OCT Review

Last updated: September 2026

OCT Review

• Time Domain OCT (TD-OCT)
  • Light echoes from each time delay measured sequentially
  • Slow acquisition speeds, high signal-to-noise ratio, poor view of the choroid
  • 400 A-scans per minute, 6 B-scans per macula, 10-μm resolution
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• Spectral Domain OCT (SD-OCT)
  • Light echoes from each time delay, all measured simultaneously (high-speed spectrometer)
  • Fast acquisition speeds, low signal-to-noise ratio, view to the choroid possible
  • 52,000 A-scans per minute, 20-40 B-scans per macula (up to 100), 5-μm resolution
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• Enhanced Depth Imaging OCT (EDI-OCT)
  • Zero delay line (ZDL) positioned at the inner retina
  • The further from the zero-delay line (ZDL), the lower the signal resolution.
  • EDI moves the ZDL closer to the choroid; negative images when the ZDL is crossed.
  • Thinner layers permit deeper tissues to be closer to the ZDL.
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• Full Depth or Combined Depth Imaging OCT (CDI-OCT)
  • 100 B-scans averaged for a single line scan.
  • 50 in standard SD-OCT mode and 50 in EDI mode
  • Maximizes resolution of vitreoretinal and choroidal structures in a single scan
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• Swept Source OCT
  • Topcon DRI OCT-1 Atlantis
  • Longer wavelength light source (1050 nm)
  • Fast (100,000 A scans per second)
  • Uniform sensitivity, allowing excellent visualization of structures from vitreous to sclera in one scan
  • Automated segmentation (7 layers)
  • 100,000 A-scans per second, 1-μm resolution
  • 12-μm wide scans (vs. 9- and 6-μm scans)
  • Image depth of 2.6 mm vs. 1.9 mm for EDI—even more ideal for choroidal tumors

OCT Angiography (OCTA) — the modality missing above

OCTA deserves its own mention alongside the structural OCT modalities above, since it has become a mainstream clinical tool rather than a research curiosity:

• Principle: detects motion contrast from flowing red blood cells across repeated B-scans at the same location — no dye injection required, unlike FFA/ICGA
• Provides depth-resolved visualization of distinct vascular plexuses (superficial capillary plexus, deep capillary plexus) that dye-based angiography cannot separate
• Can quantify foveal avascular zone size and vessel density objectively, rather than relying on qualitative visual assessment
• Current clinical role: increasingly first-line for detecting diabetic retinopathy microvascular changes and monitoring choroidal neovascular membrane activity — see our FFA Indications page for the fuller current comparison of when OCTA has replaced dye-based angiography and when it hasn't (particularly for leakage assessment, where FFA/ICGA remain necessary)