The Evolution of Spectral Domain OCT

Speed, detail and precision make SDOCT the new gold standard.

BY ERIN MURPHY, CONTRIBUTING EDITOR

The FDA approved the first spectral domain optical coherence tomography (SDOCT) system in 2006,¹ and four others soon followed. In the past year, two more devices reached the market, for a total of seven SDOCT options.

SDOCT, sometimes called Fourier-domain or high-definition OCT, is faster, more detailed and more accurate than time domain OCT. These clinical advantages have made it the gold standard for OCT imaging, and advances in software, networking and device integration promise additional capabilities and uses as we move forward.

The New Standard

When it comes to imaging and measuring the eye, from corneal mapping and angle measurement in the anterior segment to retinal and optic nerve imaging in the posterior segment, SDOCT has replaced standard OCT as the machine of choice for most physicians and technicians.

SDOCT devices have 65 times higher resolution than time domain OCT. In fact, instead of producing 400 A-scans per second, the SDOCT captures a staggering 18,000 to 40,000 A-scans per second. Physicians get highly detailed images very quickly — often fast enough to be unaffected by a patient's movement or blink. According to many physicians, this makes the SDOCT their preferred diagnostic tool, and one that researchers may be required to use in future studies.

"Once you've experienced the higher resolution of SDOCT, there's no turning back," explains Nalin J. Mehta, M.D., who practices at the Colorado Retina Center in Denver. He's had the RTVue-100 (Optovue) since November 2006. "I can't deny a patient access to the enhanced diagnostic capabilities possible with the 90% to 95% increase in data density. SDOCT has become an integral part of our practice, superceding the use of time domain OCT. I'm certain that in the not-too-distant future, large-scale studies will require that we use SDOCT technology to generate more accurate and reproducible data."

Of course, the more data you incur, the more you must manage and interpret. That's why manufacturers are focusing on developing more complex SDOCT software to analyze large amounts of data and help turn the data into useful information. The software makes it simple for physicians to perform measurements and even compare results to normative databases. Physicians see a 3-D representation of the eye, measure thickness and volume more accurately, make earlier diagnoses and detect disease progression more easily. What's more, SDOCT can use software to automatically track the eye's movements, so even patients who are unable to focus can return a clear image.

The RTVue-100 (Optovue) allows physicians to review a 3-D presentation, rotate the images and rapidly scroll through B-scans.

Big Gains

If you're considering implementing SDOCT into your practice, let your colleagues be your guides. According to early adopters in the field, SDOCT demonstrates its advantages in three key areas: speed, resolution and registration.

1. Speed. "The real advantage of SDOCT is scanning speed," says Richard F. Spaide, M.D. In private practice at Vitreous-Retina-Macula Consultants of New York, Dr. Spaide has been using the 3D OCT-1000 (Topcon Medical Systems Inc.) since July 2007. "The A-scan rates for most SDOCT devices are 60 or more times faster than time domain OCT imaging systems, so we can obtain a detailed B-scan in hundredths of a second, compared to more than 1 second. This creates a number of new opportunities. The digital images are less affected by subject motion.

"However, the most useful attribute to me is that the rapid scan speed allows us to scan a large number of contiguous B-scans to obtain an area of mapped retina," Dr. Spaide continues. "I get 128 B-scans in a few seconds, and it only takes me a few seconds in the exam room to flip through those 128 B-scans. I find edema or subretinal fluid much faster and more accurately than I did with time domain OCT. We've also found that we can map out areas where there's boundary loss between the inner and outer segments of the photoreceptors."

2. Resolution. SDOCT produces images of such high visual quality that physicians are able to see parts of the eye they couldn't see before. "By sampling several images during eye tracking, SDOCT gives us unprecedented resolution to visualize the microstructural details of the retina," explains Frank G. Holz, M.D., professor and chairman of the department of ophthalmology, University of Bonn, Germany. He's been using the Spectralis (Heidelberg Engineering Inc.) for more than 2 years. He began working with the initial prototype.

The 3D OCT-1000 (Topcon) captures 18,000 A-scans per second at an axial resolution of 6 microns.

