Device focusSwept-source OCT and glaucoma
by Maxine Lipner EyeWorld Senior Contributing Writer
An example of the type of image that can be acquired with the Deep Range Imaging OCT-1 Atlantis Source: Topcon
A technology being investigated for use in posterior segment imaging may have a role in glaucoma diagnosis, according to the literature
First there was time-domain optical coherence tomography (TD-OCT), then spectral-domain OCT (SD-OCT); now a newer technologyswept-source OCTmay be able to image structures that the previous technologies could not, according to researchers involved in clinical studies of the device.
"With swept-source technology, a frequency swept light source and a high speed detector are used to detect the interference signal as a function of time, instead of a spectrometer and camera as in spectral-domain technology," explained James G. Fujimoto, PhD, Elihu Thomson Professor of Electrical Engineering and Computer Science, Massachusetts Institute of Technology, Cambridge.
"The light source emits different frequencies sequentially and is acquired at a very rapid speed. Also, the average wavelength is longer than SD-OCT. The premise is that you get deeper penetration into the tissue without the shadowing artifacts from overlying blood vessels like you do with SD-OCT," said Lucy Q. Shen, MD, Massachusetts Eye and Ear Infirmary, Boston, and instructor in ophthalmology, Harvard Medical School, Boston. Both swept-source and SD-OCT detect A-scans in the Fourier domain and have superior sensitivity compared to TD-OCT, Dr. Fujimoto said, adding that swept-source has less sensitivity roll off with range, and therefore the imaging range is longer than in SD-OCT.
Dr. Fujimoto's group developed a prototype instrument in 2010 that uses swept-source OCT at long wavelengths of 1050 nm "to achieve imaging speeds of 100,000 A-scans per second and improved image penetration," he said. The Casia SS-1000 (Tomey, Nagoya, Japan) is a Fourier-domain, swept-source OCT designed specifically for imaging the anterior segment; the device performs 30,000 A-scans per second, allowing diagnosticians to image 360 degrees around the anterior segment in 2.4 seconds. The Topcon Deep Range Imaging (DRI) OCT-1 Atlantis 3D SS OCT (Topcon Medical Systems, Oakland, N.J.) performs retinal imaging at 100,000 A-scans per second. Recently, however, Dr. Fujimoto's group has developed "an even higher performance swept-source OCT that can perform high speed retinal, anterior, and full eye length imaging," he said. "The newest swept-source OCT has the advantage that the light source frequency sweep range and repetition rate can be adjusted to tailor the resolution, imaging range, and axial scan repetition rate for the specific imaging application," Dr. Fujimoto and colleagues wrote.1
In the group's latest prototype, vertical-cavity surface emitting laser (VCSEL) technology is used to enable 3D OCT imaging spanning the entire eye "from the cornea to the retina," Dr. Fujimoto said.
Imaging the optic nerve head
At Massachusetts Eye and Ear, Dr. Shen is performing a research study using the DRI OCT-1 Atlantis 3D SS OCT in patients with glaucoma. "We want to determine if we can see more of the optic nerve head beyond what a clinician can see on slit lamp exam or beyond what the current commercially available technology (SD-OCT) can do," she said. While her group has been able to see more structures of the optic nerve with the swept-source technology, "it has not been consistent."
As she explained, "We're trying to image structures that are deep in the optic nerve head that are covered by blood vessels and other non-transparent structures, which have made imaging more difficult in general. Swept-source OCT can theoretically render those deep optic nerve structures in better resolution without much shadowing artifacts, compared to SD-OCT."
Dr. Shen did stress, however, that requesting these high tech devices to image a structure that is only 1.5 mm to 2 mm in diameter is analogous to finding the proverbial needle in the haystack. Ideally, Dr. Shen hopes her group's research provides answers about who may be more vulnerable to glaucoma or to glaucomatous progression based on what the optic nerve head changes may be. In another study at the University of Pittsburgh,2 swept-source OCT was used to examine in vivo the three-dimensional microarchitecture features of the lamina cribrosa in both healthy and glaucomatous eyes. In that study, the device used has a 100,000 A-scan/second scanning rate, a 1050 nm light source, and a 5-micron axial resolution, the authors wrote. The study, the first to automatically quantify the lamina cribrosa microarchitecture in 3D, found beam thickness to pore diameter ratio and pore diameter standard deviation were statistically significantly higher in glaucomatous eyes. The parameters were not statistically significantly associated with age (healthy eyes averaged 41 years old, while glaucomatous subject eyes averaged 71 years old). The authors wrote that because the microarchitecture differences are not readily apparent between glaucomatous and healthy eyes, "an automated quantification method is required in order to identify differences that may not be obvious."
The posterior lamina cribrosa "is the presumed site of optic nerve damage in glaucoma. Structural changes in that area are thought to impede flow within the axons," said Barry A. Schechter, MD, director of the cornea and cataract service, Florida Eye Microsurgical Institute, Boynton Beach, Fla. If swept-source OCT can discern details of more deeply located tissues, it might "provide earlier and more detailed views to help clinicians better treat our glaucomatous patients," he said. In the anterior segment, Dr. Schechter said the technology might prove useful in visualization of synechiae and angle structures. An improved ability to ascertain where microincisional glaucoma surgery devices have been placed is a potential use as well, he said. For now, Dr. Shen said she's "cautiously optimistic" about the future of swept-source technology.
1. Grulkowski I, Liu JJ, Potsaid B, et al. Retinal, anterior segment and full eye imaging using ultrahigh speed swept source OCT with vertical-cavity surface emitting lasers. Biomed Opt Express. 2012;3(11):2733-51.
2. Wang B, Nevins JE, Nadler Z, et al. In vivo lamina cribrosa microarchitecture in healthy and glaucomatous eyes as assessed by optical coherence tomography. Invest Ophthalmol Vis Sci. 2013;54:82708274.
Editors' note: Dr. Fujimoto has financial interests with Optovue (Fremont, Calif.) and Carl Zeiss Meditec (Jena, Germany). Drs. Schechter and Shen have no financial interests related to their comments. None of the swept-source devices mentioned in this article are commercially available in the United States.