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August 2007Currently, the use of optical coherence tomography (OCT) in otolaryngology is somewhat reminiscent of those early Apple days, but it will not stay this way for long. OCT is the next great transformative medical imaging modality, said Brian J. F. Wong, MD, PhD, Professor and Associate Director of the Division of Facial Plastic Surgery, Department of Otolaryngology-Head and Neck Surgery at the University of California, Irvine.
OCT is a rapidly evolving, noninvasive, high-resolution optical imaging technique that produces cross-sectional images (7-10 μm) of living tissues to depths of up to 3 mm, using light in a manner similar to ultrasound.1
OCT technology is based on interferometry, which was invented by Michelson more than 100 years ago, said Zhongping Chen, PhD, Professor and Vice Chairman of the Department of Biomedical Engineering at the Beckman Laser Institute at the University of California, Irvine.
To produce images of tissue structure at the histological level, OCT combines a broadband, low coherent light source with interferometry and signal processing.2 An optical beam is focused into the biological tissue and the echo time delay of light reflected from the internal microstructure at different depths is measured by interferometry (see sidebar).3
At UC Irvine, our earlier devices captured images at 1 frame/second [fps], but now we are imaging at 8 to 10 fps and will soon be able to image in real time or 30 fps, said William B. Armstrong, MD, Associate Professor of Clinical Otolaryngology-Head and Neck Surgery at the University of California, Irvine. We are keeping a database of the images in digital form, or bitmap, and can convert them to JPG or TIF files and access them through our electronic medical records.
The first clinical applications of OCT were in ophthalmology to measure structures within the globe and image retinal pathology.4 Most people recognize Professor James G. Fujimoto at Massachusetts Institute of Technology as the first to use OCT to image biological tissue in the early 1990s, said Dr. Chen. OCT applications have expanded into cardiology, dermatology, gastroenterology, and urology.
OCT allows you to analyze subsurface structures without doing a biopsy, said Dr. Armstrong. This concept of ‘optical biopsy’-that is, sampling the tissue to see what’s inside without invasively removing it-is the ‘holy grail’ of much of the current OCT research.
OCT Applications in Otolaryngology
Our group at UC Irvine is leading the way in otolaryngological OCT research, as we have constructed different devices and used OCT on different patient populations, said Dr. Wong.
We see OCT as a hammer and we’re looking for nails, continued Dr. Wong. We are trying to determine the most important ENT problems that OCT can address and then match the technology to the application.
Other researchers, such as those working with Johannes F. De Boer, PhD, at Massachusetts General Hospital in Boston and those at the Nizhny Novgorod Medical Academy and the Institute of Applied Physics of the Russian Academy of Sciences, are also studying OCT, focusing primarily on imaging the larynx and radiation-related changes for head and neck cancer, respectively.
OCT has most commonly been used to image laryngeal tissue microstructure and determine minute changes in benign, premalignant, and malignant laryngeal pathologies of anesthetized adults who are undergoing surgical head and neck endoscopy in the OR. In most studies, images are acquired by inserting a fiberoptic OCT imaging probe through the laryngoscope, and then placing the tip near the region of interest.
We are looking to see if the basement membrane is intact, checking for scarring and determining the thickness of the epithelium, said Dr. Armstrong.
The Irvine group has also developed an office-based OCT sampling device that can be used in tandem with a conventional rigid laryngeal endoscope to image human vocal cords while the patient is awake.5 This technology provides otolaryngologists with information that can help them decide whether or not to pursue surgery, guide and direct biopsies, and potentially diagnose early laryngeal cancer.
One of the problems we deal with in cancer is sampling errors, said Dr. Armstrong. OCT has the potential to help you select your biopsy so that you get the best diagnostic yield.
Additionally, these researchers have used OCT to image the mucosa overlying structures in the nasal cavity to obtain information regarding normative in vivo tissue microstrucure. By inserting the scanning tip of the fiber-based handheld probe into the patient’s nasal cavity, either in the office or in the OR during surgical endoscopy, they are able to identify the distinct boundaries between the epithelium, lamina propria, and underlying bone/cartilaginous tissue.6
We are extremely excited about our most recent work in neonatal and pediatric patients, said Dr. Wong. In neonates, we are looking at factors and features that are related to extubation criteria-in particular, the evolution of subglottic stenosis due to prolonged mechanical ventilation.
Imaging the airways of pediatric patients may allow us to determine why the child has an airway problem and determine the prognosis for recovery, said Dr. Armstrong. Is the narrowing of the airway due to edema, scar tissue, or cartilage? Is it reversible or no? Will the child need a tracheotomy? These are some of the questions we are able to answer by using OCT.