Dr. Spaide agrees. "Having higher resolution for research is helpful because it allows us to see more detail of the ocular structures," he says. "In clinical practice, it's possible in some patients to detect subtle accumulations of subretinal fluid in a single B-scan that aren't visible with time domain OCT."

David Huang, M.D., Ph.D., is director of Doheny Laser Vision Center in Los Angeles, and the Charles C. Manger III, M.D., chair in corneal laser surgery at the University of Southern California. He's been using the RTVue-100 since 2005, during its prototype development. "The resolution (5 microns) and speed (26 kHz) of the RTVue-100 allows corneal mapping (thickness, curvature), macular mapping (ganglion cell, retinal thickness, RPE elevation), and optic disc and nerve fiber layer mapping, which are important for both clinical and research applications."

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3. Registration. SDOCT devices can register images based on ocular structures, rather than patient fixation, and they can track the retina to compensate for a patient's movement. The results: fewer artifacts due to patient movement and better registration to help physicians compare the same locations over time.

"Time domain is slow, and it's not clear how images are related positionally, except in the appearance of the macula," says Yale L. Fisher, M.D., who's practiced at Vitreous-Retina-Macula Consultants of New York, in New York City for 40 years, and is clinical professor of ophthalmology at Bascom-Palmer Eye Institute in Miami. "In age-related macular degeneration (AMD), the macula gets so distorted over time that it becomes difficult to register the images. By comparison, some SDOCT devices have point-to-point registration, and some have pixel-to-pixel registration." According to Dr. Holtz, "If you rescan a patient after anti-VEGF therapy, you can align the vertical SDOCT scan precisely at the site where the image was taken before."

Some SDOCT devices integrate other imaging modalities, which then are precisely registered to each other. "Cross-sectional images of the retina can be simultaneously mapped to fluorescein or indocyanine green angiography, autofluorescence, red-free images or infrared images," Dr. Holz says. "These images offer clinicians an optical biopsy with pixel-to-pixel correlation of the various scanning modes, helping with the diagnosis of retinal disease and the monitoring of therapy."

Conversely, Dr. Mehta says there are a few drawbacks with combination devices. "If something goes wrong and the device requires repair, you lose all of the functions simultaneously," he explains. "This also impacts patient flow. Instead of patients undergoing multiple tests at multiple stations, patients have to line up and wait for a single machine."

Price is also a concern, according to Dr. Mehta. "Multifunction instruments may be too expensive for many individual practitioners who are trying to get the most value out of each newly-purchased device," he says. "And since we're used to making correlations between different types of imaging modalities in a retina practice, instantaneous point-to-point correlation isn't a necessity."

Clinical Application

Despite these concerns, SDOCT has proven clinical relevance in AMD, cataract and glaucoma. In fact, retinal specialists emphasize the dramatic difference in their ability to detect early changes in AMD.

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"Whether I'm looking at a patient with diabetic macular edema (DME) or AMD, the ease at which I can review each line of the scan allows for an efficient and accurate assessment of any pathology," says Charles W. Mango, M.D., clinical assistant professor of ophthalmology at Weill Cornell Medical College of New York Presbyterian Hospital. Dr. Mango has used the 3D OCT-1000 for a year at his office in Bronxville, N.Y.

SDOCT enables physicians to measure 3-D volume, retinal pigment epithelium (RPE) height and thickness and central thickness. With time domain OCT, physicians could infer that choroidal neovascularization (CNV) was occurring when they saw cystoid macular edema and subretinal fluid. Now physicians can see topographical analysis of the choroid and RPE before those other changes occur, so they're able to diagnose and treat CNV earlier. And finally, the tracking and registration features of SDOCT make point-to-point comparison possible. Paired with advanced AMD drugs, SDOCT is a potentially sight-saving advance.

