of DR progression 2). VD was 12.90% (progressors) vs 14.90% (non-progressors), p=0.032, hazard ratio 0.825, AUC=0.643. With improved sensitivity and specificity, future application to personalized follow-up is expected.
In addition to conventional fundus OCTA, the application of OCTA to the anterior segment and conjunctiva is expanding.
Mgboji et al. (2022) evaluated vascular morphology in SCD patients using conjunctival OCTA and demonstrated its potential for non-invasive monitoring of systemic vascular complications 3).
The development and dissemination of ultra-widefield OCTA exceeding 12×12 mm is expected to improve the detection sensitivity of peripheral retinal vascular lesions and neovascularization in preproliferative retinopathy.
Clinical studies are underway to investigate whether anti-VEGF treatment for subclinical macular neovascularization detected by OCTA can prevent progression to exudative AMD6).
The main directions are wider field, higher speed, AI-based automated analysis, and standardization of quantitative biomarkers. Establishing standardization criteria to eliminate inter-device quantitative differences is also an important research topic.
