OCT Technologies Compared: TD, SD, EDI, Swept-Source and OCTA
Last updated: October 2026
Pages on this topic
- How to read an OCT, step by step
- Macular OCT reference values and staging
- RNFL and GCIPL in glaucoma
- This page: OCT technologies compared (TD, SD, SS, OCTA)
How the main OCT technologies differ, in one place: time-domain, spectral-domain, enhanced depth imaging, swept-source and OCT angiography. For reading the scans, see how to read an OCT and Retinal Layers & OCT Made Easy.
OCT technologies compared
| Technology | How it works | Speed and resolution (typical) | Strengths and limits |
|---|---|---|---|
| Time-domain (TD-OCT) | A moving reference mirror measures each depth one after another | About 400 A-scans per second; about 10 µm axial | Historical (Stratus); slow, motion artefacts, poor choroid view |
| Spectral-domain (SD-OCT) | All depths measured at once with a spectrometer (Fourier domain) | About 20,000–85,000 A-scans per second; 5–7 µm axial | Much faster and more sensitive than TD-OCT; image averaging; choroid limited without EDI |
| Enhanced depth imaging (EDI-OCT) | SD-OCT with the zero-delay line moved to the choroid (sensitivity is highest near the zero-delay line) | As SD-OCT | Shows choroidal thickness (pachychoroid, VKH, tumours); image is inverted if the zero-delay line is crossed |
| Swept-source (SS-OCT) | A tunable laser sweeps wavelengths; a photodetector records them | 100,000–400,000 A-scans per second; about 8 µm axial at 1050 nm | Deeper penetration (vitreous to sclera), less fall-off with depth, wide scans (12 mm or more); also used for biometry |
| OCT angiography (OCTA) | Motion contrast from moving red cells on repeated B-scans; no dye | Depends on the platform | Depth-resolved plexuses, FAZ measurement, CNV detection; no leakage information, projection and motion artefacts |
| Anterior segment OCT | Longer wavelength (1310 nm) or swept-source | Platform-dependent | Angle, corneal thickness and epithelial maps, LASIK flaps, anterior chamber depth |
Terms worth knowing
| Term | Meaning |
|---|---|
| A-scan | One depth profile at a single point |
| B-scan | A cross-section made of many A-scans side by side |
| Axial resolution | Set by the light source bandwidth (broader band = finer resolution) |
| Lateral resolution | Set by the beam spot size at the retina (about 15–20 µm) |
| Signal strength / quality index | Low values (cataract, dry eye, small pupil) give unreliable thickness maps |
| Segmentation error | Software misplaces layer boundaries: always look at the B-scan, not just the map |
Where residents slip
- Saying SD-OCT has a lower signal-to-noise ratio than TD-OCT: it is more sensitive, which is why it replaced TD-OCT.
- Reading a thickness map without checking the B-scans for segmentation errors.
- Forgetting OCTA shows flow, not leakage: use FFA when leakage matters.
Related: How to read an OCT, OCT macula reference, OCT angiography, Retinal layers and OCT made easy.
Revise with flip cards
Key sources
- Huang D, Swanson EA, Lin CP, et al. Optical coherence tomography. Science. 1991;254:1178-1181
- Spaide RF, Koizumi H, Pozzoni MC. Enhanced depth imaging spectral-domain OCT. Am J Ophthalmol. 2008;146:496-500
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