Optical coherence tomography (OCT)

Optical coherence tomography finds the depth of reflections inside a sample by interfering the light they return with a reference beam. The source is broadband, so its coherence length is short, and light from the sample interferes with the reference only when the two optical paths match within that length. Each reflection is therefore located in depth to within roughly the coherence length, and scanning the beam across the sample builds a cross-sectional image. The arrangement is a Michelson interferometer, usually in fiber with a coupler or circulator, and the detection is typically balanced.

The axial resolution depends only on the source spectrum. For a Gaussian spectrum of center wavelength $\lambda_0$ and full width $\Delta\lambda$, it is $\delta z = (2\ln 2/\pi)\,\lambda_0^2/\Delta\lambda$ in air. A source at 840 nm with 50 nm of bandwidth gives 6.2 µm, and one at 1310 nm with 100 nm gives 7.6 µm; in tissue with a refractive index near 1.38 these become 4.5 and 5.5 µm. The lateral resolution is set separately by the focusing optics, as in any microscope, which is the main difference from confocal microscopy, where the objective sets both. Scattering and absorption limit the imaging depth in tissue to a millimeter or two; longer wavelengths penetrate further in skin and vessel walls, and the 800 to 1060 nm range is used for the eye, where water absorption at longer wavelengths would weaken the signal.

Three generations use the same principle. Time-domain OCT moves the reference mirror and records the interference envelope depth by depth. Spectral-domain OCT keeps the reference fixed and measures the interference spectrum with a spectrometer; a Fourier transform returns the whole depth profile at once. Swept-source OCT sweeps a narrow tunable laser across the band and records the spectrum in time with a single detector. The Fourier-domain methods acquire every depth in parallel and are one to two orders of magnitude more sensitive than the time-domain method, which is why they replaced it. Their depth range is limited by spectral sampling: a spectrometer resolving 50 nm into 2048 pixels at 840 nm reaches 7.2 mm in air, from $\lambda_0^2/(4\,\delta\lambda)$. For a swept source the limit is the laser's instantaneous coherence length.

Sources follow from the bandwidth requirement: superluminescent diodes, whose amplified spontaneous emission is broad and smooth, for spectral-domain systems; swept lasers for swept-source systems; and supercontinuum sources where the finest resolution is wanted. Beyond ophthalmology, OCT is used inside blood vessels through catheters, in dermatology, and in industry for measuring coatings and multilayer films.

References: D. Huang et al., Science 254, 1178 (1991); R. Leitgeb, C. K. Hitzenberger, A. F. Fercher, Opt. Express 11, 889 (2003); M. A. Choma, M. V. Sarunic, C. Yang, J. A. Izatt, Opt. Express 11, 2183 (2003).