Current Options

In 2007, only four SDOCT devices were on the market: the Cirrus HD-OCT (Carl Zeiss Meditec), the Spectralis OCT and Spectralis HRA+OCT (Heidelberg) and the RTVue-100 (Optovue). In July 2007, the FDA cleared the 3D OCT-1000 (Topcon Medical Systems Inc.). The SDOCT Copernicus (Reichert Inc.) hit the market in February 2008. Following are the features of these devices (listed alphabetically by manufacturer):

• Cirrus HD-OCT (Carl Zeiss Meditec)² — In less than 3 minutes, a clinician or technician can use the Cirrus to scan and analyze both eyes and print the results. The speed is an asset, as is the small footprint. But perhaps more interesting for practices transitioning to SDOCT is the device's data compatibility with Stratus OCT. Clinicians can compare data from both devices side by side, integrating historic data into subsequent exams with SDOCT. The Cirrus HD-OCT also stores more than 100,000 scans internally.

The Cirrus HD-OCT (Carl Zeiss Meditec) simultaneously captures 27,000 A-scans per second and line-scanning ophthalmoscope fundus images.

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With an axial resolution of 5 microns in tissue, the Cirrus HD-OCT simultaneously captures 27,000 A-scans per second and line-scanning ophthalmoscope (LSO) fundus images. Physicians can see axial cross-sectional and 3-D images and measurements.

Using proprietary Real-Time Registration and Auto Patient Recall software, the HD-OCT uses internal and external registration functions to ensure accurate registration and repeatability of high-definition images. The device's AutoCenter feature identifies the center of the optic nerve head for precise retinal nerve fiber layer (RNFL) thickness maps, provides RNFL superior-nasal-inferior-temporal (TSNIT) analysis, and can compare results to normative data.

• Spectralis OCT and Spectralis HRA+OCT (Heidelberg)³ — Both SDOCT devices capture 40,000 A-scans per second. They use new proprietary TruTrack eye-tracking technology, which locks onto a specific location on the retina and relocates the site at later exams to enhance the monitoring of disease progression and treatment decisions. For image clarity, the proprietary Heidelberg Noise Reduction feature takes the axial resolution from 7 microns to 3.5 microns. And the device's dual-beam imaging captures a reference scan and cross section simultaneously for reliably accurate registration.

Heidelberg initially offered the Spectralis HRA+OCT, which combines SDOCT technology and the Spectralis HRA in a single device. Spectralis HRA+OCT has six modes: SDOCT, infrared, autofluorescence, red-free imaging, fluorescein angiography and ICG angiography. The system simultaneously registers the cross-sectional SDOCT images to reference images from any of these modalities.

The Spectralis OCT (Heidelberg) can lock onto a specific part of the retina and relocate the site at later exams to better monitor disease progression.

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In 2007, Heidelberg introduced the Spectralis OCT in response to requests from clinicians at smaller practices who wanted an economical, but expandable system. The Spectralis OCT has two modes — SDOCT and infrared — and offers the flexibility to upgrade at a later date.

• RTVue-100 (Optovue)⁴ — The RTVue-100 completes 51,700+ A-scan 3-D imaging in 2.2 seconds. Physicians can review a 3-D presentation, rotate the images, rapidly scan through B-scans and zoom in for a better view. The system offers full retinal, inner retinal and outer retinal thickness maps and metrics, as well as a 0.35-second complete nerve head analysis for glaucoma diagnosis. It's the only commercially available OCT system that's FDA-approved for corneal and retinal imaging.

Technicians can choose from two scan-depth ranges (2 mm for imaging the macula and 2.3 mm for imaging a tilt disk), as well as two image modes for enhanced results (vitreo mode for vitreoretinal tissue and choroid mode for chorioretinal tissue). The RTVue-100 has an in-tissue depth resolution of 5.0 microns and a transverse scan resolution of 15 microns.

The device has internal and external fixation, as well as a video fundus camera that delivers high quality, near-infrared images of the retina. The fundus image can import and register from any digital system. And technicians can export scans or reports from the RTVue-100 to image management or electronic medical record (EMR) applications.

• SDOCT Copernicus (Reichert)⁵ — Introduced by Reichert in February 2008, the SDOCT Copernicus produces 25,000 A-scans per second with an axial resolution of 6 microns. Automatic retina tracking and spectrometer alignment help ensure optimum image quality. Physicians can perform 3-D retinal imaging, map retinal thickness, RNFL and RPE, use automated software for nerve measurement, and compare the RT, RNFL and RPE with a normative database.

