16.3 Optical Coherence Tomography Angiography (OCT-A) Principles & Flow

OCT Angiography Principles, Slabs, and Artifacts

Optical coherence tomography angiography (OCT-A) compares repeated B-scans at the same location. Static tissue produces similar backscatter, while moving erythrocytes change amplitude or phase and generate decorrelation. Algorithms convert that motion contrast into flow maps.

What OCT-A shows and does not show

OCT-A is noninvasive and depth-resolved. It can depict capillary networks, foveal avascular zone contour, neovascular complexes, and areas with reduced detectable flow. It does not directly show fluorescein leakage, pooling, staining, or transit time. Absence of signal does not prove absence of a vessel; flow may be too slow, too fast for the algorithm, attenuated, or outside the slab.

Algorithm names and scan speeds vary by manufacturer. Learn the device's quality metric, scan pattern, motion correction, and export conventions rather than assuming one platform's numbers apply to another.

Slab anatomy

Common displays include superficial and deep retinal plexuses, an outer-retinal slab, and choriocapillaris or deeper choroidal slabs. The exact boundaries differ among devices and can shift when edema, PED, atrophy, traction, or segmentation failure distorts anatomy.

Always inspect the structural B-scan with segmentation lines and flow overlay. An en face image without cross-sectional confirmation can place superficial vessels into a deeper slab or omit real flow.

Neovascular flow

The normal outer retina lacks intrinsic vessels, so genuine flow there is concerning for neovascular tissue. However, apparent outer-retinal flow may come from projection of superficial vessels onto reflective deeper tissue, incorrect slab boundaries, motion, or noise. Compare vessel shape with overlying plexuses, toggle projection-removal tools, adjust segmentation when authorized, and check structural location. The clinician integrates OCT-A with OCT fluid, examination, photography, FA, or ICG.

Do not tell the patient that any colored pixel confirms CNV. Likewise, absence of a visible network does not exclude activity when hemorrhage, slow flow, poor signal, or segmentation error is present.

Common artifacts

• Projection: superficial vessel pattern appears in deeper slabs.
• Motion: horizontal or vertical bands, duplicated vessels, or displacement from blinks and saccades.
• Segmentation: boundaries cut through the wrong tissue, especially with PED, edema, or atrophy.
• Shadowing or attenuation: hemorrhage, pigment, fluid, opacity, or lashes reduce signal and mimic nonperfusion.
• Unmasking or hypertransmission: atrophy increases deeper signal and changes apparent choroidal texture.
• Threshold artifact: slow flow falls below detection and appears absent.
• Tail or decorrelation artifact: bright structures or motion extend signal beyond its true location.

Acquisition workflow

1. Confirm eye, scan size, fixation target, and clinical question.
2. Center the region and optimize focus, polarization, and signal using device instructions.
3. Reduce blink and motion without exhausting the patient.
4. Review every slab with structural B-scans and segmentation.
5. Repeat a poor scan when authorized and feasible, preserving the reason.
6. Use the same device and scan dimensions for serial quantitative comparison when possible.
7. Save en face images, flow overlays, and source structural data.

Quantitative measures such as vessel density or FAZ area are affected by scan size, magnification, axial length, segmentation, thresholding, and software version. Do not compare numbers across platforms as though they were interchangeable.

Clinical comparison

OCT-A is powerful when acquisition and artifact review are disciplined.

Acquisition strategy and cross-sectional confirmation

Select the ordered scan size and fixation target, optimize focus and polarization when available, and keep the patient aligned. Smaller fields generally provide denser sampling, while wider fields trade density for coverage; use the same protocol for follow-up unless the order changes. Review the motion-correction result rather than assuming it repaired blinking or saccades. Reacquire when possible if vessel doubling, horizontal displacement, fringe washout, or poor signal makes the slab unreliable.

Never interpret an en face slab alone. Scroll the B-scans with flow overlay to confirm that a signal lies in the expected tissue and is not projection from a superficial vessel. Inspect segmentation boundaries across pathology because fluid, pigment-epithelial detachment, atrophy, and traction can displace automated lines. Absence of visible flow does not exclude a lesion: slow flow, small caliber, shadowing, and threshold settings can produce false negatives. Conversely, a flow signal outside normal layers is not by itself proof of active neovascular disease. Save relevant slabs and B-scans and route the study for clinician interpretation.