Clinical Imaging with Optical Coherence Tomography

Abstract

Recent advances in optical fiber and wave-guide technologies have led to the availability of clinically viable optical coherence tomography (OCT) systems. OCT provides cross-sectional images of tissue with a resolution of approximately 10 m. Its relatively shallow depth of penetration (approximately 2–3 mm) requires the use of fiberoptic catheters and endoscopes for accessing internal organs, but flexible, narrow-diameter OCT probes are readily constructed with standard telecommunications optical fiber (diameter, 250 m). Recent studies have described the use of OCT for imaging in several human organ systems. This article reviews the current technology and provides an overview of recent OCT studies in gastroenterology and cardiology. Imaging techniques such as radiography, computed tomography, magnetic resonance (MR) imaging, and ultrasound (US) allow noninvasive investigation of largescale structures in the human body, with resolutions ranging from 100 m to 1 mm. For many disease processes, however, including cancer in its early stages, higher resolution is necessary for accurate diagnosis. In addition, some clinical screening procedures, such as random biopsies for the detection of high-grade dysplasia in Barrett esophagus, could be improved by the use of a high-resolution, noninvasive imaging technique to determine which biopsy site corresponds to the most severe disease. Finally, in locations where biopsies cannot be performed because the results would be catastrophic (eg, the coronary arteries), high-resolution, noninvasive imaging is necessary for diagnosis. To address these and other clinical problems in situ requires a noninvasive imaging technology with a resolution approaching that used in conventional histologic studies. OCT, first introduced in 1991 (1), is a cross-sectional optical imaging technique that can obtain images with an axial (depth) resolution of 10 m. OCT measures the path length traveled by the interrogating beam incident on the tissue sample by using an optical technique known as interferometry (1,2). This process is commonly accomplished by dividing the source light into two identical beams with a Michelson interferometer and directing one beam to the sample and the other to a reference mirror with a known location. When light returns from both the sample and the reference mirror it is recombined at a detector, and the interference between the two beams is registered. The use of temporally incoherent light allows the distance traveled by the sample beam to be determined, since interference can occur only when light from both beams arrives at the detector simultaneously. The axial or depth resolution of the OCT system is determined by the property of light referred to as the coherence length. With advanced femtosecond laser systems, OCT resolution can be as low as a few micrometers (3–5). A single line or axial scan in an OCT image is obtained by changing the delay in the reference arm and recording the interference modulation amplitude as a function of reference arm delay. Transverse scanning of the sample arm beam across the specimen during recording of the axial data for each lateral location allows an entire two-dimensional OCT image to be formed. OCT, performed with a source center wavelength of 850 nm, was first used to image posterior and anterior Acad Radiol 2002; 9:942–953