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An advance in posterior segment imaging technology (Cirrus HD-OCT, Carl Zeiss Meditec) brings clinicians and researchers useful advantages compared with previous-generation optical coherence tomography (OCT) equipment and holds promise for valuable expanded capabilities in the future.
"[The device] is based on spectral-domain imaging technology that represents the wave of the future. This new generation of OCT instrumentation already is powerful technology, and as new software is developed to harness the enormous volume of data it captures, we can expect the utility of this device for management and research purposes will increase," said Dr. Chang, who is the Edward S. Harkness Professor of Ophthalmology, chairman of the Department of Ophthalmology, and director of the Edward S. Harkness Eye Institute, Columbia University College of Physicians and Surgeons, New York.
The instrument provides a live fundus image during scanning and high-resolution (5-μm) axial cross-sectional images. In addition, it performs 27,000 A-scans per second to generate a densely sampled macular cube scan.
"Scanning through the volumetric images, clinicians also can better localize macular pathology and quantify changes over time," he added.
By generating a more accurate and higher-resolution image, the spectral-domain device may have some advantage for improving diagnosis relative to time-domain OCT, Dr. Chang said. Nevertheless, he added, a device using time-domain imaging technology (Stratus OCT, Carl Zeiss Meditec) remains highly useful diagnostic technology. With its precise three-dimensional layer segmentation algorithms, the spectral-domain OCT clearly has a benefit for providing more information about the internal structure of the retina, Dr. Chang said.
"With this new-generation OCT device, it is possible to separate out the various layers of the retina. For example, in an eye with macular degeneration, one can 'peel away' the retinal layers and focus on the choroidal elevation and surface of the retinal pigment epithelial layer. With this capability, the clinician can get a better feel for the topography of the affected surfaces," he said.
Compared with time-domain OCT, the new spectral-domain device already provides greater accuracy in retinal thickness determination that should enable monitoring of disease-related change. Software now being developed to take advantage of the volumetric data captured, however, will provide information on retinal volume rather than thickness alone.
"With this information, we will be better able to assess changes in edema-such as in eyes with diabetic macular disease or vein occlusion-than we can now with the analysis of single sections," Dr. Chang said.
Better registration of serial images is another beneficial feature of the spectral-domain unit, he said, because it enhances the ability to compare changes from one session to the next.
"One problem that may be encountered with time-domain OCT is that the center of the fovea may vary from image to image depending on where the photographer puts it," Dr. Chang said. "With its registration capabilities, that issue is avoided with spectral-domain . . . OCT."
The new device also is more patient- and practice-friendly, he said. Because the images are obtained so rapidly, problems with motion artifact are reduced. As a result, it is possible to acquire images in some patients where it previously may have been difficult to obtain a good scan using the time-domain OCT device, Dr. Chang said.
In addition, the spectral-domain unit has a smaller footprint relative to the time-domain unit. That feature makes it easier for the patient to approach the instrument for the examination and is a welcome advantage considering office space constraints, he said.