Optical coherence tomography (OCT) has developed during the last 15 years toward becoming a gold standard for evaluating the layered structure of the retina. Within glaucoma, in particular, the technology has provided specific benefits in examining glaucomatous effects on the retina. Recently, a new method of acquiring OCT scans was developed, called Fourier-domain OCT, also known as spectral or high-speed high-resolution OCT. The speed and resolution of this method hold considerable promise for the improvement of glaucoma assessment using OCT.
COMPARISON OF TIME-DOMAIN AND FOURIER-DOMAIN OCT
Briefly, OCT applies low-coherence interferometry, using light to map the layers within the retina in a cross-sectional image. Each image is made up of a series of A-scans scanned through the depth of the tissue and when aligned side by side, create a B-scan two-dimensional cross-sectional image. In standard time-domain OCT, each A-scan is acquired by moving a reference mirror to correspond to each point along the depth of the A-scan, and the signals from the reference arm and from the retina are interfered to determine the signal at that point in the A-scan. However, in Fourier-domain OCT, reflectance interference between the reference arm and retina at each A-scan location is Fourier transformed to simultaneously acquire all points along the depth of the A-scan without reference arm movement.
The advantages of the Fourier-domain OCT are improved speed and resolution. Because each A-scan is acquired all at once in Fourier-domain OCT, the acquisition rate is much higher at 16,000 to 40,000 A-scans per second, as opposed to the time-domain OCT's rate of 400 A-scans per second; the latter technology is limited by the mechanical movement of the reference mirror. This acquisition rate allows for much faster scanning times, reducing motion artifacts, and enabling denser and other novel scan patterns. Fourier-domain OCT is also able to improve axial resolution from 8 to 10 µm to 3 to 6 µm, which improves the ability to visualize intraretinal structures.
APPLICATION OF FOURIER-DOMAIN OCT TO GLAUCOMA
The current standard clinical system for OCT imaging is the Stratus OCT (Carl Zeiss Meditec, Inc., Dublin, CA) time-domain system. The primary scan patterns used to assess glaucoma damage with this device are: (1) a circular scan around the optic nerve head with a radius of 3.4 mm and 256 A-scans, which are repeated three times to assess the thickness of the retinal nerve fiber layer (fast retinal nerve fiber layer scan); (2) a six-line pattern centered on the optic nerve head with a diameter of 4.0 mm with each line made up of 128 A-scans (fast optic nerve head scan); and (3) a six-line radial pattern centered on the macula with a radius of 6.0 mm with each line made up of 128 A-scans (fast macula scan). When evaluating the three scanning protocols for glaucoma discrimination, optic nerve head parameters of rim area had the best discriminatory ability followed by horizontal integrated rim width and vertical integrated rim area, fast retinal nerve fiber layer, peripapillary nerve fiber layer thickness, and finally, all macular parameters.1 This finding demonstrates that these measures take into account the areas that include all retinal nerve fibers, as opposed to the macula, where only a specific subset of fibers are sampled.
Fourier-domain OCT can provide distinct advantages when scanning both the optic nerve head and circumpapillary retinal nerve fiber layer. The faster scanning speed allows for acquisition of denser patterns across the optic nerve head. Rather than a simple six-line radial scan pattern, radial scans with more lines, or a combination of radial and concurrent circular scans around the optic nerve head can be acquired to better characterize the structure of the optic nerve head and peripapillary retinal nerve fiber layer with more complete tissue mapping. This faster scanning speed allows the collection of larger data sets that provide many more parameters to be analyzed, as well as a more accurate assessment of disc shape and structure. Because of the increased speed, B-scans with higher numbers of A-scans can also be acquired, resulting in improved transverse resolution. Additionally, a raster scan can be created, scanning a line down across the optic nerve head to create a precise 3-D data set of its structure. From this data set, an OCT fundus image can be created by averaging through each A-scan. The fundus image allows for precise image registration, for comparisons through time of optic nerve head changes (Figure 1).
