Optical Coherence Tomography

Optical Coherence Tomography (OCT) is a new noninvasive optical diagnostic technique that provides depth-resolved images of tissues with resolution of about 2-10 µm at depths of up to several mm. This technique was introduced in 1991 to perform tomographic imaging of the human eye by Fujimoto's group at MIT. Since then, OCT is being actively developed by several research groups for many clinical diagnostic applications. The basic principle of the OCT is to detect backscattered photons from a tissue of interest within a coherence length of the source by using a two-beam interferometer. OCT imaging is somewhat analogous to ultrasound B-mode imaging except that it uses light, not sound. Briefly, light from a broadband source (a laser with low coherence) is aimed at objects to be imaged using a beam splitter. Light scattered from the tissue is combined with light returned from the reference arm, and a photodiode detects the resulting interferometric signal. Interferometric signals can be formed only when the optical path length in the sample arm matches the reference arm length within coherence length of the source. By gathering interference data at points across the surface, cross-sectional 2D and 3D images can be formed in real time with resolution of about 2-10 µm at depths up to several millimeters, depending on the tissue optical properties.

Currently, the OCT-based imaging technique may be divided into two classes: time domain OCT (TDOCT) and spectral domain OCT (SOCT). While conventional TDOCT methods are widely used in clinical and research laboratories, recently, the significant signal-to-noise (SNR) advantage of SOCT over TDOCT has been demonstrated. Also, simplicity of the interferometer construction, the absence of mechanical in-depth scanning, and the SNR advantage of SOCT are facilitating its further development for many applications.

SOCT may be further divided into two distinct methods of acquiring spectral information from an object under study: 1) Fourier-domain OCT (FDOCT) – using a broadband laser source (typical FWHM > 75 nm) with a grating and a photodiode array in the detection arm; and 2) swept-source OCT (SSOCT) - using a rapidly tunable, narrow-linewidth laser source over a broad optical bandwidth with a single photodiode in the detection arm. While FDOCT and SSOCT have similar sensitivity advantages over conventional TDOCT systems, the SSOCT system can detect interference fringes over a substantially longer range of time delays between reflections from reference and sample interfaces.

Time-Domain OCT

Spectroscopic OCT

Swept Source OCT