Optical Coherence Tomography - An Overview

Optical Coherence Tomography (OCT) is an emerging state-of-the-art imaging modality capable of providing micron-scale images of subsurface biological tissue. It is analogous to ultrasound, but instead of using sound waves, it uses low-coherence light. OCT performs cross-sectional imaging by measuring the backscattered intensity of light from structures in tissues.

On the right there is a graph showing the scattering and absorption properties of skin at various wavelengths. There is a "biological window" into tissue for wavelengths ranging from 800 to 1300 nm. In this window, attenuation of light is due largely to scattering, rather than absorption. OCT utilizes a low-coherence light source within this range of wavelengths to image deep into tissue. Simply stated, by measuring the time it takes the reflected light to return to the detector, an image can be created. It is possible to measure this time through low coherence interferometry using a Michelson type interferometer. This, in turn, allows us to deduce the precise location of a reflection.

Profio & Doiron, Photochem. Photobiol 46:591, 1987

The schematic shown below illustrates the general organization of a time-domain OCT system. Output from a broadband light source is split by a coupler or beam splitter into a reference beam and a sample beam. The reference beam is projected onto a mirror mounted on a rapidly moving reference arm stage. Other reference arm scanning techniques can also be used. The sample beam is focused through scanning equipment and lenses to a point on the tissue. Then backscattered light is combined with the reference beam and is captured by a photodetector.

In recent years, advances in the fundamental OCT system design have been made using Fourier-domain OCT techniques.  These are divided into spectral-domain OCT and swept-source OCT.  In spectral-domain OCT, as shown in the figure below, the photodetectors are replaced by a spectrometer, composed of a grating, lens system, and line scan camera.  The reference arm mirror is pathlength matched, but held fixed.  In this technique, backscattered signals are captured from all depths in a single column, without the need to mechanically scan the reference arm mirror.  This results in a higher signal-to-noise ratio, improved phase-stability since no mechanical reference-arm scanning is involved, and faster acquisition rates dependent on the read-out rate of the camera.

In swept-source OCT, the broad bandwidth optical source is replaced by a rapid-scanning laser source. By rapidly sweeping the source wavelength over a broad wavelength range, and collecting all the scattering information at each wavelength and at each position, the composition of the collected signal is equivalent to the spectral-domain OCT technique. Collected spectral data is then inverse Fourier transformed to recover the spatial depth-dependent information. Swept-source OCT systems are advantageous for their extremely fast scan rates, on the order of 50,000 to 300,000 axial scans per second.

The OCT technology has found wide application over the span of the last decade by imaging tissue cross-sections at depths exceeding 2 cm in transparent tissues (including images of the eye and frog embryo) and 2-3 mm in highly scattering (non-transparent) tissues, such as the skin. In addition, the recent development of an OCT needle fitted with fiber-optics has allowed much deeper penetration of solid tissue masses, such as breast tissue. Further development of beam delivery systems is continuing. The advantages to OCT are numerous; the portability, high resolution, low cost and non-invasive nature of OCT make it an extremely attractive technology.