The focus of my current research is on Optical Coherence Tomography. Specific imaging interests include intravascular stents and grafts, ligaments, and leg muscle perfusion.

See:

Stent images

Skin images

Experiments with microbubbles as a contrast agent

What is optical coherence tomography (OCT)?

 OCT is a non-destructive imaging technique which uses infrared light to visualize subsurface structure in biological tissues. Depths of a few millimeters can be imaged with about 20 micrometer (µm) resolution. OCT is analogous to ultrasound imaging, but the magnitude of reflected light is measured instead of reflected sound waves. OCT achieves relatively high resolution and large depth of imaging by combining characteristics of confocal microscopy and white light interferometry. OCT uses a special light source with a short coherence length in a fiber Michelson interferometer arrangement (Figure 1). Only light which travels the same distance in both the sample and the reference arms of the interferometer will interfere and be detected. When the reference arm is lengthened, light reflected from deeper in the tissue is selectively collected. Thus, images of subsurface tissue structure can be built up pixel by pixel, by moving the reference arm mirror and by scanning the sample arm across the tissue.

OCT has successfully be used to image many types of hard and soft tissue. One limited system is commercially available; generally, each instrument is hand-built. The current instrument contains an inexpensive source and sample arm optics suitable for in vitro imaging. Improvements to source quality (thus image resolution and depth of imaging) are currently underway. A catheter system for in vivo imaging will be available by summer 1999.

What do you see in OCT images?

OCT is a map of the reflectivity of the sample. Reflections are generated at index of refraction mismatches. In most tissues, main sources of reflection are collagen fiber bundles, cell walls, and cell nuclei. Dark areas on the image represent homogeneous material with low reflectivity, such as air or clear fluids.

The imaging light is attenuated in the sample, so there is an exponential decrease in the intensity of the image with depth. Blood attenuates the signal faster than collagenous tissues, fat and fluids attenuate the signal the least.

The unprocessed image scale is in terms of optical depth. It is normal to divide the z axis of the image by the average index of refraction of tissue (1.4) to convert the image to physical depth. This is accurate when the air-tissue interface is flat. (Bumps at the air-tissue interface will appear exaggerated by a factor of 1.4.)