What is an OCT?

What is optical coherence tomography (OCT)? For a clear, layer-by-layer view of retinal structure, clinicians use imaging beyond standard fundus photos. Optical coherence tomography (OCT) is a noninvasive imaging technique based on low-coherence interferometry that produces high-resolution cross-sections of the eye. 

By mapping retinal layers and the optic nerve/nerve fiber layer, OCT helps clinicians diagnose and monitor diabetic eye disease (including diabetic macular edema), glaucoma, and age-related macular degeneration.

Quick Answer: What is Optical Coherence Tomography (OCT)?

OCT is a noninvasive, noncontact test that uses near-infrared light and low-coherence interferometry to rapidly create high-resolution, cross-sectional images of the retina and optic nerve head (retinal nerve fiber layer). It helps clinicians detect and monitor conditions including age-related macular degeneration, glaucoma (via RNFL/ONH analysis), and diabetic retinopathy/diabetic macular edema.

OCT: How It Works and What It Shows

Optical coherence tomography (OCT) uses near-infrared light to create cross-sectional images of the retina and the optic nerve head (retinal nerve fiber layer) with micrometer-level detail.

Key points:

• A-scans: axial (depth) reflectivity profiles from a single sampling point.
• B-scans: a series of adjacent A-scans forms a 2-D cross-section.
• Axial resolution depends on the light source’s spectral bandwidth and center wavelength (inversely related to coherence length).

This enables imaging of ~2–3 mm of retinal depth at micrometer-level axial resolution (device-dependent), revealing fine layers within the retina and the optic nerve head. OCT’s depth-resolved imaging shows layer-by-layer retinal structure that fundus photography cannot provide. It is noninvasive and contact-free, making it safe for repeated use.

How OCT Works

Optical coherence tomography (OCT) uses low-coherence interferometry with broadband near-infrared light split into sample and reference arms; back-reflected signals interfere only when path lengths match within the coherence gate, producing a depth-resolved reflectivity profile (A-scan). This noncontact method acquires high-resolution B-scans in fractions of a second and provides layer-by-layer views of the retina.

Time-domain OCT (TD-OCT) scans depth by mechanically moving the reference mirror, which limits speed and sensitivity compared with newer methods.

• Spectral-domain OCT (SD-OCT): measures the full interference spectrum simultaneously at a spectrometer and reconstructs depth with a Fourier transform, delivering higher speed and SNR than TD-OCT.
• Swept-source OCT (SS-OCT): uses a tunable (swept) laser, typically at longer center wavelengths (e.g., ~1050 nm), providing greater imaging depth and higher acquisition speeds.

Types of OCT

Clinicians can choose from several OCT modalities to match specific clinical needs:

• Time-domain OCT (TD-OCT): first-generation OCT that scans depth by mechanically moving the reference mirror, resulting in slower speed and lower sensitivity/resolution than newer methods.
• Spectral-domain OCT (SD-OCT): records the full interference spectrum simultaneously at a spectrometer and reconstructs depth with a Fourier transform, enabling higher speed and axial resolution than TD-OCT.
• Swept-source OCT (SS-OCT): uses a tunable (swept) laser, typically near ~1050–1060 nm, providing deeper tissue penetration and higher acquisition speeds.
• OCT Angiography (OCTA): a dye-free technique that uses motion contrast from repeated OCT scans to render depth-resolved maps of the retinal and choroidal vasculature (including the choriocapillaris).

Each modality balances speed, depth, and resolution, so clinicians can match the technology to the clinical question. Selection depends on image-quality needs, patient tolerance, and diagnostic goals (for model options, see our OCTs lineup).

Recommended OCT Systems We Carry

Choosing the right OCT starts with how you practice. Below are some of our OCT systems, clinician-trusted systems:

• Optovue RTVue — The Optovue RTVue supports high-speed retinal imaging and analysis for busy clinics.
  
• ZEISS CIRRUS 5000 — The CIRRUS 5000 provides HD-OCT for retina and glaucoma analyses, with AngioPlex OCTA available on select configurations.
  
• ZEISS CIRRUS 400/4000 — The ZEISS CIRRUS 400/4000 offers core OCT essentials in a compact footprint, ideal for everyday diagnostics.
  
• Topcon 3D OCT-1 Maestro — The Topcon 3D OCT-1 Maestro combines automated SD-OCT with an integrated color fundus image to streamline capture and review.
  
• Heidelberg SPECTRALIS HRA+OCT — The Heidelberg SPECTRALIS HRA+OCT is a modular platform that pairs confocal scanning laser imaging with OCT and upgradeable options.
  
• ZEISS CIRRUS 6000 — The ZEISS CIRRUS 6000 adds faster acquisition and a wider field to speed high-detail scans without slowing clinic flow.
  

What to Expect During an OCT Exam

Here’s what to expect during a typical OCT exam: simple, quick, and noncontact:

• Prep: Your provider may use dilating drops, but OCT can be performed with or without dilation; if drops are used, expect ~15–20 minutes for them to take effect.
• Positioning: Rest your chin and forehead on the support.
• Imaging: Focus on a target while the scanner captures images; acquisition is seconds per scan (fractions of a second per B-scan), and total imaging is a few minutes per eye, depending on the protocol.
• Comfort: The test is noncontact and painless

Afterward, if dilating drops were used, you may have temporary light sensitivity and blurry near vision for ~4–6 hours; if no dilation was used, most people resume normal activities immediately.

Conditions Diagnosed with OCT

Optical coherence tomography (OCT) helps detect and monitor a wide range of retinal and optic nerve head conditions, providing high-resolution, repeatable images for baseline and follow-up. 

Key categories include:

• Retinal disorders

• Age-related macular degeneration
• Diabetic retinopathy and macular edema
• Macular holes and epiretinal membranes
• Central serous chorioretinopathy (CSCR)

• Glaucoma & optic nerve diseases

• RNFL thinning and GCC/ONH changes on SD-OCT support diagnosis and progression monitoring.

• Vitreoretinal interface issues

• Vitreomacular traction
• Posterior vitreous detachment

• Structural abnormalities

• Retinal detachment care (macular status, subretinal fluid): OCT helps assess macular involvement and fluid, but peripheral tears are diagnosed clinically with dilated exam ± ancillary imaging.
• Drusen and outer retinal/RPE changes in AMD

Final Thoughts on How OCT Works

For most clinics, choosing between SD-OCT and SS-OCT comes down to speed, penetration depth, and the analyses you run most often; SS-OCT typically uses longer wavelengths (~1050 nm) for deeper penetration, while SD-OCT prioritizes high speed and axial resolution. If you’re comparing systems, review device-specific modules (e.g., ZEISS AngioPlex OCTA on CIRRUS) and workflow features, then add a few candidates to your quote to compare side by side. 

Explore our OCT lineup or contact us to get pricing, availability, and a side-by-side recommendation tailored to your workflow.