Tracking the OCT Revolution
Tracking the OCT Revolution
How OCT has transformed the clinical landscape and earned its standing as an established pillar of eye care at all levels.
BY FRANK CELIA, CONTRIBUTING EDITOR
OCT arrived at an opportune moment in medical history… just in time to play handmaiden to the success of anti-VEGF retinal therapy, arguably the most significant vision-care achievement of our time. Because anti-VEGF drugs, and therapies such as lasers and steroids, produce limited durations of effect and better outcomes on early-stage disease, there exists a need for heightened monitoring of existing retinal patients and quicker diagnosis of new ones. OCT excels at both.
OCT also continues to impact glaucoma, though advancement here has been slower. This is likely due not only to glaucoma’s more elusive pathogenesis, but also to the more gradual progression of the disease. Whereas a condition such as diabetic retinopathy develops over a period of months, glaucoma can take years or even decades to diagnose or progress. So, for example, though incremental breakthroughs have been made in glaucomatous OCT progression analysis, thus far, none have involved spectral domain machines. Studies are ongoing.
Outside subspecialties, OCT can help resolve vague, general vision complaints as well. Optometrists tell of patients with no known refractive error who demonstrate 20/25 vision and upon routine OCT examination reveal vitreomacular traction or an incipient macular hole.
In short, this fast, non-invasive imaging technique holds an important place in a wide range of eyecare endeavors. This article explores some of the practical uses for OCT and how they might evolve in the future.
“Prior to the advent of OCT, VMT was a rare diagnosis, imperceivable using fundoscopic evaluation alone. Today, OCT aids in the diagnosis of VMT, which is now recognized as the pathogenesis of a spectrum of maculopathies including macular holes and macular edema.”
— Diana Shechtman, OD, FAAO Associate Professor of Optomertry Nova Southwestern University
Most Eyes Have It
Like any maturing industry, OCT manufacturers have begun to move toward a state of equilibrium wherein a handful of established brands dominate the market (OPKO Health, Inc. sold its OCT assets to retinal photography company Optos, Inc., in 2011, for example). In such environments, products begin to resemble each other as industry standards emerge. One such standard has been the rise of gaze-tracking systems.
Although they employ widely different methodology, all tracking systems recognize when a patient’s eye move-ment or blink has skewed an OCT scan, and then auto-matically perform an additional scan to correct the error. Eye tracking is often advertised as a “time saver,” but the technology’s true value lies in guarding against operator error. A skilled, attentive OCT operator should notice when eye movement distorts a scan, and then simply re-take the scan. Unfortunately, this doesn’t always happen. By automating this task, eye tracking removes image-quality decisions from human hands, which provides more accurate results and simplifies the training process for staff.
Perhaps the most noteworthy clinical advantage of eye tracking is its proven precision in calibrating the registration of images from one office visit to the next, sometimes down to the micron,1 creating enormous monitoring potential over successive scans. “If I’m treating a patient with the goal of decreasing retinal thickening, it’s ideal to have each scan image at the exact same location,” says Ronald C. Gentile, MD, FACS, FASRS, professor of ophthalmology at The New York Eye & Ear Infirmary. “This provides a more accurate and quantitative map of the location of the decrease in retinal thickening and also the opposite, indicating where the thickening may be getting worse. This technology can actually tell you in microns how much better or worse the retinal thickening is.”
Gaze tracking holds so much appeal, almost every major OCT manufacturer now offers it, the sole exception being Bioptigen (See “Coordinated Movement,” page 14-S). This was a conscious decision based on Bioptigen’s unique market niche, says CEO Eric L. Buckland, PhD. “There are many numbers thrown around about imaging, but we feel safe saying our machines are unique in their ability to scan at high density, at high speed,” Dr. Buckland explains. “With high-density scans captured without a need for ‘averaging’ images, we better sample pathology that is within our field of view. Typically, in the systems that offer eye tracking, they take some of the computer memory we expend on collecting data and put it toward averaging the images to reduce image noise.” This creates a high-contrast image well suited to a busy private practice, he concedes — but also an image containing less information. “It’s fair to say eye tracking can add real value to a busy clinical practice, but Bioptigen’s research-oriented customers want unbiased data and richer information content.”
