Over the last several years, the technology that enables optical coherence tomography (OCT) has significantly improved. When it first debuted, OCT was a natural fit for ophthalmologists to utilize the near-infrared light-based technology to create high-resolution images of the far reaches of the eye. Since eye tissue is translucent, OCT can provide images revealing retinal pathology and can also be used to diagnose and monitor many retinal diseases like glaucoma and macular edema. Now, a raft of medical applications for OCT has emerged and several exciting new applications are in development.
What is OCT?
OCT imaging is similar to ultrasound, but it employs reflected near-infrared light, rather than reflected sound, as the medium to create its image. The near-infrared source (typically 800–1300 nanometers) is split into two paths: One path is applied to the sample tissue and the other to a reference mirror. Interferometry is used to coherently gate the back-scattered reflections from the sample tissue with the light from the reference arm as the sample arm is scanned across the tissue. Digital signal processing algorithms are performed on the coherently gated light to achieve a depth-resolved axial scan. Stacking such scans results in 2D or 3D images of the tissue. OCT can typically resolve an image to a tissue depth of 3–5 millimeters (mm) at a very high resolution of less than 10 microns.
A key component in OCT systems, the reference mirror, was mechanical in first-generation time domain systems. As a result, the machines were slow and the resolution of the images was limited. Second-generation OCT systems replaced the mechanical reference mirror with a fixed mirror and employed a spectrometer and powerful digital signal processing techniques like Fast Fourier Transforms (FFT), magnitude computation, and log compression to resolve the embedded depth information and combine it with the lateral scan data in real time. This enabled dramatically reduced imaging times along with improved image resolution.
The majority of today's OCT medical systems are used for ophthalmologic purposes; however, several new and promising applications have emerged over the last few years. For example, Ear/Nose/Throat (ENT) physicians and pediatricians may make up a new group of doctors who adopt OCT technology as a diagnostic tool. Typically, the otoscope is employed by physicians to examine the ear, ear canal, and tympanic membrane for signs of redness that would indicate a bacterial infection. OCT could improve the certainty of diagnosis by imaging surface skin as well as subcutaneous membranes to determine the presence of infection-causing bacteria. Following several doses of an antibiotic, OCT systems could then be used to analyze whether the antibiotic has been effective. If the infecting biofilm has been removed, the patient could cease taking the antibiotic.
Other emerging OCT medical applications include dental diagnostic systems and inter-operative uses of the technology. Dentists could employ OCT imaging to identify early-stage cavities and certain gum disorders that might otherwise be missed by X-rays and visual inspection, resulting in more effective preventive procedures.
As an inter-operative technique, OCT could analyze the presence or absence of cancer during the surgical procedure to remove a tumor. Typically, surgeons remove tissue around a tumor with the hope that all cancer cells have been removed. The removed tumor and its surrounding margin tissue are then analyzed in a pathology lab — a process that typically requires weeks after the operation. Because OCT images offer physicians access to histology-resolution levels in real time, an OCT system could enable the surgeon to determine exactly how much tissue to remove with an exacting safe margin. By helping surgeons make better decisions during the first surgical procedure to remove a tumor, this technology could reduce the likelihood of costly and painful follow-up surgeries to remove the missed cancer tissue.
Several other medical applications of OCT will soon be available. For example, OCT can be employed in conjunction with needle biopsy to remove small, early-stage tumors. For a breast cancer patient, OCT could guide the needle to the precise location of the tumor with both visual and “smart” signal processing techniques to identify suspect tissue, minimizing the intrusive nature of the procedure. For a cardiovascular patient, OCT could be utilized with a very small catheter to better position a stent or to check plaque build-up. Advanced digital signal processing techniques will not only enable superior image quality but also make possible tissue classification in these types of applications.