What Is Optical Coherence Tomography?

Optical coherence tomography (OCT) is a non-invasive, high-resolution imaging technique that produces cross-sectional images of the internal microstructure of biological tissues and materials by measuring the echo delay and magnitude of back-reflected or backscattered light. The technique is analogous to B-mode ultrasound imaging, but uses light rather than sound waves, achieving axial resolutions of 1 to 15 micrometers compared to the 150-micrometer resolution typical of clinical ultrasound. OCT draws on the physics of low-coherence interferometry, optical fiber technology, and signal processing, and it was first demonstrated for ophthalmic imaging by Huang and colleagues in 1991.

Because light cannot be timed with electronic detectors at the nanosecond precision that ultrasound flight-time measurement requires, OCT determines depth by interfering the returning signal with a reference beam: constructive interference occurs only when the optical path lengths of the sample and reference arms match to within the coherence length of the light source, which defines the depth resolution.

Low-Coherence Interferometry

The core measurement principle of OCT is a Michelson interferometer illuminated by a low-coherence light source, typically a superluminescent diode or a broadband femtosecond laser. Backscattered light from different depths within the sample returns with different delay times; each depth produces an interference fringe only when it matches the reference path length. The interference signal encodes the complex reflectivity profile of the sample as a function of depth, a quantity called the A-scan or depth-profile. The bandwidth of the light source determines the axial (depth) resolution, while the focused beam diameter determines lateral resolution. A foundational description of OCT principles is given in the PMC article on OCT as an emerging technology for biomedical imaging and optical biopsy, which details the interferometric basis and compares OCT performance with other modalities.

Time-Domain and Fourier-Domain OCT

Early OCT systems acquired A-scans in the time domain (TD-OCT) by mechanically scanning the reference mirror length while detecting the interference signal envelope. This approach achieves depth scanning but is relatively slow, limiting the number of A-scans per second and therefore the imaging frame rate. Fourier-domain OCT (FD-OCT), developed in the early 2000s, replaced the scanning reference mirror with either a spectrometer that records all depth information simultaneously (spectral-domain OCT) or a rapidly tunable laser source (swept-source OCT). FD-OCT provides a sensitivity advantage of 20 to 30 dB over TD-OCT at equivalent acquisition speeds, enabling imaging rates of tens to hundreds of thousands of A-scans per second. The NCBI Bookshelf chapter on OCT principles and technical realization provides a detailed treatment of the sensitivity advantage derivation and the design trade-offs between spectral-domain and swept-source variants.

Functional Extensions

Beyond structural imaging, OCT has been extended to quantify functional tissue properties. Doppler OCT maps blood flow velocity by measuring the phase shift between successive A-scans at the same location, with the phase shift proportional to the flow component along the beam direction. Polarization-sensitive OCT uses the polarization state of the back-reflected light to map tissue birefringence, which correlates with collagen fiber organization and stress. OCT angiography (OCTA) generates volumetric maps of the retinal and choroidal vasculature without injectable contrast agents, by detecting motion-related signal changes between repeated B-scans at the same cross-section. These functional variants have expanded the diagnostic range of OCT considerably, particularly in ophthalmology where OCTA provides capillary-resolution maps of the foveal avascular zone. Recent advances in OCT instrumentation are reviewed in the PMC article on OCT retinal imaging and clinical applications.

Applications

Optical coherence tomography has applications in a wide range of clinical and industrial fields, including:

• Ophthalmology, for retinal disease diagnosis and monitoring of glaucoma, macular degeneration, and diabetic retinopathy
• Cardiology, for intracoronary plaque characterization and stent placement guidance
• Dermatology, for non-invasive skin lesion assessment
• Gastroenterology, for esophageal and colorectal tissue characterization
• Non-destructive evaluation of coatings, semiconductors, and composite materials