In patients with too much movement, eye tracking can fail, but the Bioptigen system continues to collect data, he says. Where others offer eye tracking, Bioptigen provides user-defined protocols for research studies that rely on a multiplicity of scans to visualize pathology, allowing accurate repetition across patients and times within a research program, according to Dr. Buckland.
Carl Zeiss Meditec, with more than 10,000 OCT devices in the field, also hesitated before developing an eye tracking system, wary about tradeoffs in data density and scan time spent re-calibrating and re-shooting. But ultimately the company decided it was worth adding it as an option to achieve better image quality and interscan registration, according to Christine Ritter, senior director of OCT imaging. “Eye tracking gives us some improvement in the reproducibility of the macular thickness measures on the dense cube rasters, on the order of two or three microns — not a huge benefit, but it’s there for difficult cases where the patients do not fixate well,” she says. “In addition to the raster cube improvements, tracking also enables a series of best-of-the-best quality b-scans that can be placed at exactly the same location every visit. When there’s a difficult patient, or when you want to examine the subtle details of a lesion using the highest quality image, that’s the time to use our FastTracTM tracking feature.”
Can Extensive Data Help Answer Enduring Questions?
Researchers remain hopeful that OCT technology’s vast computer memory and large population databases may one day help untangle some of the mysteries surrounding glaucoma. Profound questions endure: What metric best defines and measures disease progression? Does progression occur linearly at a fixed rate, or stepwise with great clumps of cells dying at variant speeds? Which computer algorithm is most useful, event-based or trend-based analysis?
Limited progress has been made in answering these questions. For example, trend-based analysis appears to be edging out event-based formulas as
the preferred metric. As one well-regarded study put it: “Measuring the rate of change in [retinal nerve fiber layer] thickness [only achievable with trend-based analysis] would serve as a new paradigm for observing and managing patients with glaucoma.”2
The same paper tentatively suggests glaucoma progresses linearly, while conceding that additional and lengthy follow up is needed to confirm that hypothesis.2
The theory that structural damage, as measured by diagnostics such as OCT, precedes functional damage, as measured by visual field testing — another hotly debated glaucoma topic — appears to be gaining ground.3 One recent study suggests “progression as measured by a decrease in RNFL thickness is more noticeable than is progression assessed by visual field measurements in early-stage glaucoma patients, whereas the reverse is true when the disease is more advanced.”3
“To perform trend analysis, we need stable software platforms, and we’re starting to see that with the spectral domain OCTs.”
— Nathan M. Radcliffe, MD Director of Glaucoma Service, Weill Cornell Medical College
Last year, the FDA cleared Bioptigen’s Envisu Spectral Domain Ophthalmic Imaging System (SDOIS) for use on pediatric patients. The system’s handheld scanner can image patients of any age, from premature and neonatal infants to adults, whether upright or supine, ambulatory or confined. The ability to capture sharp images rapidly and at high density without averaging is what makes handheld OCT feasible, according to the company.
“Our system is enabling new levels of research,” says Dr. Buckland. “There are a number of trials using our system to evaluate how the retina develops right after birth.” A deeper understanding of how healthy retinas grow may help researchers develop ways to treat retinopathy of prematurity (ROP), which continues to be a problem for premature infants, he notes.
The retina undergoes rapid development up to age two, and then significant growth up to age five. Here too OCTs could reveal practical information, says Dr. Buckland: “Certainly a 2-year-old toddler cannot be imaged by a standard clinical lane [OCT] system. And for subjects, young or old, who are under anesthesia, our mobile handheld systems extend the application to the operating theater.”
