Optical coherence tomography in the gastrointestinal tract
- Shai Friedland, MD
Shai Friedland, MD
- Assistant Professor of Medicine
- Palo Alto VA and Stanford University Medical Center
- Jacques Van Dam, MD, PhD
Jacques Van Dam, MD, PhD
- Professor of Medicine
- USC School of Medicine, Los Angeles, CA
Components borrowed from the telecommunications industry have been applied to medical imaging to improve resolution as never before. Optical coherence tomography (OCT) is an emerging medical imaging technology that relies on the backscattering of light to obtain cross-sectional images of tissue. Many of the early applications of OCT were in ophthalmology, where the transparency of anterior structures of the eye facilitated high-resolution imaging of the retina. More recently, there have been several pilot studies using OCT in the gastrointestinal tract. These have demonstrated the feasibility of this technology to enhance endoscopic imaging of the superficial layers of the esophagus, stomach, bile ducts, pancreatic duct, and colon. OCT imaging has demonstrated anatomic structures such as crypts and glands that could potentially permit endoscopists to diagnose mucosal abnormalities such as Barrett's esophagus. In 2013, an OCT system designed for imaging the esophagus became commercially available. (See 'Technical advances' below.)
OCT is similar in principle to ultrasonography but uses light waves rather than acoustical waves. As in B-mode ultrasonography, a quantitative measurement of backscattering is performed at each axial depth, and the measurements are repeated at different transverse positions. In this manner, a linear or radial two-dimensional map of backscattering strength is acquired [1-3].
Measurement of optical backscattering is performed by low coherence interferometry . This method uses a low coherence light source such as a superluminescent diode, which typically has a coherence length of approximately 20 micrometers. The incident light is split in two by an optical beam splitter, with one beam directed to the tissue via an optical fiber and the other beam directed to a mirror located at a precisely controlled distance. The backscattered light from the tissue is combined with the reflected light from the mirror. This results in interference only when the path lengths match to within the 20 microns of the coherence length of the light source. A quantitative measurement of optical backscattering at different depths is obtained by measuring the degree of interference at each mirror position as the mirror is moved.
The coherence length of the light source determines the maximal axial resolution that can be obtained. Transverse resolution is determined by the spot size of the focused beam directed at the tissue and the amount that the apparatus is translated at each during the scan; it is typically also approximately 20 microns. OCT is typically performed with near infrared light because tissue is relatively transparent; longer wavelengths penetrate deeper into biological tissues at these frequencies. Scattering of light in tissue limits the depth of scanning to approximately 1 to 2 mm in the gastrointestinal tract, generally restricting OCT imaging to the mucosa and submucosa when performed during endoscopy.
OCT is typically performed using catheters passed through the accessory channel of standard gastroscopes, colonoscopes, or duodenoscopes (picture 1):
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