Understanding the Advanced Features of SD-OCT
Understanding the Advanced Features of SD-OCT
Learn of the unique benefits manufacturers offer for this state-of-the-art imaging technology.
Frank Celia, contributing editor
Optical Coherence Tomography (OCT) holds an indispensable place in the research, screening, diagnosis and management of ocular diseases, such as glaucoma. Some of the latest Fourier domain, or spectral-domain (SD) units, in particular, generate scans of such high resolution and cross-sectional detail, they essentially constitute a form of in-vivo histopathology. Specifically, these devices simultaneously assess all the light waves that bounce off the back of the eye. The time saved by this simultaneous measurement increases image resolution, as the noise of the patient's eye movement is reduced. Hence, complex disease structures once only visible to pathologists can now be seen by you during a routine office exam —an ideal way to increase your involvement in glaucoma and retinal care.
This article summarizes the various SD-OCT devices available, with each offering different imaging strategies and computer software capabilities, among other features, that may benefit your practice.
Topcon 3D OCT-2000
The 3D OCT-2000 (Topcon Medical Systems, Oakland, NJ) is an SD device with 5-6µm axial resolution and performs 27,000 A-scans per second.
The device can combine OCT data with digital color or red free fundus photographs, allowing for overlay alignment comparisons of the OCT scan with areas of pathology found in those other tests.
In addition, the 3D OCT-2000 provides synchronized playback, which Topcon says essentially functions as a synchronized B-scan. Specifically, this feature permits you to position two separate OCT images adjacent to each other on one screen. These images can then be scrolled through —simultaneously —one frame at a time while you study the data for small, incremental changes that could signify disease progression.
Further, the device comes with the company's FastMap software, which, among other components, automatically tracks the center of the optic disc to improve accuracy in detecting glaucoma progression.
Finally, the 3D OCT-2000 comes with a one-year license of the company's i2k Retina Software. This software automates the creation of wide-angle retinal mosaics and can align images from any retinal camera.
CZM Cirrus HD-OCT
The Cirrus HD-OCT (Carl Zeiss Meditec, Dublin, Calif.) is an SD-OCT that features Zeiss optics for high-density 2D images and 3D cube volumes of the retina, optic nerve head and anterior segment. It offers 5-µm axial resolution and performs 27,000 A-scans per second.
Also, it includes Automatic Fovea Finder, which centers the macula analysis on the fovea. Further, proprietary algorithms provide high-quality layer segmentation —critical in retinal pathology identification and monitoring —the company says.
Values are compared with normative data. For evaluation of change due to disease progression or therapy, Macular Change Analysis with automatic direct data cube registration provides a qualitative and quantitative comparison of two visits. For optic nerve head and retinal nerve fiber layer (RNFL) evaluation, AutoCenter aligns the optic disc cube analysis post-acquisition. Key parameters are compared with normative data.
Guided Progression Analysis compares RNFL thickness measurements over time and determines whether statistically significant change has occurred.
Anterior segment imaging provides visualization of the angle and central corneal thickness measurement, with no add-on lens required.
Bioptigen HHP SDOCT
The Bioptigen SDOIS (Spectral Domain Ophthalmic Imaging System, Bioptigen, Durham, NC) is designed for routine clinical examinations in perioperative, research or hospital settings. The unit can be rolled from room to room, and its handheld scanner —tethered to the device —allows for ergonomic testing flexibility, the company says. The device offers 3-µm axial resolution in ocular tissue and captures 34,000 A-scans per second. Typical B-scans consist of 1,000 A-scans acquired, processed and displayed at 32 frames per second, Bioptigen says.
“Our patients are not just the cooperative, alert, adults who can put their chin on the rest and fixate,” explains Joe Vance, Bioptigen's vice president for sales and marketing.
Bioptigen plans to release advanced multi-layer thickness measurement tools for research applications mid-year. A release of such features for clinical applications would be dependent upon future regulatory clearance.
Also, the company serves the pre-clinical market via custom optics, mechanical stages and methodologies for obtaining high-density volumes of OCT data from small vertebrates, such as rhesus monkeys, pigs, rabbits, cats, dogs, rats, mice and zebra fish.
