Ultra high-speed OCT provides minute structural details, potential applications

Ultra high-speed optical coherence tomography (OCT) offers the potential to follow minute changes in patients with various pathologies. Johannes de Boer, PhD, the Wellman Center of Photomedicine, Harvard Medical School, Massachusetts General Hospital, Boston, reviewed the principles of OCT and the status of ultra high-speed OCT.

"OCT is analogous to ultrasound imaging that uses infrared light instead of sound-the difference between light and sound being that light propagates at five orders of a magnitude faster than sound," Dr. de Boer explained.

"Spectral domain offers a significant advantage over the previous-generation OCT because it provides two to three times better sensitivity," Dr. de Boer said. "Spectral domain OCT allows faster scanning and maintenance of image quality."

One of the first retinal images of 6-µm resolution obtained in the early 1990s provided a 72-fold improvement over the commercial system in speed and 1.5 times in depth resolution. One disadvantage is that the sensitivity of spectral domain OCT is depth-dependent, and the sensitivity decreases by a factor of four at a depth of 1 mm compared with Stratus OCT (Carl Zeiss Meditec), in which the depth response of the system is uniform and the sensitivity is the same. Swept-source OCT can overcome this drawback because the technique uses a laser to obtain the tissue profile.

Dr. de Boer described the potential clinical applications of the technology in glaucoma. He demonstrated that thinning of the superior nerve fiber bundle could be visualized. This may allow clinicians to follow patients with glaucoma over time and offers a comparison of the retinal nerve fiber layer thinning from one time point to another on difference maps.

Clinical application also can be used in patients with age-related macular degeneration because blood clots, subretinal fluid, and suspected choroidal neovascularization can be imaged as well as the scattering of the photoreceptors-leading to a consideration of the changes in the scattering properties of the photoreceptors in these patients and whether these changes are indicative of photoreceptor survival.

Dr. de Boer said that there will be an emergence of increasingly fast OCT systems, but he believes that the technology is limited by the amount of light that is currently introduced into the eye and the resultant quality of the images. He suggested that adding more power in the retina in a shorter amount of time might be a solution.