Optical Coherence Tomography (OCT)

Optical Coherence Tomography (OCT) is an interference-based imaging technique that performs high-resolution, cross-sectional tomographic imaging of the internal structure in samples by measuring backscattered or backreflected light.

The axial (depth) resolution of the method is given by the coherence length of the OCT system light source, which is usually within the 1-30µm interval. This spatial resolution places OCT in a niche between confocal microscopy and ultrasound.

OCT vs. standard imaging

Figure 1

OCT has an extensive clinical history of being used in opthalmology but more recently the potential of OCT is being explored to detect cancer, dental caries and image vascular disease.

The level of detail provided by its high spatial resolution makes OCT suitable for investigating structures that are not revealed by other imaging methods. For example, in cardiovascular research OCT offers the potential to image in great detail not only the surface of an artery but also the layers beneath the surface of the vessel. OCT with its spatial resolution in the micrometer range offers the possibility to investigate the structural details of an artery to detect the presence of atherosclerotic disease.

NRC’s Institute for Biodiagnostics, the Industrial Materials Institute, and the Institute for Microstructural Sciences are developing OCT hardware systems to target medical applications in cardiac and oral health .

How does OCT work?

TIME DOMAIN OCT

A schematic of the principle behind a time domain OCT (TDOCT) system is presented in the figure below. Usually OCT systems are fiber-optic based where a source providing low coherence light is transmitted into a 2x2 splitter where the light is split between the sample and reference arms.

Figure 2

In the sample arm, light is directed toward the sample of interest. At the surface of the sample and at various interfaces within the sample some of the light is reflected back into the sample arm. In the reference arm light is reflected off a moving mirror. The reflected light from both the sample and reference arms returns through the 2x2 fibre splitter to a detector. At the detector, the reflected light from the two arms generate interference patterns.

Light back-reflected from the sample that has travelled the same distance as the pathlength of light reflected from the mirror in the reference arm will interfere constructively leading to a signal on the detector. As the mirror moves, the pathlength that light travels in the reference arm changes, which in turn changes the interference pattern at the detector. By capturing the detector signal as a function of mirror position one can map the optical interfaces within a sample. By then scanning the light beam over the sample, one can form a 3D image of the sample. An OCT image constitutes a cross-sectional map of light intensity back-reflected at various depths within the sample (figure below).

Swept Source Frequency Domain OCT

OCT image of a multi-layered sample

Research on OCT aims towards faster and more sensitive systems. In the recent years, this quest has led to the development of another form of OCT: Fourier-Domain OCT (FDOCT).

FDOCT does not rely on a variable optical delay line (moving mirror) but on the variation of the interference pattern with wavelength. The spatial information is recovered from a Fourier transform.

FDOCT comes in two modes: Spectral Domain OCT (SDOCT) and Swept Source OCT (SSOCT). In SDOCT, a broadband source and a setup very similar to time domain OCT is used, but different wavelengths are separated upon detection. In SSOCT, a tunable laser varies the wavelength.

At NRC-IMI, the focus is on the development of SSOCT systems. Figure 4 illustrates a basic SSOCT setup. The source is a tunable laser that sweeps the wavelength over a certain range.

As in TDOCT, light is split by a coupler and sent in a sample arm and a reference arm. In the reference arm, the mirror is fixed. In the sample arm of Figure 4, two partial reflectors (blue and red) have been placed. They are located at a distance Δ (blue and red) from the dotted line. The dotted line represents the zero-delay position; the optical path length from the coupler to the dotted line is the same as that from the coupler to the mirror.

Light reflected by both arms are combined in the coupler and sent to the photodetector and an interference phenomenon is observed. It contains a term in the form cos(4 π Δ/λ), where λ is the wavelength.

If the wavelength is varied (inverse wavelength in the graph of Figure 4), this term will lead to an oscillation. For the blue reflector, the frequency of the oscillation is low. For a reflected located further from the zero-delay position, like the red one, the frequency of the oscillation is higher.

The position of a reflector is encoded in the frequency of the oscillations recorded on the detector as the wavelength is varied. If one takes the Fourier transform of the combined signals from both reflectors, one extracts the position and strength of the reflectors.

As shown in the bottom right of Figure 4, since the information is obtained from the Fourier transform of a real signal, one obtains a mirror image relative to the zero-delay position.

Fortunately, there are techniques to remove this artifact. A source operating at 1300 nm swept over 100 nm will provide an OCT resolution of about 7 microns. A TDOCT system with a very fast delay line can provide 10 000 depth scans per second. A SSOCT system with a commercial swept source can reach about 50 000 depth scans per second. Sources developed in research laboratories can reach more than 200 000 depth scans per second. SSOCT is thus much faster than TDOCT and is also more sensitive.

Figure 4

How can OCT be used?

OCT developments will focus on developing hardware and applications for in vivo studies such as those listed in the Oral-Dental Health Program and Coronary Artery Disease sections. Future work will also target multimodal applications.

Related Information

Read more about Optical Coherence Tomography (OCT):

• Bioluminescence Imaging
• Diagnostic Technologies
• Diffuse Reflectance Spectroscopy
• Fluorescence Imaging
• Frequency Modulated Spectroscopy
• High Throughput Diagnostics
• Molecular Imaging - Spectroscopy
• MRI Studies
• Non-linear Optical Microscopic Imaging
• Raman Spectroscopy
• Speckle Imaging
• Thermal Imaging

Related Programs:

• Medical Photonics

Institutes:

• NRC Institute for Biodiagnostics