Instead of relying on a flash of white light to illuminate the retina, confocal scanning laser ophthalmoscopy (cSLO) employs a laser light, which is reflected back through a pinhole. This allows only passage of light from a narrow focal plane, resulting in a focused, high-contrast, clearly defined image.
The Spectralis (Heidelberg Engineering, Vista, Calif.) is a combination SD-OCT/cSLO and offers simultaneous dual-beam imaging. This simultaneous imaging produces OCT and fundus images with point-to-point mapping. This combination also allows for video angiography with fluorescein and indocyanine green, blue laser fundus autofluorescence (BluePeak) and infrared reflectance images.
Also, the device includes Tru-Track technology, which creates a detailed map each time the patient is imaged to ensure accurate and repeatable alignment of both OCT and fundus imaging. Tru-Track enables the AutoRescan function, which ensures precise follow-up scan placement.
BluePeak technology is considered a valuable tool to study the success of new therapies on dry AMD patients, as the FDA is accepting BluePeak data as the clinical endpoint in a number of drug studies.
The RTVue-100 (Optovue, Fremont, Calif.) is a SD-OCT that is able to measure both RNFL and ganglion cell complex (GCC). GCC consists of the outer layer of axons (nerve fiber), the cell body (ganglion cell layer) and the dendrites (inner plexiform layer). GCC thins in patients who have glaucoma. As a result many practices that have a large population of glaucoma patients use this device.
The device has a patented GCC Significance analysis map with focal and global analysis, says Peter Naismith, Optovue's director of marketing.
In addition, the instrument provides a 5-µm axial resolution and completes 26,000 A-scans per second.
Further, the RTVue-100 features a large FDA-approved normative database, so you can analyze glaucoma and retinal pathology. In addition to the patient's age, the software also adjusts for scan signal strength and optic disc size in its analysis. Finally, the device offers corneal pachymetry mapping and angle imaging with measurement.
A portable version of the device, known as iVue, is available with the new iHealthCheck fee-per-visit program.
Opko Spectral OCT/SLO
The Spectral OCT/SLO (Opko/OTI Miami, Fla.) features microperimetry, offering you a quantifiable measurement of a patient's visual field. The device then matches this visual field to the patient's inner retinal pathology. Specifically, microperimetry can test certain areas of the macula and evaluate how their appearance changes through time via a normative database and the one-to-one correspondence between SLO retinal surface localization and the cross-sectional detail provided by OCT. The device has a 5-µm axial resolution and provides 27,000 A-scans per second.
The Spectral OCT/SLO's combined function and the point-to-point registration between the two have also contributed to vision care's understanding of the role of the vitreous in retinal pathology.
In addition, you can use the instrument for imaging the vitreous and vitreoretinal interface. Specifically, it can help you identify conditions, such as vitreo-papillary adhesion in patients who have macular holes and macular pucker. (See “Bridging the gap,” below.)
As you can see, an array of SD-OCT devices that offer several advanced features are currently available to meet your imaging needs. By understanding these needs, you can take the firsts steps toward finding the right system for your practice. OM
|Bridging the Gap|
|Despite the awesome power of the instruments described, a discrepancy still persists between patients' visual function and the structures that can be revealed through imaging. But this perennial problem need not always exist. Researchers continue to explore ways of assessing function at a biological level.|
For example, a high-resolution OCT technique called optophysiology takes advantage of the fact that physiological changes in dark-adapted retinas caused by light stimulation can result in local variations of tissue reflectivity. Though simultaneous measurement of both retinal morphology and visual function is theoretically possible in a laboratory setting, it is not currently feasible in everyday practice. The level of resolution and accuracy necessary for such devices would create a situation in which a patient's breathing or heartbeat would generate enough noise to skew results. Yet many believe the future of imaging lies in bridging the gap between function and structure with a routine office exam.
“That's where things are probably going to go,” says Richard Rosen, M.D., Opko consultant, vice chair of ophthalmology, director of ophthalmic research and chief of retinal services at the New York Eye and Ear infirmary in New York City. “Hopefully in the not-too-distant future, we will develop some functional signals that will bypass the subjectivity of our current technology.”
|Mr. Celia is a freelance healthcare writer based in the Philadelphia area.|
Optometric Management, Issue: March 2011