New techniques and advances in imaging technologies have been a topic of great interest as of late, particularly improvements in optical coherence tomography (OCT) and confocal microscopy (CM) for the anterior segment. Of course, one key consideration for any new technology is its clinical practicality. This month, we’ll unravel the intricacies of these technologies, and consider how they are being employed as tools for both patient care and clinical research.
The OCT of today comes in two basic formats: time-domain (TD-OCT) and spectral domain (SD-OCT). TD-OCT employs a technology that captures image scans serially according to depth, and uses them to reconstruct an image of the scanned tissue. On the other hand, SD-OCT separates reflective signals according to wavelength, and can collect scans for subsequent tissue re-construction in parallel. The result of these distinctions is that SD-OCT devices can collect scans as much as 100 times faster than TD-OCT devices, thus allowing for greatly improved image clarity and detail.
Though traditionally used for vitreoretinal evaluation, recent OCT advancements allow for anterior segment imaging as well. An example of an emerging application for OCT is in the diagnosis of keratoconus. In its early stages, keratoconus can often go undetected. OCT has the ability to identify those individuals with early corneal thinning or other changes in corneal shape.
One of the newer experimental applications of OCT is in dry eye assessment, where it can be used to measure tear film volume by means of tear meniscus height.
In addition, treatment protocols for other anterior segment conditions— such as corneal scars or dystrophies, corneal transplants and anterior segment malignancies— have all benefitted from advances in OCT imaging technologies.
OCT also has become an essential tool in anterior chamber biometry, whether used to help better fit IOLs or to obtain more accurate angle measurements. Unlike ultrasound-based methods, there is no need for direct contact with the ocular surface using OCT. These measures are completed very rapidly, minimizing motion artifacts.
While the use of OCT for imaging the retina and anterior segment continues to expand, another methodology—confocal microscopy— has also made the move from the back to the front of the eye, becoming a standard technique for assessment of the conjunctival and corneal surface.
In vivo imaging of conjunctival blood vessels before (top) and after (bottom) allergen challenge. White cells are clearly visible following challenge, and some of these can be see migrating out of vessels into extravascular space.
When compared to traditional slit lamp imaging, confocal microscopes can provide a higher magnification and depth of view, which allows for visual diagnosis of conditions caused by Acanthamoeba or Fusarium infections. This is significant because diagnostic confirmation via culturing can take several weeks. CM also can be used to assess and monitor therapeutic efficacy for these and other infections, such as herpes keratitis.
Several recent studies have explored the use of corneal imaging with CM as a means to track and diagnose diabetic retinopathy by longitudinal assessment of corneal nerve morphology.
The use of CM imaging has become particularly useful in studies of new treatments for ocular surface disorders. The real-time aspect of this imaging allows researchers to directly assess the efficacy of new therapies in terms of macrophage and lymphocyte infiltration into the conjunctiva as well as in measuring changes in epithelial integrity.
The very latest technological advancements typically are found in experimental settings, and the same goes for the newest methodologies in assessing and monitoring dry eye disease. Researchers at our company, Ora, Inc., have been addressing the sometimes baffling assortment of metrics used in diagnosing dry eye by focusing on new, objective criteria in combination with patient subpopulation analysis.
One example of this is our computer-based imaging method, which is used to define and track dry eye signs and symptoms.
Dr. Abelson is the founder and scientific advisor for Ora, Inc. Dr. McLaughlin is an employee of Ora, Inc.