The SDOCT Copernicus (Reichert) performs 3-D retinal imaging, maps retinal thickness and uses automated software for RNFL measurements.

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To view the images, the SDOCT Copernicus offers zooming, rotating, sectioning and surface reconstruction for accurate diagnosis and disease management. RPE deformation maps, AVI animations of retina cross-sections and direct fundus preview during scanning are also available.

• 3D OCT-1000 (Topcon Medical Systems)⁶ — The FDA cleared the 3D OCT-1000 in July 2007, when it became the first device to combine SDOCT with a color nonmydriatic retinal camera. The device captures 18,000 A-scans per second at an axial resolution of 6 microns. Physicians see a 3-D "virtual microscopic view" of the targeted area, a color 45° nonmydriatic retinal photo documentation and can pinpoint registration between OCT data and fundus images.

Topcon released software updates to the 3D OCT-1000 in February 2008, including Version 2.12 of its TrueMap software, which features a new retina module with four-layer segmentation, an enhanced compare function and a new OCT automosaic module. A new direct interface between the 3D OCT-1000 and Topcon's EyeRoute Image Management System enables physicians to view 3-D images in real time. The SDOCT data goes to the EyeRoute system within minutes of capture, and physicians can view, edit and manipulate a variety of diagnostic images together, including B-scans, digital retinal images, visual fields and 3-D OCT images.

Software and Networking

SDOCT device manufacturers continually update their software for enhanced performance and better networking with digital image systems and EMRs.

"Optical resolution among SDOCT devices is pretty much equivalent," says Dr. Mehta of Colorado Retina Center in Denver. "And all the SDOCTs present their results in an aesthetically pleasing way, so physicians need to look to software as the differentiating factor.

"All SDOCT units can extract enormous amounts of data — it's a matter of what they do with it and how they convert incredible pictures into useful numbers that counts. With normative databases and quantified comparisons, software can elucidate qualitative changes in these images, substantially impacting clinical outcomes."

Scott W. Cousins, M.D., the Robert Machemer professor of ophthalmology and director of the Duke Center for Macular Diseases at the Duke University Eye Center in Durham, N.C., has been a Spectralis user for 9 months. Dr. Cousins has found software development at Heidelberg to be responsive to change. "The company upgrades software frequently. When a clinician has a profound insight, it's in the software 6 months later," he says. "Software reports provide robust quantitative measurements, so I know if a patch of edema is responding to treatment."

Image networking is a hot topic right now, and SDOCT manufacturers are responding. In the future, data from SDOCT devices may need to meet Digital Imaging and Communications in Medicine (DICOM) standards, which certainly will help integrate SDOCT data with image management systems and EMRs.

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Making the Investment

Undoubtedly, SDOCT has changed diagnostic imaging. The speed, resolution and accuracy are unprecedented — which is a major step forward in diagnosis and disease monitoring. So, just a few years into this revolution, what do colleagues say about making the investment?

"It's a major expense, and OCT machines take time to show a return on investment," says Dr. Mango of Weill Cornell Medical College of New York Presbyterian Hospital. "But it's worth it in terms of the clinical information you receive."

Dr. Spaide of Vitreous-Retina-Macula Consultants of New York recommends that physicians look past the specs to the more practical aspects of SDOCT devices. "I recommend that people forget about small differences in image quality and compare how various devices will help improve their work," he says. "Software capabilities can be great for surgical cases. And make sure data accessibility and safety are at the core of any SDOCT purchase decision. The scans must be quick, cover everything and be safely and easily networked for review anywhere in the office."

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According to Dr. Mehta, the investment and transition can be made effortlessly — especially as SDOCT devices remain physically similar while the software and networking change. "Many people who don't have SDOCT are thinking about purchasing it, but they're concerned about making such a large investment or getting rid of their current equipment," he says. "But there are options to fit individual needs and patient flow. With existing devices, the hardware is unlikely to undergo a major change soon. I envision keeping the same instrument and updating software, data interpretation and the size and characteristics of the normative databases in the years ahead." OM

REFERENCES

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