IMAGE REGISTRATION
Registration of images can also be used to assess and improve the reproducibility of the data sets. The physician can slice the 3-D data set in a variety of ways to allow for visualization of any specific cross-sectional slice, as well as slice down through the layers of the retina in a C-scan mode. C-scan visualization can allow a physician to examine specific layers and may aid in the visualization of focal defects in the retinal nerve fiber layer, as well as visualization in the cup, even at the level of the lamina cribrosa. These 3-D data sets allow for postprocessing, such as the selection of a precisely centered 3.4-mm circle for retinal nerve fiber layer measurements, to minimize the problems of patient motion causing decentration of the retinal nerve fiber layer circle. These 3-D data sets would not be possible with time-domain OCT, because patient eye motion would render them useless (ie, scanning time would be far too long).
IMPROVED RESOLUTION
The other main advantage of Fourier-domain OCT is its increased resolution (Figure 2). By improving axial resolution from 8 to 10 µm to 3 to 6 µm, OCT can now be used to distinguish structures that were previously indistinguishable, such as the ganglion cell layer in the retina. Focusing on retinal layers that are specifically prone to glaucomatous damage rather than assessing the entire retinal thickness might further improve the ability to detect disease. The improved resolution also allows for more precise location of the boundaries between structures, such as the posterior surface of the retinal nerve fiber layer. The improved resolution permits more accurate segmentation, allowing for localization and identification of very narrow focal defects. Current segmentation techniques in the time-domain OCT require substantial A-scan averaging and smoothing, which can hide localized abnormalities.
FOURIER-DOMAIN OCT CLINICAL DEVICES
Now that clinical benefits of resolution and speed differences are being realized, multiple clinical OCT instruments are beginning to join the market. Each of these devices has their own value and focus, whether it is clinical quantitative measurements from segmentation algorithms for glaucoma or 3-D scans to provide more qualitative information about retinal pathologies. Because this is a new field, these clinical devices are also at different levels of development, from early prototyping to FDA-approved products on the market. As various clinical devices arrive on the market, they will provide a wide range of options to ophthalmology clinics.
CONCLUSION
Fourier-domain OCT is the next step in the evolving field of clinical ocular imaging. Its speed and resolution are an improvement over current OCT technology. High speed, high, or ultrahigh resolution OCT using Fourier/spectral detection captures image 60 to 100 times faster than standard time-domain OCT. This speed creates a high-definition OCT and permits the acquisition of 3-D OCT data sets. The 3-D data enable the presentation of an OCT fundus image, which is used for precise image registration. Because of the high resolution and scan density, segmentation and mapping of intraretinal layers is feasible. These improvements in scan speed, analysis algorithms, and resolution are expected to lead to increased reproducibility, sensitivity, and specificity, but this must be proven in clinical studies.
Kelly A. Townsend, BS, is a research specialist from the UPMC Eye Center, Eye and Ear Institute, Ophthalmology and Visual Science Research Center, University of Pittsburgh School of Medicine, and Department of Bioengineering, University of Pittsburgh School of Engineering. She acknowledged no financial interest in the products or companies mentioned herein. Ms. Townsend may be reached at townsendka@upmc.edu.
Gadi Wollstein, MD, is Assistant Professor of Ophthalmology and Director of the Ophthalmic Imaging Laboratory at the UPMC Eye Center, Eye and Ear Institute, Ophthalmology and Visual Science Research Center, University of Pittsburgh School of Medicine, and Assistant Professor of Bioengineering, University of Pittsburgh School of Engineering, Pittsburgh, PA. He has received research funds from Carl Zeiss Meditec, Inc. Dr. Wollstein may be reached at (412) 647-0325; wollsteing@upmc.edu.
Joel S. Schuman, MD, is the Eye and Ear Foundation Professor and Chairman of the Department of Ophthalmology at the University of Pittsburgh School of Medicine and Director of the UPMC Eye Center. He is also Professor of Bioengineering at the University of Pittsburgh School of Engineering and Professor, Center for the Neural Basis of Cognition, Carnegie Mellon University and University of Pittsburgh. In the past 3 years, Dr. Schuman has received research funding, research equipment, honoraria, and/or payment of faculty travel expenses from Alcon Laboratories, Inc., Allergan, Inc., Carl Zeiss Meditec, Inc., Heidelberg Engineering GmbH, Lumenis Inc., Merck & Co., Inc., and Pfizer, Inc. Dr. Schuman receives royalties from intellectual property licensed by M.I.T. to Carl Zeiss Meditec, Inc. Dr. Schuman may be reached at (412) 647-2205; schumanjs@upmc.edu.