Raising the Refractive Bar
OCT technology can play a significant role in refractions, especially in cases of vitreomacular traction (VMT), which often begin as vague visual complaints. “Prior to the advent of OCT, VMT was a rare diagnosis, imperceivable using fundoscopic evaluation alone,” says Diana Shechtman, OD, FAAO, an associate professor of optometry at Nova Southwestern University. “Today, OCT aids in the diagnosis of VMT, which is now recognized as the pathogenesis of a spectrum of maculopathies including macular holes and macular edema.”
Co-management opportunities abound. The early and intermediate stages of dry AMD can last for years, even decades, Dr. Shechtman notes. “In the early or even moderate stage of the disease, so long as the optometrist has the capabilities and instruments, we can certainly follow these patients and play a critical role in their management by making the proper nutritional supplementation recommendations and educating them about lifestyle modification.”
Moving Toward Standardization
As trend-based analysis emerges as the likely preferred algorithm for monitoring glaucoma, one physician is glad to see the OCT industry moving toward agreed-upon industry standards and common functionalities such as eye tracking. “To perform trend analysis, we need stable software platforms, and we’re starting to see that with the spectral domain OCTs,” says Nathan M. Radcliffe, MD, director of glaucoma service at Weill Cornell Medical College. Accurate trend analysis requires at least five scans (ideally over many years) with an identical acquisition protocol. “If these software developers change their minds every 6 months, we’ll have to keep going back to square one,” he says. “I’m happy the CIRRUS has had the same data cube since 2007. That’s the kind of stability we need.”■
1. Wolf-Schnurrbusch UE, Ceklic L, Brinkmann CK, et al. Macular thickness measurements in healthy eyes using six different optical coherence tomography instruments. Invest Ophthalmol Vis Sci. 2009;50:3432-3437.
2. Leung CK, Cheung CY, Weinreb RN, et al. Evaluation of retinal nerve fiber layer progression in glaucoma: a study on optical coherence tomography guided progression analysis. Invest Ophthalmol Vis Sci. 2010;51:217-222.
3. Lee EJ, Kim TW, Weinreb RN, Park KH, Kim SH, Kim DM. Trend-based analysis of retinal nerve fiber layer thickness measured by optical coherence tomography in eyes with localized nerve fiber layer defects. Invest Ophthalmol Vis Sci. 2011;52:1138-1144.
As OCT technology matures and becomes a permanent part of the vision care world, a prevailing trend has been the addition of gaze-tracking software designed to compensate for patients’ eye movements during the scanning process. Gaze tracking produces consistently cleaner images and increases the visit-to-visit reproducibility of scans. To date, nearly every major OCT manufacturer has added some version of this technology to their devices.
3D OCT 2000. Topcon’s latest OCT device comes standard with infrared tracking and automatic motion correction software. If eye movement is detected, the 3D OCT will automatically rescan the affected area, according to the company.
Cirrus HD-OCT 5000. CZM introduced its latest CIRRUS iteration last year, which includes FastTracTM, the company’s inaugural retinal tracking software. As the name implies, the tracking software rapidly acquires crisp, precise images, without sacrificing the data density for which the CIRRUS is known, according to the company. Additionally, this gaze tracking feature can be switched off at the operator’s convenience.
RTVue FD-OCT. Also released last year, Optovue’s VTRAC Eye-Tracking upgrade employs a novel method of real-time video image processing that tracks at 30 frames per second for following patient eye movement. VTRAC also expands the scan depth to nearly 3 mm for visualizing both the vitreomacular interface as well as the choroid with great detail, according to the company. The tracking upgrade is software-based, so it doesn’t require additional hardware.
Spectralis. Heidelberg led the pack in eye tracking, having included it as standard for years. TruTrack, the name of the company’s proprietary software algorithm, recognizes and uses the retina and other eye structures to align images. It was first used in the HRT to align tomographic retinal scans for reconstructing 3-D images of the optic nerve head. The software was later applied to retinal imaging on Heidelberg’s HRA platform and then to edema and thickness measurements on the HRT Retina Module, according to the company.
Optometric Management, Volume: , Issue: July 2013, page(s):