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OCT Is Becoming Even More Versatile
Robert W. Dunphy, OD
There have been tremendous advancements in spectral-domain optical coherence tomography (SD-OCT) technology during the past few years. One trend has been the combination of OCT with additional imaging modalities and the incorporation of anterior segment imaging capability in devices that were previously limited to retinal evaluation (Figure 1). One company has combined OCT with a fundus camera (Figure 2) , and more than one company has made OCT available on a surgical arm for intraoperative applications. Yet another company has created a multimodality imaging system by combining SD-OCT with a multispectral confocal scanning laser ophthalmoscope (cSLO). Combining SD-OCT with ancillary imaging greatly enhances the clinical utility of the images that the platform generates. I find the greatest clinical utility in the combination of OCT with cSLO imaging for identification of important qualitative features and localization of OCT scans.
The clinician needs to understand exactly where he or she is scanning in the fundus and, therefore, registration between the overview imaging data and the OCT data is very important. This reliable registration also allows the clinician to be sure that the same reference point is being imaged in serial examinations.
OCT is now the mainstay of assessment for patients’ response to therapy and for interpreting clinical change in a condition over time. The ability to reassess individuals reliably is of paramount importance in making clinical decisions.
A WISH LIST FOR OCT
Because it has become such an integral part of patients’ evaluation, it would be helpful if OCT technology were more readily integrated into the examination. It would be useful for OCT technology to be incorporated into an examining stand-mounted device that would allow for the evaluation of a patient’s condition during the clinical examination. I would like to see a cSLO and OCT combined and mounted on the examining stand so that the patient could be scanned while he or she is in the chair. OCT and cSLO need to become integral elements of anterior and posterior segment clinical surveillance in addition to specialized evaluation for suspect disorders.
The future of OCT in clinical practice is promising. The acquisition speed and the sophistication of image processing are progressing rapidly. It is hoped that the near future will bring greater acceptance of this technology by general eye care practitioners. OCT should migrate from the realm of the specialist into that of the generalist. Many signs associated with retinal pathologies are subtle in nature and may be identifiable earlier in their course using SD-OCT and cSLO compared with conventional examination techniques. Therefore, clinicians may be able to employ diagnostic imaging to diagnose and intervene earlier in the course of a disorder to improve patients’ outcomes. In the end, the wider application of OCT and enhancements designed to improve diagnostic effectiveness will help all eye care practitioners provide greater service to patients.
Robert W. Dunphy, OD, practices in Boston where he manages the ocular diagnostic imaging for a tertiary care referral center. He has received speakers’ honoraria from Heidelberg Engineering and Optos and is a member of the clinical advisory panel for Optovue. His institution, VA Boston Healthcare System, has received the loan of equipment for research purposes from Heidelberg Engineering, Optovue, and Optos. Dr. Dunphy may be reached at firstname.lastname@example.org.
Better Quantitative Output Is Needed
Larry J. Alexander, OD
The most popular benefit of using SD-OCT is its offering of a detailed picture of the retina, but this may be one of the technology’s least important features. A pretty picture is not going to improve diagnostic or patients’ care capabilities; analysis of numbers will, and it will enable eye care physicians to diagnose pathologies like diabetic retinopathy, age-related macular degeneration, and glaucoma earlier in the disease’s natural history. In truth, the full potential of SD-OCT devices has not yet been truly realized. It is analogous to an incredible computer stocked with applications and software such as Word, PowerPoint, the Internet, and Excel, but it is only used to write letters. In the same way, clinicians are really only using about 10% of the material stored in SD-OCT instruments.
SERIAL OCT IMAGING
The true utility of SD-OCT is harnessed when it is used to identify whether pathology is progressing or regressing. Furthermore, it is in quantifying elevation, depression loss of fibers, and loss of tissue when the instrument is taking the management of ocular issues to a new level. SD-OCT can also quantify what is occurring in the anterior chamber, which has long been difficult to evaluate objectively—gonioscopy has many limitations, not the least of which is that it is difficult to perform, and there is great interclinician variability in its interpretation.
Two SD-OCT devices currently on the market provide the ability to look at the ganglion cell complex on the neurological level, which is beneficial for understanding the implications of conditions such as type 1 diabetes, multiple sclerosis, dementia, Alzheimer disease, branch artery occlusion, ischemic optic neuropathy, and even sleep apnea. This is a technology that is going to allow clinicians to detect diseases at a much earlier phase.
