HIGH RESOLUTION IMAGING OF MUSCULOSKELETAL DISEASE, PARTICULARLY CARTILAGE PATHOLOGY, WITH A NEW IMAGING TECHNOLOGY

INTRODUCTION Over the years orthopedic surgeons have used modalities such as plain radiography, magnetic resonance imaging (MRI), and ultrasound to assess joints and other musculoskeletal structures. Each of these imaging modalities has its own advantages; however, there are still instances when these technologies do not possess adequate resolution to effectively assess the relevant pathology. An example is the current inability to assess cartilage during treatment for osteoarthritis. We have developed a new technology, optical coherence tomography (OCT), for the assessment of articular cartilage, tendons, and ligaments. OCT is analogous to ultrasound, but measures the intensity of backreflected infrared light rather than sound.[1-3] These efforts in orthopedic imaging have received several awards, including the Presidential Award in Science and Engineering from President Clinton in 1998. OCT has several advantages for the assessment of musculoskeletal pathology. First, OCT has a resolution of 10 – 25 times that found in other clinical imaging technologies. Laboratory-based state-of-the-art OCT systems have attained resolutions as high as 4 μm.[4] Second, OCT has a faster speed of acquisition.[5] OCT can image with an acquisition rate of up to 16 frames per second, which could allow this technology to image surgical procedures in near real time. Third, since OCT is based on fiber optics, imaging instruments utilizing OCT technology can be built with cross-sectional diameters as small as 0.014 inches.[6] This opens the potential of designing OCT catheters to be incorporated into arthroscopic instruments or bedside needle-based devices. Fourth, the entire unit is compact, similar in size to an ultrasound unit, and can be readily transported into a surgical ward or clinic. Finally, since OCT is based on optics, it can be combined with other spectroscopic techniques to assess the optical and biochemical aspects of the tissue being imaged. TECHNICAL ASPECTS OF OCT The details of OCT have been previously described.[1-3] As stated, OCT is analogous to B-mode ultrasound, measuring the backreflection of near-infrared light rather than sound waves. Due to the high speed of light, the echo delay time cannot be measured electronically (as it is with ultrasound) and therefore OCT relies on a technique known as low coherence interferometry. Figure 1 depicts a schematic of a general OCT system and illustrates the principle of low coherence interferometry. The broad bandwidth light, which can be thought of as a series of pulses, is split into two separate arms, referred to the reference and sample arms. Light that passes down the reference arm is reflected back from a movable mirror. The sample arm directs the light toward the tissue being imaged. Once the light reaches the tissue it can be absorbed or scattered. Light backreflected from the tissue will ultimately be recombined with the light from the reference arm at the beam splitter. If the light has traveled the same path length in both arms, to within the coherence length (or in the context of our analogy, the pulse length), interference will occur when the light is recombined at the beam splitter. Therefore, OCT measures the intensity of this interference and uses it to represent backreflection within tissue. The beam in the sample arm scans the tissue to generate twoand three-dimensional images. Debra L. Stamper PhD is an Instructor in Orthopedic Surgery at Harvard Medical School and Scientist at Brigham and Women’s Hospital.

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