OCT arrives in the Laboratory

Optical Coherence Tomography is attracting a lot of interest in the medical community due to its ability to generate high resolution images in real time. Here, we learn why it is no longer considered the DIY option.

FOR more than a decade, Optical Coherence Tomography or OCT has been confined to application in research by specialists with the skills to design and build their own equipment from the core optical components. Now, due to improvements in available light sources and advances in the speed of capture and quality of OCT images, practical instruments are commercially available and finding application.

OCT is a technique for obtaining sub-surface images of translucent or opaque materials at a resolution equivalent to a low-power microscope. It is attracting a great deal of interest in the medical community, because of its potential to provide images of tissue morphology at a much higher resolution than is possible with imaging modalities such as MRI, or ultrasound. For example, the basal membrane in epithelial tissue can be seen in OCT images (Figure 1). Disruption of this basal membrane is an indicator of malignant cancer. Currently, medics have no alternative to the often-lengthy biopsy process - i.e. surgically removing the tissue and sending it to a histopathology laboratory for analysis. It is not surprising therefore that they are interested in the potential of OCT imaging to provide informative images, in real time.

OCT can deliver much higher resolution than other imaging modalities because it is based on light and optics, rather than sound or radio frequency radiation. With OCT a laser beam is projected into the subject, and light is reflected from the layers and sub-surface artefacts as the beam penetrates. Most of this light is scattered on its way back to the surface. Scattered light has lost its original direction and therefore cannot be used for imaging - this is why scattering material such as tissue appears opaque. However, a very small proportion of the reflected light escapes scattering and it is this non-scattered light that is detected and used in an OCT microscope.

	Figure 1: OCT image of skin tissue of palm of the author. The horizontal bar is 1mm. Note the stratus corneum (A) and basal membrane (B)
	Figure 2: OCT image of cancerous oesophagus tissue with superimposed histopathology image at the same scale, courtesy Dr. N.Stone, F. Bazant-Hegemark, Gloucestershire Hospitals NHS Foundation Trust

Non-scattered light retains its coherence, and can be made to interfere with the original source light in the interferometer of an OCT instrument, and the resulting interference fringes can be detected and filtered from the scattered light signal. The magnitude and position of the fringes provide the intensity and depth of the reflected light. Then by scanning the beam sideways through the material, a picture of the subsurface detail can be built up. (Figures 1, 2)

The technique is limited to imaging 1 to 2mm below the surface in tissue, because at greater depths the proportion of light that escapes without scattering is vanishingly small. However, this is deep enough for many applications. New, commercially available OCT instruments (Figure 3) can provide images in real time, and to an optical resolution of better than 10μm. Image capture time is 1/10th second or faster. They can also provide 3D images. No special preparation of the specimen is required, and images can be obtained ‘non-contact’ or through a transparent window or membrane.

Figure 3: 3D rendered image of the tip of the author’s forefinger. Note the sweat gland (arrowed)

At a practical level, OCT instruments are built from the same sort of components as a confocal microscope - a laser or superluminescent diode as light source, focusing and scanning optics, and a photodetector. There can be specialist components, for example some instruments use a high-speed tuneable laser as the light source, however, in broad terms, the cost (and size) of the equipment is more akin to a high-end microscope than to, say, an MRI scanner. It is also important to note that the laser output from the instruments is low – usually eye-safe near-IR is used – and no damage to the sample is therefore likely.

OCT has already reached mainstream application in retinal imaging1. The complex, layered structure of the retina is an ideal subject for OCT, and the race is now on to apply OCT to other applications. There are a large number of research groups working on a very wide variety of applications . Each year, people think of something else to image with OCT - as the saying goes, when you have a hammer, everything looks like a nail!

In the clinical sector, in-vivo applications are being driven by the design of the scanning device. For each clinical condition, a different type of scanning probe is required in order to get scan beam to the organ of interest. Many different probes are under development, including rigid endoscopic probes, flexible probes, and catheter-type probes.

However, the characteristics of a suitable application for OCT imaging do not limit it to use in the clinical field. These characteristics are:

• The specimen material scatters light, preventing use of conventional microscopy
• The specimen material allows light to penetrate – e.g. tissue and some ceramics and plastics, but not metals
• The features of interest are within 1 or 2mm of the surface
• The features of interest can be resolved with an optical resolution of ~10μm (similar to a low power microscope)
• Conventional analysis by cutting a cross-section of the sample is too slow, expensive or damaging to the subject

The future for OCT looks rosy, due to the flexibility of the technique and the power of imaging as a research tool. We expect further advances in light source and detector technology to drive improvements in image resolution and contrast during the next decade. So-called “functional” OCT uses Doppler processing or polarised light to produce images that are colour-coded to produce “velocity maps”2 and “birefringence maps”3 of the specimen. Most recently, there has been exciting work in developing nano-particle contrast agents4 that could be used to highlight tumour tissue in the live OCT image.

OCT imaging has now arrived in the laboratory in the form of commercial products that can be used for scientific research. OCT imaging fills a hole in the applications space by providing the researcher with images of sub-surface structure in translucent or near-opaque materials, at much higher resolution than ultrasound or MRI, and far greater depth than is possible with conventional microscopy. The prospects are bright for this technique, as further improvements in the underlying technology will bring ever-improving image quality and more ways to extract data from OCT.

References
1. Huang, D. et al. (2005) “Retinal Imaging” Elsevier
2. Yang, V. et al (2003) “High speed, wide velocity dynamic range Doppler
optical coherence tomography”, Optics Express Vol .11(19) pp2416-25
3. Nadya Ugryumova et al (2005) “The collagen structure of equine articular cartilage, characterized using polarization-sensitive optical coherence tomography”, J. Phys. D: Appl. Phys. 38 2612-2619  
4. Barton, J.K. et al. (2004), “Nanoshells as an optical coherence tomography contrast agent”, SPIE Proceedings, Vol. 5316, pp. 99-106


 By Jon Holmes, Chief Executive, Michelson Diagnostics Ltd, UK