USING CURRENT TECHNOLOGY MORE EFFECTIVELY
Any general practitioner, optometrist, or ophthalmologist who employs an SD-OCT device as a pretesting modality is providing beneficial, effective, efficient, and economical care to his or her patients. Optometrists, because they are typically the initial point of care in eye health, have the very real potential to minimize vision loss on a population scale if SD-OCT is used for the early detection of diseases that ultimately result in loss of acuity.
There is a need, hoewever, for better utilization of the data acquisition and normative databases associated with SD-OCT technology. As they exist today, the databases may give clinicians a false sense of security, and I think that is problematic. There may be a lack the motivation to figure out how to properly use all of the data that are available in every SD-OCT that is already on the market. Still, SD-OCT technology is likely the most important technological advance that has come about in eye care during the 41 years that I have been in practice. It is the standard of care. For example, the early stages of plaquenil toxicity can only be detected using SD-OCT (Figure 3), focal electroretinogram, or fundus autofluorescence. If any other technology is used, the early phases of the condition will be missed, which is potentially negligent and medical malpractice.
Larry J. Alexander, OD, operates the free, noncommercial website eyelessons.com. Previously, he served in the US Navy, was a professor at the University of Alabama at Birmingham School of Optometry, practiced in a referral center in Louisville, Kentucky, and is the immediate past president of the Optometric Retina Society. He is a consultant to Optovue, Inc. Dr. Alexander may be reached at email@example.com.
Current Advances and Future Concepts in SD-OCT
Nalin J. Mehta, MD
Most commercial SD-OCT (or Fourier-domain OCT), instruments incorporate near-infrared imaging wavelengths in the 800-nm range. Longer wavelengths (ie, 1,310-nm SD-OCT) are being developed for better penetration of the sclera, which would be particularly useful for imaging the anterior segment.
The acquisition speed of SD-OCT is also increasing from the tens of thousands to more than 100,000 scans/ second. Although presently cost prohibitive, these higher speeds may provide more data points with improved image resolution.
Simultaneous posterior pole imaging is usually limited to pseudocolored scanning laser ophthalmoscopic images. Although some units will take a photograph just before or after the OCT is acquired, I believe that simultaneous true photographic acquisition of the OCT target is not commercially available.
The open nature of intellectual property surrounding SD-OCT technology in ophthalmology has resulted in numerous companies with similar hardware configurations. The biggest differentiating factors among the SD-OCT instruments currently on the market are, therefore, the unique, proprietary software features of each manufacturer. These include features such as posterior pole tracking during image acquisition, which improves resolution and reproducibility of images and enhances the accuracy of comparative software used to determine change in macular thickness and optic nerve tissue loss. Another such feature, retinal ganglion cell complex analysis, is becoming useful for monitoring glaucoma patients for changes, which some investigators feel may precede those in the visual fields. Some devices include software that quantifies macular degenerative changes, such as the area of geographic atrophy (which will become increasingly important as drugs are being developed to slow progression) as well as the area and volume of drusen and retinal pigment epithelial detachments.
The study of intraoperative SD-OCT, a relatively new concept, has intensified during the past few years. This OCT application will allow the surgeon to evaluate surgical outcomes in real time, possibly avoiding the need for a return visit to the OR for things like iatrogenic retinal holes and residual epiretinal membranous traction, as well as the assessment of corneal graft placement. The surgeon will be able to address such issues during the initial surgery. Optimal visualization of retinal structures with OCT while simultaneously operating upon them, however, still eludes investigators.
Future needs to be addressed also include a better determination of retinal lesion composition in macular degeneration (ie, retinal pigment epithelial detachment vs subretinal neovascularization), as well as improved three-dimensional visualization of the choroid (Figure 4). These improvements may require advances in analysis software as well as the ability to enhance structures, possibly by using optical “dyes” or other enhancing contrast media.
Nalin J. Mehta, MD, is in private practice at The Colorado Retina Center in Denver. He is a consultant and speaker for Optovue, Inc. Dr. Mehta can be reached at (303) 893-5138; or via e-mail at firstname.lastname@example.org.