Advances in Ocular Imaging Technology

Improvements on many fronts are creating a more unified picture.

BY DIANE DONOFRIO ANGELUCCI, CONTRIBUTING EDITOR

These are heady times for diagnostic imaging. Whether you're monitoring a patient at high risk for AMD, contemplating what looks to be subtle signs of glaucoma progression or handling countless other day-to-day clinical tasks, a multitude of sophisticated ocular imaging devices are at your disposal — and their capabilities advance each year.

Today's imaging tools enable a sharper, more complete view of the eye, so you can assess structure, diagnose disease earlier, track progression and evaluate response to treatment. And the trend toward digitally integrating multiple imaging modalities helps create a “complete picture” of a patient's visual status and ocular health. Read on to learn what experts are saying about just some of these impressive devices.

OCT Advances

Since its debut in the early 1990s, OCT has evolved from a specialized tool largely confined to academic institutions and retinal subspecialists to a highly versatile mainstream imaging device widely used today. Time-domain OCT started the trend, but spectral domain OCT took imaging to another level by offering better resolution, faster acquisition speeds, and the potential for earlier diagnosis and monitoring of disease.

All agree that SD-OCT was revolutionary. “There was a huge hardware improvement from measuring 750 points on the retina to 45,000 points in the same amount of time — two seconds — and the jump from time domain to spectral domain OCT provided us with more data at a higher resolution,” says Donald Budenz, MD, MPH, professor of ophthalmology at Miami's Bascom Palmer Eye Institute.

Experts anticipate SD-OCT hardware and software will continue to improve and expand. This will be particularly important in keeping pace with research on potential treatments for retinal diseases such as AMD. In particular, as researchers test potential dry AMD drugs, enhancements in technology will enable them to assess how treatments affect drusen volume and atrophy area, says Peter Kaiser, MD, professor of ophthalmology at Cleveland Clinic.

Carl Zeiss Meditec will soon introduce software that helps to quantify the area and volume of drusen, and area of geographic atrophy. The Advanced RPE Analysis module will make use of an algorithm developed by Giovanni Gregori, PhD, at Bascom Palmer to create a map showing elevations in the RPE corresponding to drusen. The package will also provide images of geographic atrophy along with an automated segmentation developed by Zeiss.

Doppler imaging is already available on some systems, enabling clinicians to image flow inside the retinal vessels. There's hope that the technology can someday be used to image flow in the choroidal vessels, Dr. Kaiser says. This will enable clinicians to detect perfusion abnormalities.

The future also may include a number of other enhancements that may be beneficial in diagnosing and monitoring retinal disease, including swept-source OCT, which is much faster and is expected to eliminate motion artifacts; longer wavelengths, for deeper imaging; adaptive optics, to increase resolution and correct for aberrations; and functional OCT, which may enable clinicians to measure changes in the retina in response to stimuli. (For more on OCT technology, see the sidebar below.)

Managing Glaucoma

New Cirrus HD-OCT optic nerve head analysis software recently introduced by Carl Zeiss Meditec will help ophthalmologists analyze and measure more structures accurately and reproducibly, which will help them manage glaucoma, Dr. Budenz says. “Using the CZM software, one is acquiring 45,000 points in a cube of data that includes the optic nerve, and is then able to determine the optic nerve head area, rim area, cup area, and cup-to-disc ratio measurements from this cube of data in a way that we're finding is very reproducible and has a high ability to discriminate glaucoma from normal,” Dr. Budenz says.

However, Dr. Budenz cautions that it's important to combine this assessment with careful clinical examination. “There's a tendency, I think, in glaucoma management to want to look at the numbers from the imaging machines, and this is one of many examples where you have to look at the patient and use your brain and your training from ophthalmology residency to assess cup-to-disc ratio, because you're going to be led in the wrong direction if you only look at the numbers,” he says.

