Spectral-domain OCT unit offers high-quality retinal and anterior segment imaging

April 15, 2009

A number of factors should be considered when choosing a spectral-domain optical coherence tomography system. Although the capabilities of many systems essentially are similar, some differences exist based on ease of image acquisition and quality, software versatility, and other factors that may make them the best choice for a specific practice's needs. Proprietary technology (RTVue-100, Optovue Inc.) has the capability to perform both retinal and anterior segment imaging, which may make it a versatile instrument for a general ophthalmology practice, according to one practitioner.

Key Points

Key Biscayne, FL-Although many practices have used optical coherence tomography (OCT) technology for years, the newer spectral-domain OCT (SD-OCT) units generate high-resolution scans that make three-dimensional (3-D) macular and optic nerve rendering and "fly-through" possible, and many practices are looking to get on board with the new technology, said Elias Reichel, MD, vice chairman for research and education, Department of Ophthalmology, Tufts University School of Medicine, and director, vitreoretinal diseases, New England Eye Center, Boston. The high-resolution scan patterns these devices produce allow for observation of fine macular pathologies.

A number of factors should be considered when choosing an SD-OCT unit for your practice, said Dr Reichel, who spoke at Angiogenesis: Exudation and Degeneration 2009.

For example, Dr. Reichel said in some clinical settings the ease of image acquisition may be the most important consideration when choosing an OCT device. In busy practices, fast acquisition time may be needed to free up "chair time." Other practices may need certain capabilities that are germane to one specific imaging unit's software.

The RTVue-100 (Optovue Inc.) is a versatile instrument for a general ophthalmology practice, according to Dr. Reichel.

"You can perform both anterior and posterior segment imaging with this device, and it also provides very high-quality retinal imaging. The capability to do both retinal and anterior segment imaging in one machine should be a very attractive consideration for a general ophthalmologist," he said. "The Optovue unit also has a proprietary normative database and allows for a 3-D 'fly-through' of the macula to provide the practitioner with a nice view of where the pathology is."

Image measurement quality varies among units; the RTVue-100's is rated in signal strength intensity (SSI). A good rule-of-thumb is that an SSI of 40 or below is too low, and a re-scan should be considered, according to Dr. Reichel.

The scanning capabilities of the SD-OCT scanner that are used by the ophthalmologist will differ depending on the suspected diagnosis.

Line or HD Line scans are qualitative and represent a general scan pattern for observing macular pathologies, whereas MM5 and ETDRS scans are used to measure and map retinal and macular thickness.

"SD-OCT units can also measure inner retinal thickness and volume, which is useful for identifying and tracking the progression of diseases that affect nerve fiber layer and ganglion cells, such as optic nerve disease and retinal vascular disease," he said. "They can also measure outer retinal thickness and volume, which allows the practitioner to identify and track diseases that affect the photoreceptors, such as macular degeneration and dystrophies, and retinal detachment."

The Optovue unit contains a normative database for macular thickness, which makes it a good choice for tracking progression of neovascular age-related macular degeneration, Dr. Reichel added.

It also has two line-scanning capabilities: a 1024 line scan captures a single section through the retina, and a 4096 HD line scan captures a higher pixel-density OCT image.

"The 1024 scan employs image averaging or 'speckle elimination,' through which 16 individual 1024 A-scan OCT images are captured through the same portion of tissue and are then averaged to create a single OCT image. This helps the user more clearly define intraretinal layers and pathologies," he said. "The 4096 scan density effectively increases the signal-to-noise ratio and enhances a single scan. Because this is a single scan, resolution is not degraded due to patient motion between images, as it may be in some cases where image averaging is employed."

Finally, Dr. Reichel said that the quality of the SD-OCT units' segmentation software differs. This software is important for measuring retinal thickness, and ophthalmologists should be aware of the differences in software when choosing a unit.

"Keep in mind, though, that the companies update their software almost continuously, so while this is an important consideration, it's also a moving target," he concluded.

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