NOV 2013

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November 2013 EW NEWS & OPINION 25 Device focus Spectral-domain OCT offers unprecedented views by Michelle Dalton EyeWorld Contributing Writer The enhanced capabilities and quick acquisition time are cited as two major advantages I n the early days (read: just a couple of decades ago), clinicians were clamoring for an objective, quantitative, and reproducible tool that could help assess glaucomatous eyes—and optical coherence tomography (OCT) achieved those goals. Fast forward to today's world, and posterior segment surgeons are just as likely to use OCT to assess the macula and choroids as glaucoma specialists are to use it for better visualization of the optic nerve. OCT is a non-invasive imaging test that uses light waves to take cross-sectional views of specific areas of the eye. Among the various conditions it is used to diagnose: macular hole, macular pucker, macular edema, age-related macular degeneration, diabetic retinopathy, and various optic nerve disorders. It cannot, however, be used if the eye has certain conditions that interfere with the light transmittal, such as dense cataracts or significant bleeding in the vitreous. Time-domain OCT (TD-OCT) measures time-of-flight of light, but has given way to spectral domain in many cases, simply because the latter has a much greater data acquisition speed, resolution, and reproducibility. Spectral-domain OCT (SD-OCT) measures wavelengths of back-reflected light. "I use OCT on every patient I see—especially on every confirmed glaucoma or glaucoma suspect," said Joel S. Schuman, MD, Eye & Ear Foundation Professor and Chairman, Department of Ophthalmology, and director of the University of Pittsburgh Medical Center Eye Center. Dr. Schuman is one of the inventors of OCT. Likewise, Stephen Jae Kim, MD, associate professor of ophthalmology, vitreoretinal diseases and surgery & ocular inflammation, and program director, vitreoretinal fellowship, Vanderbilt Eye Institute, Nashville, Tenn., said the advances in OCT technology have granted posterior segment specialists the ability "to image individual layers of the retina with so much more definition. Because of that, we're learn- ing so much more about aspects of diseases we can now predict." For one, retinal structures in vivo can be imaged with a resolution of 4-10 microns. Using OCT in glaucoma What Dr. Schuman appreciates about SD-OCT is that "whether you're an expert observer of the optic nerve or a novice, SD-OCT gives you an accurate measurement of the retina," he said. "It does require that you interpret the information, but it can allow you to operate at the level of the expert observer whether you are one or not." TD-OCT "encoded the location of each reflection in the time information relating the position of a moving reference mirror to the location of the reflection. SD-OCT, instead, acquires all information in a single axial scan through the tissue simultaneously by evaluating the frequency spectrum of the interference between the reflected light and a stationary reference mirror," Dr. Schuman wrote in 2008.1 At that time, SD-OCT "shows better reproducibility than TD-OCT, but glaucoma discrimination is similar" between the two techniques, he wrote.1 SD-OCT "has been extremely helpful in reducing the variability inherent in subjective assessment," he told EyeWorld. "It is a tool, but it's not a perfect one." For example, inter-patient assessments will find variability with the retinal nerve fiber layer (RNFL), the optic nerve head, macula, and ganglion cell layer. He advises clinicians to avoid becoming "slaves to the technology," and believes a clinical assessment of the patient is as important as the OCT readings. In his hands, he uses SD-OCT to assess the progression of glaucoma (or from glaucoma suspect to true glaucoma). "It has to do with the intensity of the next intervention required," he said. "If I know a patient is on one or two meds, I'll use SD-OCT to determine patient disease progression to see if I'm going to add a med or use a laser." He relies equally on OCT and visual field data, acknowledging that he "may or may not see simultaneous changes in both," but that changes in the visual field would be necessary before commit- Spectral-domain OCT can be used to view even the most subtle of changes in the macula. Source: Joel S. Schuman, MD ting a patient to the next successive step. "Hiroshi Ishikawa, MD, had developed the segmentation algorithm used on TD-OCT years ago and published on that development. We had looked at the correspondence between ganglion cell analysis and glaucoma and compared that to RNFL analysis and glaucoma and we found similar discriminating ability," Dr. Schuman said. Ganglion cell analysis can be used as an "internal quality check on your findings," he said. When a patient presents with an RNFL abnormality, ganglion cell analysis "should be abnormal supratemporal in the macula as well." "We sometimes forget that Ram Ziemer was the person who first proposed looking at the macula for glaucomatous abnormalities," Dr. Schuman said. "When he first described it, it was considered heresy. Yet now we know the macula is an extremely valuable place to look for glaucomatous damage." Using OCT in retina With SD-OCT, "we can predict outcomes much better, after treatment or even after surgery for some retinal disorders," Dr. Kim said. Being able to image the retina as quickly as the SD-OCT devices is helping clinicians and researchers alike determine why some people have specific visual complaints or problems with distortion while others who have the same visual acuity do not have those same complaints or problems. "Before the popularity of SDOCT, you'd look and the OCT would look normal, but there was not enough definition to see the individual layers of the outer and inner retina," Dr. Kim said. What retina specialists had was a "vague and blurry view of the retina," but the details were not specific enough to determine why some treatments may work on some individuals but not on others. "Now with SD-OCT, we can see layers and much more definition," he said. "Now we can even predict who will do well after a specific treatment and who may not." TD-OCT's resolution was close to 10 microns, but SC-OCT has a much greater resolution, up to about 4 microns, "and because the images are captured so much more quickly, you'll get a sharper image," Dr. Kim said. Before SD-OCT, retina specialists used fluorescein angiography (FA); SD-OCT has replaced "about 90%" of what FA had been used for, Dr. Kim said. However, FA is still a valuable tool to analyze perfusion and flow through the arteries, "which OCT cannot do. OCT doesn't give you the actual leakage. It's much faster and safer and there's no worry about the risk of an injection." SD-OCT is more cost effective, and more quantitative than FA, which is a "good tool," but a qualitative test. Down the road, Dr. Kim predicts there will be a considerable amount of published papers on the use of SD-OCT and its benefits with imaging the choroid. "We'll start to learn more about how choroidal health is associated with certain disease," Dr. Kim said. Choroidal health is significantly associated with diseases like age-related macular degeneration. He believes SD-OCT may have some benefit in dry AMD as well. "Some patients see 20/30 or 20/40 and others will see 20/80 or 20/90; some may have 20/20 vision but horrible distortion. It's been difficult to clinically look at [patients] and predict how well they are doing. continued on page 26

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