One novel idea recently introduced to aid glaucoma specialists is the MatchedFlicker software from EyeIC, which converts time-series digital fundus photos into a movie of two rapidly-alternating images, allowing clinicians to track progression by detecting changes over time, which can be difficult to see when studying still photographs.

Zeiss's GDx scanning laser polarimeter is also evolving, says Douglas R. Anderson, MD, professor emeritus of ophthalmology at Bascom Palmer in Miami. The new GDx Pro uses enhanced corneal compensation to provide more reliable images. “It improves the way the polarimeter analyzes the polarized light, making corrections for the irrelevant tissues such as the cornea, and helps you see more reliable images of the nerve fibers,” he says.

In addition, GDx Pro offers a number of user-friendly upgrades, such as autofocus, a touchscreen, a DVI port for external monitor use, DICOM-based connectivity, and an alternate fixation target for patients who have central vision defects. An RNFL map created with the GDx Pro is shown in the right-hand image below.

Refined Photography

As camera technology advances, fundus photography will continue to improve, enabling clinicians to photograph images with higher resolution and less flash, Dr. Kaiser says. “The ISO is improving to the point where we can take really great images without a super bright flash, which makes the whole process much easier on the patient,” he says.

Autofluorescence has gained attention for its ability to provide functional information about the health of the retinal pigment epithelium without the need for dye injection. “We now have the ability to perform widefield autofluorescence using systems such as the Optomap and can also obtain autofluoresence images at different wavelengths and by using regular fundus cameras,” says SriniVas Sadda, MD, associate professor of ophthalmology at USC's Doheny Eye Institute. “Researchers are conducting studies to determine which wavelengths of autofluorescence are most useful and the best ways to perform it.”

With improvements in ultra-widefield imaging — allowing imaging out to the peripheral retina — clinicians can identify neovascularization, nonperfusion or other abnormalities in the periphery. “Now there are several devices to do that to varying degrees. The one we've used the most is the Optomap (Optos), which has been perfected over the last couple of years, says Mathew MacCumber, MD, PhD, associate professor of ophthalmology and associate chair for research at Chicago's Rush University. “We can image the periphery for color imaging, but what I find particularly helpful, being a retina specialist, is using it for fluorescein angiography.”

“There's some evidence that you can have peripheral abnormalities in principally macular disease, perhaps more than we previously suspected,” Dr. MacCumber continues. “Some investigators have found leakage in the periphery with widefield-fluorescein angiography that we wouldn't have detected otherwise. I've noted some areas of peripheral abnormality — for instance, leakage — in cases of epiretinal membrane where there could be peripheral traction.”

Clinicians also have discussed using widefield imaging and widefield angiography to examine areas of non-perfu-sion to perform selective laser treatment, according to Dr. Sadda. “Maybe you can cause less damage with the laser and still achieve a benefit,” he says. “This creates an opportunity to change the way certain diseases are managed.”

Added Convenience

Non-mydriatic cameras have become popular, as they allow the convenience of non-dilated exams without compromising image quality. An example be found above; the top image is a diabetic retinopathy fundus photo taken with Topcon's TRC-NW8 nonmyd camera. The device is ideal for telemedicine applications such as diabetic screenings, Topcon says. The company's newest camera, the TRC-NW7SF Mark II, combines myd and nonmyd image acquisition in one device, a common trend in camera design.

Stephen Sinclair, MD, of Media, Pa., has used the Canon CX-1 Digital Retinal Camera, which also combines mydriatic and non-mydriatic technologies in one digital system. In addition to the usual methods of retinal photography (color and monochromatic filters to enhance various retinal components or fluorescein angiography), the system allows fundus autofluorescence photography in both mydriatic and non-mydriatic modes. This will be particularly helpful for monitoring patients with AMD or a family history of AMD so clinicians can detect disease early and work to prevent progression.

A new system awaiting FDA clearance is the Wx 3D from Kowa Optimed. The device is a non-mydriatic camera that enables clinicians to perform normal photography, small-pupil photography and simultaneous stereoscopic photography.

