SPECTRAL DOMAIN OCT

The Spectral Domain Advantage

With high-speed scan acquisition and higher resolution, spectral domain OCT reveals a whole new level of detail in patients with glaucoma.

Reviewed by Joel S. Schuman, M.D., F.A.C.S.

► When diagnosing glaucoma, you have a variety of tools at your disposal: clinical examination, automated perimetry, photography and imaging technology. Used in tandem, these tools help you piece together a complete picture, understand your patient's condition and fine-tune your treatment strategy. An advancement in one of these tools has taken glaucoma diagnosis to a whole new level. Spectral domain optical coherence tomography (OCT) allows you to view the eye with greater detail and precision, allowing for a more accurate diagnosis and treatment strategy.

Higher-speed, higher-resolution spectral domain OCT acquires scans faster and gathers more information in a shorter amount of time than time domain OCT. Spectral technology also generates 3-D images.

Spectral domain OCT may detect very small changes in a patient's condition, which is particularly important in diagnosing and assessing glaucoma progression. The smaller the changes we can detect with a high level of certainty, the earlier we can detect disease progression and intervene with appropriate treatments.

In this article, I'll discuss the features and capabilities of spectral domain OCT and how the technology is helpful in glaucoma diagnosis and management.

Faster Image Acquisition

OCT is similar to ultrasound, but OCT uses light instead of sound waves to reveal an image. There are A-scans that show reflections along a single point, and B-scans that show the cross section of the tissue. The B-scans are simply a collection of A-scans along a given line.

Spectral domain OCT increases the speed of image capture by 50% to 125% compared with time domain OCT. Further, spectral domain OCT captures an entire scan at once, acquiring a high-density OCT scan with 8,000 A-scans in approximately one-third of a second, whereas time domain OCT captures the scan pixel by pixel, with 512 A-scans in a high-density scan.

Therefore, we can capture much more information in less time — a clear advantage when we're imaging the eye, where shorter times result in fewer eye movements and fewer motion artifacts.

3-D Imaging

The paradigm-changing feature of spectral domain OCT is 3-D imaging. Because the technology captures more A-scans per OCT scan, we can acquire 3-D datasets in the amount of time required to obtain 3 to 6 time domain OCT scans. We can evaluate 3-D datasets in a number of ways.

With time domain OCT, we examine circumpapillary scans, but spectral domain OCT extracts circumpapillary scans from the 3-D datasets. In examining the retinal nerve fiber layer thickness with spectral domain OCT, we extract a 6-mm circle from the 3-D datasets with many more data points and less interpolation compared with time domain OCT. When we use spectral domain OCT to assess the optic nerve, we capture a rich 3-D dataset from which we can extract optic nerve information.

In contrast, time domain OCT centers six radial lines on the optic nerve, requiring extrapolation.

With roughly twice the resolution as time domain OCT, spectral domain enables us to examine finer details. I think this will allow us to better assess a patient's condition if we can discern tissue layers and identify glaucoma at the cellular layer, where damage is occurring.

With 3-D imaging, we can examine tissue interfaces so we can see the 3-D relationship between the vitreous and retina or various retinal tissue layers, and this may be useful in diagnosis and surgical planning.

In addition, with spectral domain OCT generating higher density individual tomograms, tissue planes come into sharper relief, enabling us to more easily identify tissue planes and small abnormalities, such as specific photoreceptor loss.

This is just the tip of the iceberg when we look forward to the benefits of 3-D datasets.

In addition to many other advantages, 3-D datasets will enable us to examine tissue contours and more closely study each specific area of interest.

Registration Capabilities

Because we need to detect small changes over time in patients with glaucoma, spectral domain OCT is especially suited to assessing this disease. Time domain OCT and other imaging technologies are excellent for detecting glaucoma, but measurement noise from scan to scan makes these technologies less sensitive and specific in detecting disease progression. However, because future software developments enable spectral domain OCT to register 3-D datasets with each other and extract the scan information from exactly the same location from each scan, we anticipate a significant reduction in noise. Therefore, scans should be less variable, with much greater sensitivity and specificity, allowing us to better identify disease and monitor progression.

Make the Most of Spectral OCT

Despite the promise of this technology, it's important to keep in mind that it remains to be proven whether scan registration, 3-D imaging and registration of 3-D scans will improve reproducibility, sensitivity and specificity. We also need to remember that, although we've been using time domain OCT for more than 5 years, spectral domain OCT is new and still needs to be validated in clinical studies and clinical practice.

A lag always exists between the time when a technology is introduced and the time when peer-reviewed studies are published. And although company representatives can provide some useful information about the new technology, it's best to obtain as much information as possible from peer-reviewed literature. You also can learn from experts at scientific meetings. Evidence-based medicine is the best way to practice medicine. But bear in mind that if you're a complete slave to evidence-based medicine, you'll never be able to try anything new until it's been on the scene for some time.

Using Spectral OCT in Practice

Spectral domain OCT and other imaging technologies standardize assessment of the ocular structure. Numerous studies have shown that the sensitivity and specificity of imaging technologies are equal to that of an expert observer. However, no matter how many technologies you use — automated perimetry, photography or imaging technology — they won't replace a physician's clinical examination. They simply provide additional details to enhance your diagnosis.

Imaging technologies allow you to look for structurefunction correspondence in a quantitative way. For example, perimetry results can vary from test to test, with most studies requiring five or six visual field tests before you can verify disease or progression. But if you see correspondence between perimetry and imaging, you've verified your diagnosis or confirmed disease progression without having to repeat the test numerous times, saving time and money for the patient and the practice.

Finally, when you identify an abnormality using imaging technologies, you should always go back and clinically examine the patient to verify the machine's findings. Technology provides objective quantitative information, but it also allows you to reexamine the patient more closely based on the key information your equipment has provided. nMD