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February 2005

How to Get the Most Out of OCT Scans
By Miriam Karmel, Contributing Writer

Choice anxiety. That’s what optical coherence tomography scanning can induce, said Greg Hoffmeyer, manager of Duke Eye Imaging at Duke University Eye Center. Mr. Hoffmeyer compares OCT, a noninvasive imaging technique used to diagnose and manage a variety of retinal diseases and glaucoma, to a Chinese menu. Newer users may be so overwhelmed by OCT’s array of scan/analysis modes, line numbers, lengths and angles that they won’t know what to order.

It is possible, however, to extract the most from the OCT with only a few basic imaging modes. “You’re probably going to find 99 percent of what you need” by using a few standard options, said Mr. Hoffmeyer, who is also project manager of Duke’s OCT Reading Center.

Consistency Counts
OCT has radically changed the way the retina is being viewed, Mr. Hoffmeyer added. Before OCT, the only way to get cross-sectional tissues of the retina was in the pathology lab. “What we have now with OCT is essentially a virtual biopsy.”

But to get the most from the device, an ophthalmic practice must establish a set of protocols for the imaging technician to follow. “Define what modes you’re going to use and try to use them correctly on every single patient, every single time,” Mr. Hoffmeyer advised.

Consistency is essential because a single scan is just one slice of tissue. OCT uses near infrared light to scan the retina and optic disc to provide 2-D cross-sectional views of layers of the retina resembling a histologic section. It is similar in concept to ultrasonography, but it uses a light source rather than sound to image tissue. OCT can identify macular holes, cysts, vitreomacular traction, subretinal fluid, pigment epithelial detachment and CNV. It can identify and quantify macular edema, and measure retinal thickness changes in response to therapy over time “with stunning accuracy,” Mr. Hoffmeyer said.

 Preferred Scan Modes
Despite the array of scan options, experts have narrowed the choices to a standard few. The protocols will be different for glaucoma and diseases of the retina, with some overlap. Duke employs the following modes for the Stratus OCT, or OCT3, by Carl Zeiss Meditec:

Retina Scans

  • 7-mm posterior pole scan. If you take only one scan, this “just may be the single most useful OCT image for macular disease,” Mr. Hoffmeyer said. It’s a 7-millimeter cut that extends from the optic nerve angled five degrees inferiorly through the fovea. Duke customized the scan, but any imaging center can do the same on the Stratus OCT. The alternatives (which Duke uses in specialized situations) are the line scan or cross-hair scan that are built into the device. But if you only do a simple horizontal scan, you may hit the macula and miss the fovea. “We want to hit the fovea,” Mr. Hoffmeyer said. “That’s your central vision. That’s where the money is.”
  • Fast macular thickness map. This mode uses lower-resolution scans through the fovea to measure macular thickness and to “preview” the macular area for defects to be rescanned with the higher-resolution modes. It acquires six good 6-mm scans centered on the fovea in less than two seconds. It also can monitor changes in thickness over time.

Glaucoma Scans

  • Fast retinal nerve fiber layer 3.4. This average of three concentric scans around the optic nerve head detects nerve fiber layer thickness. It comes with a built-in age-corrected normative database (3.4 refers to the circle diameter in millimeters).
  • Fast retinal nerve fiber layer map. This provides a topographic image of the nerve fiber layer emanating from the optic nerve. It illustrates NFL thickness in a pseudocolor rendering with assigned colors for thicknesses similar to the macular thickness maps.
  • Fast macular thickness map. Glaucoma produces macular thickness loss, but generally it is easier to detect using the fast retinal nerve fiber layer strategy.

When to Order a Scan
It’s easy to get most of what you need with a few scan modes, but should you order a scan—and for which patients?

Retina patients. Peter K. Kaiser, MD, a retina specialist and director of the Digital OCT Reading Center at the Cleveland Clinic’s Cole Eye Institute, finds OCT “indispensable for retinal diagnosis and follow-up.” In AMD patients, for example, a scan can give a very high-resolution view of CNV and help differentiate occult CNV from dry AMD. The presence of intraretinal or subretinal fluid around the CNV helps him decide whether to treat with photodynamic therapy or newer antiangiogenic inhibitors.