Structure and Function

The recently upgraded Eye Cubed ultrasound system from Ellex acquires images at a rate of up to 25 frames per second, allowing a clinician to view ocular activity, such as vitreous hemorrhage movement, in real time. “The quality of the images is excellent, and the ability to store real-time movie segments is very useful for review for teaching,” says Yale Fisher, MD, of Vitreous-Retina-Macula Consultants of New York. To learn more about contact diagnostic B-scans, visit Dr. Fisher's Web site at www.ophthalmicedge.org.

Researchers are investigating ways to increase resolution of ultrasound. They're also exploring photoacoustic imaging, but this remains in a research stage. “I think those are things that hopefully will be part of our armamentarium at some point, but it's not clear exactly when,” Dr. Sadda says.

Richard Rosen, MD, vice chair, surgeon director and director of ophthalmology research at New York Eye and Ear Infirmary, has been working with the Retinal Functional Imager (RFI) by Optical Imaging Ltd., which recently formed a partnership with Topcon Medical Systems.

The computerized fundus camera, which uses a stroboscopic light source, enables clinicians to examine retinal blood flow in vessels that range between 100 microns and 4 microns in diameter as a video or as still photos. “It's very revealing in terms of flow patterns along with the circulation of the retina without any injection of fluorescein dye,” Dr. Rosen says. “It actually uses the hemoglobin in red blood cells as the contrast agent.” In addition, he explains, the device generates very fine capillary perfusion maps with details similar to those a clinician would see in a good quality fluorescein angiogram.¹ Researchers are also studying how the RFI can be used for retinal oximetry, to differentiate areas of low oxygenation from high oxygenation.

Dr. Rosen and his colleagues have been using the device clinically in a couple of ways. They found a characteristic pattern in macular telangiectasia, which is often difficult to diagnose.² “Most clinicians don't see enough of those to easily recognize them,” he says. “Sometimes it's mistaken as early diabetic retinopathy or some variation.” In addition, they've been using it to examine patient recovery after vascular occlusion.

He anticipates the procedure might replace fluorescein angiography in some instances, allowing clinicians to assess blood flow, oxygenation and perfusion patterns without dye injections. “The idea would be that you could actually do this on a regular basis every time you saw the patient if you were concerned about changes, because the data is very reproducible from one time to the next,” Dr. Rosen says.

Choosing an Imaging System

In choosing an ocular imaging system, it's important to consider the space in your office as well as the needs of your patient population. For example, Harvard's Daniel Laby, MD, has found Kowa Optimed's Handheld Digital Camera to be valuable in a pediatric practice because children don't have to be examined using a machine that is too large for them, and portability is also attractive. “It's almost like a radar detector type of gun, and I can use it to take posterior segment and anterior segment photos, so that's really the only imaging system I have in the office,” he says.

It's essential to acquire hands-on experience with a system before you purchase it. Attend local or national society meetings, visit exhibitors, and examine devices carefully, without depending solely on the advice of others. OM

Editor's Note: Dr. Anderson is a consultant for Carl Zeiss Meditec. Drs. Fisher, Budenz, Laby, Rosen and Sinclair have no financial interest related to their comments. Dr. Kaiser is a consultant for Heidelberg and Carl Zeiss Meditec, and has received honoraria from Topcon. Dr. Sadda shares royalties from intellectual property licensed by the Doheny Eye Institute to Topcon. He also serves on the scientific advisory board for Heidelberg and has received or will receive research support from Zeiss, Optovue and Optos. Dr. MacCumber's department has received equipment from Optos and research funding from Heidelberg. He has been a consultant and speaker for Optos.

References

1. Landa G, Rosen RB. New patterns of retinal collateral circulation are exposed by a retinal functional imager (RFI). Br J Ophthalmol. 2010;94:54-58.
2. Landa G, Rosen RB. A new vascular pattern for idiopathic juxtafoveal telangiectasia revealed by the retinal function imager. Ophth Surg Lasers Imaging. 2010;2:1-5