Introduction to Optical Coherence Tomography (OCT)

Background

Optical coherence tomography (OCT)^1,2 is an imaging modality that can produce two- or three-dimensional images by combining lateral point beam scanning with the depth-sectioning ability of low-coherence interferometry. Unlike comparable techniques such as ultrasound, the depth (or axial) resolution depends almost entirely on the optical bandwidth of the light source, not the aperture of the optical system. For this reason, OCT systems may combine high axial resolutions with large depths of field, so their primary applications have included in vivo imaging through thick sections of biological systems, particularly in the human body. The technique is a relatively recent development in the biomedical field: the term optical coherence tomography was first coined in the seminal paper by David Huang (and co-authors)¹ that appeared in Science in 1991.

Basic description

Fig. 1 shows a schematic diagram of the basic optical fibre-based OCT system setup. The Michelson interferometer splits the light from the broadband source into two paths, the reference and sample arms. In the simplest configuration, the reference arm is terminated by a mirror; in the sample arm, the light is weakly focussed into a sample. The interference signal between the reflected reference wave and the backscattered sample wave is then recorded. The axial optical sectioning ability of the technique is due to the following reason: Because the light is emitted from a broadband source (large range of optical wavelengths), a strong interference signal is only detected when the light from the reference and sample arms has travelled the same optical distance! Specifically, coherent interference is observed only when the optical pathlengths differ by less than the coherence length of the light source, a quantity that is inversely proportional to its optical bandwidth. The act of translating (axially scanning) the reference arm reflector is equivalent to performing optical sectioning of the sample, allowing for the generation of map of optical reflectivity versus depth. Transverse scanning of the sample (to build up a two- or three-dimensional tomographic image) is achieved via rotation of a sample arm galvonometer mirror.

Figure 1. Schematic of an OCT system based on a Michelson interferometer, showing its key components in their simplest forms.

After 15 or so years of research, the modality is continuing to establish itself as a medical imaging technology suitable for routine clinical use. Sample types that have garnered the most attention to this point have been largely transparent tissues such as the human eye and developmental biological models, and the human gastrointestinal tract and cardiovascular system (via endoscopic imaging).

Comparison with alternate techniques

OCT may be directly compared with alternative techniques in terms of several different criteria: resolution, imaging depth, acquisition time, complexity, and sample intrusiveness. With regard to the first two, OCT occupies a niche represented in Fig. 2: its imaging depth is typically limited to a few millimetres, less than ultrasound, magnetic resonance imaging (MRI), or X-ray computed tomography (CT), but its resolution is greater. This comparison is reversed with respect to confocal microscopy. Like ultrasound, the acquisition time of OCT is short enough to support tomographic imaging at video rates, making it much more tolerant to subject motion than either CT or MRI. It does not require physical contact with the sample, and may be used in air-filled hollow organs (unlike ultrasound). OCT uses non-ionising radiation at biologically safe levels, allowing for long exposure times, and its level of complexity is closer to ultrasound than to CT of MRI, allowing for the realisation of low-cost portable scanners.

Figure 2. Comparison of OCT resolution and imaging depths to those of alternative techniques; the "pendulum" length represents imaging depth, and the "sphere" size represents resolution.

OCT has many advantages when compared to alternative modalities:

• The strength of the optical sectioning capability is conveyed by heterodyne interferometric detection. Because of this OCT can image to greater depths than confocal microscopy in highly-scattering tissues.
• The axial resolution of the optical section is not dependent on the numerical aperture of the optical system, which bestows an advantage when the numerical aperture is limited, e.g., in the human eye or in endoscopic imaging.
• Point scanning avoids crosstalk from neighbouring lateral sites in the sample.
• In common with confocal microscopy, high-speed scanners make real-time operation at video rates feasible.
• Images in depth, which match the orientation of conventional histological sections in many fields of medicine, may be naturally produced, making the interpretation of OCT images more readily accessible to clinicians than en face images.
• The point-scanning feature can be implemented in fibre optics, which makes endoscopic and catheter-based imaging possible.

Common to other optical techniques, OCT is sensitive to the distorting effects associated with a ligth wave propagating through a turbid, scattering sample. The signal is strongly attenuated, and multiple scattering effects tend to corrupt the detected signal. However, OCT also possesses one important disadvantage that is limited to coherent imaging systems: OCT images are subject to the corrupting effects of speckle, the coherent interference of multiple lightwaves, which can limit image fidelity, resolution and depth range.

Recent developments

Recent developments in the OCT field³ have included the deployment of sources of extremely broad bandwidth, primarily based on supercontinuum generation in various forms of optical fibre, but also employing incandescent sources, to achieve axial resolutions of about 1μm. (Standard current-generation semiconductor sources typically achieve resolutions of about 10μm.) The advantages of polarisation-sensitive imaging are being realised; sample birefringence (dependence of the phase velocity on polarisation) and dichroism (dependence of the amplitude on polarisation) are being exploited as contrast mechanisms. Finally, it has been recognised in the last few years that there is a significant sensitivity advantage in performing OCT in the spectral domain; instead of translating the reference arm, the source wavelength is scanned, or full spectral detection is employed.

OCT research at OBEL

OCT research in OBEL is broad, encompassing strictly theoretical modelling, fundamental-level experimental research, system design and implementation, software development, and clinical measurements.

References

1. D. D. Sampson, T. R. Hillman, Optical coherence tomography, Lasers and Current Optical Techniques in Biology, G. Palumbo and R. Pratesi, eds. (ESP Comprehensive Series in Photosciences, Cambridge, UK, 2004), pp. 481-571.
2. D. Huang, E. A. Swanson, C. P. Lin, J. S. Schuman, W. G. Stinson, W. Chang, M. R. Hee, T. Flotte, K. Gregory, C. A. Puliafito, and J. G. Fujimoto, Optical coherence tomography, Science, 254, 1178-1181 (1991).
3. D. D. Sampson, Trends and prospects for optical coherence tomography, in 2nd European Workshop on Optical Fiber Sensors, edited by J. M. López-Higuera, B. Culshaw, Proc. of SPIE, Vol. 5502, (SPIE, Bellingham, WA, 2004), pp. 51-58.