Polarization-sensitive OCT (PS-OCT), which is more sensitive to collagen fiber orientation and density, has been used in conjunction with OCT to image human vocal fold tissue in vivo in a study at Mass General.7
The purpose of our study was to test the technology’s ability to better delineate microscopic vocal fold structure and/or disease, said Adam M. Klein, MD, who is now Assistant Professor at the Emory Voice Center in the Department of Otolaryngology-Head and Neck Surgery at Emory University in Atlanta. Although we were successful at imaging the human vocal folds in the asleep and awake patient, there is still a substantial amount of development that must occur before these technologies can be a viable clinical tool.
Not Ready for Prime Time
Whenever I explain the concept of OCT and its potential in the field of otolaryngology, most ENTs think it’s very exciting and wonder why we don’t have a self-contained unit ready for them to use right now in their office, said Dr. Armstrong.
Although things are progressing faster than we predicted, we are still in the early stages of development, he continued. For other specialties, such as ophthalmology, OCT is fairly straightforward, as the eye is easily accessible for observation. Otolaryngology, on the other hand, requires more mechanical expertise, as we are trying to put a probe down someone’s throat, while dealing with patient and physician movements, vibrations, and gag reflexes.
Each time we use OCT, we have to have an engineer present, tweaking the equipment. We have had to overcome many technical hurdles, like delivery, access, ergonomics, resolution and speed of image acquisition, just to get this far. Further refinement of the prototype is necessary before it can be commercially available.
One of the biggest problems we had initially was condensation on the optical probe, said Dr. Chen. The hot air from breathing condenses on the window surface and smears the image. We developed a special coating to solve the problem.
Right now, we sterilize the probe each time, but in the future, a disposable device would be better, he added.
In order for this technology to be useful in the average clinician’s hands, engineers will have to design a much smaller, user-friendly piece of equipment with an on/off button that will automatically gather the signal, calibrate itself, and give you a report and visual images for analysis, said Dr. Armstrong.
Physicians treating voice disorders will be interested in OCT because it provides information about the microstructural anatomy of the vocal fold mucosa, said Dr. Klein. To the surgeon and patient, every millimeter matters.
One of the promising things you can do with OCT is map out scars on the vocal cords, he continued. On the horizon, there are businesses trying to develop a permanent gelatinous material that can be injected into a stiff vocal cord so that it will regain its vibration. When this capability arrives, an instrument like OCT can help us to identify the size and location of a scar area, so that we can precisely inject the substance.
Some may think OCT is like ‘Star Wars’ and too far off into the future, said Dr. Armstrong. But the technology is here today and improving rapidly. OCT has great promise in how we, as otolaryngologists, will clinically manage our patients. It will allow us to plan, target, follow, and manage a disease in a more effective and noninvasive manner.
Interferometry
Interferometry is the science and technique of superposing two or more waves (i.e., putting one on top of the other), resulting in an interference fringe pattern. In a Michelson interferometer, a light beam is split into two beams, a reference and sampling beam. Light reflected from the reference mirror and sample, forms an interference fringe as the detector. If a low coherent broadband light source (i.e., light with multiple colors) is used, interference fringe is formed only when the light in the reference and sampling arms travel at the same distance.
References
- Armstrong WB, Ridgway JM, Vokes DE, et al. Optical coherence tomography of laryngeal cancer. Laryngoscope 2006;116:1107-13.
- Ridgeway JM, Armstrong WB, Guo S, et al. In vivo optical coherence tomography of the human oral cavity and oropharynx. Arch Otolaryngol Head Neck Surg 2006;132:1074-81.
- Klein AM, Pierce MC, Zeitels SM, et al. Imaging the human vocal folds in vivo with optical coherence tomography: a preliminary experience. Ann Otol Rhinol Laryngol 2006;115(4):277-84.
- Armstrong WB, Ridgway JM, Vokes DE, et al. Optical coherence tomography of laryngeal cancer. Laryngoscope 2006;116:1107-13.
- Guo, S, Hutchison R, Jackson RP, et al. Office-based optical coherence tomographic imaging of human vocal cords. J Biomedical Optics 2006;11(3):030501 -1/3.
- Mahmood U, Ridgway J, Jacson R, et al. In vivo optical coherence tomography of the nasal mucosa. Am J Rhinol 2006;20:155-9.
- Klein AM, Pierce MC, Zeitels SM, et al. Imaging the human vocal folds in vivo with optical coherence tomography: a preliminary experience. Ann Otol Rhinol Laryngol 2006;115(4): 277-84.
©2007 The Triological Society