When treating macular edema, he orders a fast macular thickness map, which shows changes over time and helps gauge how well the treatment is doing. Dr. Kaiser orders a linear cross-hair scan to evaluate whether a patient would benefit from laser or not. When cysts are seen on a scan, he often will use steroids instead of laser.

Glaucoma patients. Sanjay G. Asrani, MD, assistant professor of ophthalmology at Duke, finds OCT very useful in glaucoma suspects. It’s also helpful as a baseline in patients with established glaucoma.

OCT is also valuable for detecting whether there’s thinning in the macula or nerve fiber layer in one eye, but not in the other. That asymmetry, which Dr. Asrani calls “a hallmark of glaucoma,” is a red flag. OCT also can be a useful instructional tool for asymptomatic patients. “One can show patients the level of damage. This is pictorially put in front of them,” Dr. Asrani said. “Compliance is helped tremendously because the patient can see what I’m treating and not just because I told them they have glaucoma.”

As helpful as OCT is, Dr. Asrani does not order a scan in advanced patients in which the visual fields are reliable. “Then by all means follow them by the visual field.” Even when you use OCT, “any of these [OCT] readings has to be correlated with the optic nerve appearance and the nerve fiber layer appearance,” he said.

While OCT has radically changed the field of ophthalmic photography, it does have its limitations:

  • It is dependent on operator technique.
  • It can be degraded in the presence of media opacity, as it relies on optical coherent light source.
  • Optic nerve atrophy and irregularly shaped nerves can cause the light to bounce off scar tissue and inhibit good scans.
  • The computer may misidentify inner and outer retina and give false information about actual retinal thickness, said Glen J. Jaffe, MD, professor of ophthalmology at Duke and director of the Duke OCT Reading Center. “You have to be able to recognize when you get an artifact and when you don’t.”
  • It can’t replace clinical judgment. For example, when interpreting scan results on patients with both diabetes and AMD, scar tissue or fluid buildup can cause actual thickenings, which may mask a true thinning of the retina (associated with glaucoma), said Dr. Asrani.

Will OCT Replace Fluorscein Angiography?

OCT, which provides an entirely new view of the retina, has resurrected the field of ophthalmic photography, said Mr. Hoffmeyer. “This has been the most significant imaging device since fluorescein angiography was introduced in the 1960s.” But will OCT replace fluorescein angiography? It depends.

 “In some cases it supplants fluorescein angiography; in some it complements,” said Dr. Jaffe. For example, PDT treatment guidelines are based on fluorescein angiography. The angiogram shows the area to treat. But OCT, which indicates any cystic changes or subretinal fluid that goes along with the CNV, “tells me whether treatment is really needed.”

For diabetic macular edema patients at baseline, Dr. Jaffe orders OCT scans and a fluorescein angiogram. “The fluorescein tells me the source of the leakage,” which determines the course of treatment, he said. And the OCT helps monitor the course of the thickening following the initial therapy. If thickening persists, OCT helps the clinician decide where to apply additional therapy.

OCT is useful for following patients. For example, an OCT scan can confirm questionable leakage that appears on a fluorescein angiogram. In that regard, Dr. Kaiser predicts a paradigm shift in which fluorescein angiography won’t play as much of a role in certain cases. There already are times when Dr. Kaiser uses OCT exclusively. For example, he doesn’t see the need to subject young patients with a pigment epithelial detachment or retinal subfluid accumulation to a fluorescein angiogram.

 Dr. Jaffe almost never uses angiograms with cystoid macular edema from uveitis or with postcataract CME. “That’s where OCT has replaced fluorescein angiography.” He prefers OCT, which he said provides the same information in a faster, less-invasive and cheaper manner. OCT answers the question: Is persistent macular edema present—and, if so, how does it look morphologically? “The correlation in CME between OCT and fluorescein angiography is very high.”

Mr. Hoffmeyer has consulted and lectured for Carl Zeiss Meditec, and Dr. Kaiser is on the company’s scientific advisory board. Drs. Asrani and Jaffe have no related financial interests.