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Spectral-domain optical coherence tomography (SD-OCT) offers several major advantages over conventional OCT: faster image acquisition time, improved resolution, and improved image processing. In addition, the technology decreases patient motion artifact, which is especially useful when obtaining images in children.
"The differences between conventional OCT and SD-OCT makes a difference in how we can view retinal disease," said Dr. Toth, professor of ophthalmology, Duke University Medical Center, Durham, NC.
In addition to being 50 times faster than conventional OCT, SD-OCT, which also is referred to as Fourier-domain OCT, uses Fourier transformation to gather depth data from the spectra of the OCT signal. This feature eliminates the need for a scanning mirror-necessary in conventional OCT-thus saving time, Dr. Toth explained.
The SD-OCT technology also offers better image resolution as the result of improved superluminescent diodes and improved processing, Dr. Toth added.
Although conventional OCT allows clinicians to view retinal lesions, SD-OCT allows greater density of sampling, thereby capturing images of several lesions in the same amount of time that would be required for conventional OCT to capture an image of one lesion, Dr. Toth noted. In addition, she said, the technology enables improved resolution of larger retinal structures. For example, SD-OCT allows the clinician to view an area of retinal atrophy to see the loss of photoreceptors and where the loss starts and ends.
SD-OCT also can cover more areas of the retina, that is, it can search for disease sites, capture numerous images of the same site, and create a three-dimensional representation of the areas of the retina scanned, Dr. Toth said.
Another advantage over conventional OCT, she added, is that SD-OCT decreases the patient movement artifact. This property is helpful especially when the ophthalmologist is attempting to obtain ocular images in children and in patients with nystagmus.
"In age-related macular degeneration [AMD], for example, there is often motion artifact," Dr. Toth said. "It is difficult to sort out drusen and differentiate them from patient motion that could be either a pigment epithelial defect or normal retinal pigment epithelium [RPE] choroid. Using SD-OCT, it is easier to isolate drusen from the underlying baseline RPE and to define whether pigment epithelial detachments or RPE abnormalities are present."
When used to evaluate AMD, the technology can image and measure the volume of drusen. It still needs to be determined whether SD-OCT is useful when viewing small foci of edema or fluid or whether the technology would result in changes in decision-making when treating neovascular AMD or other diseases, she said.
A major advance in the technology, Dr. Toth said, is that SD-OCT data can be integrated with information obtained from angiography and fundus examinations. This integration allows the location of a retinal abnormality found on OCT to be identified.
"One benefit of SD-OCT is the creation of the summed voxel projection [SVP], which is helpful for orientation of individual OCT scans. One can collapse three-dimensional OCT volumes along the depth axis to form a two-dimensional plane, summing pixels to calculate one representative pixel intensity alone each line in the projection," she explained.
As a result of prominent shadowing from the retinal vasculature, the SVP image is similar to a fundus photograph. This image can be used to orient the features on a SD-OCT image to those on a fundus image.
System pros and cons
Is SD-OCT useful in day-to-day practice? The drawbacks of the technology, Dr. Toth said, include large, three-dimensional data sets requiring storage and backup; the necessity of data transmission between the photographer and the clinician; the need for lengthy image processing; the need to integrate data with conventional imaging; and the requirement of normative data for the new systems. In addition, the clinician must choose between several systems and must integrate data across clinical trials and offices. An advantage of the system, however, is the availability of portable systems that allow images to be acquired in children more easily, she added.
"Three-dimensional SD-OCT offers the potential to identify biomarkers and measure changes in disease over time. For clinical and research use, improved methods are needed to export, analyze, summarize, and manage data. These are on the way," Dr. Toth concluded.