Optical coherence tomography or OCT, well established as a diagnostic tool for ophthalmic applications, continues to gain interest as a tool for following patients with neurological diseases. In simplest terms, OCT involves the emission of near-infrared light that is reflected by target tissue and then collected by the device, which then interprets patterns of reflectance to create an “image” of the specific biological target. The concept is in many ways akin to ultrasound, says Ari Green, MD, Debbie and Andy Rachleff Chair in Neurology and Assistant Director of the Multiple Sclerosis Center at University of California San Francisco, using light in place of sound waves. Ahead, Dr. Green, who is also Director of the Neurodiagnostics Center at UCSF, addresses current and potential uses for OCT in neurology.
How does OCT work?
Infrared light is poorly absorbed by tissues, Dr. Green notes, making it an optimal, safe, and effective tool for use in imaging. However, he says “the major limiting factor of OCT” is resolution. The best realizable resolution is in the range of microns, “hundreds of times higher than Tesla MRI,” Dr. Green notes. However, currently, the data can only be decoded at a macro level. OCT cannot be used to visual structure at the cellular level—yet. As such, it has proven particularly useful for visualizing and quantifying the thickness of the Retinal Nerve Fiber Layer (RNFL).
Those who use OCT are still learning how to fully mine images for information, Dr. Green acknowledges, noting that this form of biomedical imaging may yet prove useful to “view” a range of events taking place in the central nervous system (CNS). One day, Dr. Green says, OCT imaging may be equivalent to “doing histology without putting a tissue sample on a slide.”
How is OCT currently used as a clinical tool?
The primary applications of optical coherence tomography in neurology currently relate to neuroophthalmology, particularly to studying the thickness of the RNFL. The OCT technique can be used to identify or monitor changes in the thickness of the RNFL, generally related to diseases like glaucoma, optic neuritis, and even MS.
Infrared light, as in OCT, has a distinct pattern of reflection from various tissues. As long as adjacent tissues have different reflection patterns, then the technique can easily distinguish between structures. This is the case in the retina, where the RNFL has a distinct signal from adjacent tissues. Given the limitations of resolution described above, OCT images do not show microscopic structures within the RNFL, but it is well suited for measuring the layer's thickness. Although the RNFL is not a pure population of axons, they form the primary component, therefore, any decrease in thickness is attributed to a loss of axons, Dr. Green explains.
What are other potential uses for OCT?
The CNS “happens to have an optical window” in the form of the axon-rich RNFL, Dr. Green states. The ability to see and monitor changes in the RNFL suggests that neurologists can indirectly “see things going on in the CNS,” he says. “We believe that OCT has the potential to show us, at least at one time point, everything that has happened in the brain.” Ongoing study may help clinicians correlate changes seen on OCT to events occurring in the CNS and with clinical observations.
There is ongoing research to correlate OCT findings with visual function, Dr. Green says. He believes this type of correlation can be especially useful for monitoring and treating patients with MS. Noting that deficits in low-contrast vision, color vision, and visual fields all may be affected by MS yet are traditionally difficult to quantify, he says that OCT may provide helpful information in this regard. “We know that vision quality of life is lower in those with decreased RNFL thickness on OCT. As a specialty, we sometimes underappreciate the degree of people's visual dysfunction,” he suggests. OCT could one day be used to predict vision changes and even long-term functional outcomes in patients with MS.
Are there risks associated with OCT? What is the experience like for a patient?
Unlike MRI or ionizing radiation, there are no exposure risks associated with OCT, Dr. Green says. He says using OCT for the RNFL is “relatively easy,” given that the eye functions to let light in and out. The procedure is painless.
Depending on the imaging sequence used, the process can take up to half a minute, but the majority of patients are imaged in just a few seconds, Dr. Green notes. Subjects must maintain a fixed gaze through the duration of data acquisition, which individuals occasionally find annoying. There are ways to adjust for patient movement in some sequences, Dr, Green adds. In rare cases, acquiring an optimal image requires dilation, but this is not the norm.
What is the neurologist's role in OCT?
“There are opportunities for clinical neurologists to be involved in the process of acquiring and interpreting images,” Dr. Green asserts. “OCT is not just going to have applications in ocular imaging.”
Dr. Green predicts that with time, researchers and clinicians will harness OCT as a tool for monitoring the brain and the CNS. “The greatest area to expand OCT is for how we can serve and care for our patients,” he adds. While many neurologists' experience with OCT currently is limited to receiving reports from consulting ophthalmologists or neuro-ophthalmologists, Dr. Green encourages clinical neurologists to learn about OCT and how to interpret data. Findings of OCT are often simplified for inclusion in a clinical report, which it may not provide all the information a clinician may desire. In reality, OCT may provide a much richer understanding of the patient's status than the report suggests. Dr. Green uses the analogy of relying solely on a radiologist's report from an MRI rather than viewing the scans personally. “Problems can arise from automated interpretations,” he asserts, “The clinician really should be seeing the scan.”
Information about OCT can be found online:
Manufacturers, such as Heideleberg Engineering, offer online educational resources and programs, as well.