Three main attributes for swept-source optical coherence tomography involve deeper range of imaging into the eye, less sensitivity roll-off with depth, and faster scanning speed.
Take-home message: Three main attributes for swept-source optical coherence tomography involve deeper range of imaging into the eye, less sensitivity roll-off with depth, and faster scanning speed.
By Lynda Charters; Reviewed by Richard F. Spaide, MD
New York-Developments in optical coherence tomography (OCT) have emerged relatively rapidly over the past 20 years-each iteration of the technology having its own pros and cons, said Richard F. Spaide, MD.
OCT angiography seems to be the direction in which OCT is moving, with its rapid scanning speeds and more detailed imaging.
Spectral-domain OCT (SD-OCT) instruments use a number of wavelengths in a particular bandwidth and the light goes through the eye. Some light goes through a reference arm for comparison with that in the eye by a spectrometer. In that reflection, the frequency of the interferogram depends on the depth within the eye from which the reflection originated.
“The greater the distance that the eye is penetrated, the more the mismatch and the higher the frequency of the interferogram,” said Dr. Spaide, in private practice, Vitreous Retina Macula Consultants of New York.
“SD-OCT is a bit less sensitive to higher frequencies,” he said. “Therefore, the sensitivity of SD-OCT decreases with increased depth.”
To compensate somewhat for this drawback, investigators who want to evaluate deeper ocular structures, such as the choroid and sclera, can move the peak of the sensitivity curve inward, referred to as enhanced-depth OCT (EDI-OCT). However, in so doing, the visibility of the vitreous is markedly decreased, he noted.
Swept-source OCT (SS-OCT) scans sequentially through a series of wavelengths to enhance the interferogram, which is decoded with Fourier transform into an A scan (many of which make a B scan) that produces an image. Depth is still an issue, but much less so than with SD-OCT, because the sensitivity roll-off of SS-OCT is less than SD-OCT.
“SS-OCT has three main attributes: deeper range of imaging into the eye, less sensitivity roll-off with depth, and faster scanning speed,” Dr. Spaide said.
The vitreous and the choroid can be imaged simultaneously because of the deeper range afforded by SS-OCT.
“This ability eliminates the need to focus on the vitreous or the retina, as was necessary with EDI-OCT,” he said.
With SD-OCT, the range where sensitivity is high is limited. The physician has to make a choice: Use conventional imaging of the vitreous and retina, or use EDI-OCT to be able to see the choroid and the retina.
Because SS-OCT has a larger “sweet spot” in terms of sensitivity, it can image longer ranges. However, the illumination beam is not at optimal focus across that range.
The longer range afforded is 2.6 mm deep with a scanner (Topcon DRI-OCT). Across the image range, the retina and choroid can easily be seen, but the choroidal details are dim.
“To evaluate the vitreous, the brightness can be increased,” Dr. Spaide said. “However, the details of the inner vitreous are not sharp and the retina is too bright.”
To overcome this, Dr. Spaide developed a method to improve imaging of the vitreous that includes dynamic sweep of focus across the vitreous with windowed averaging and imaging processing as the final result.
“There is a great deal of detail seen in the vitreous and the details are sharp throughout the entire depth of the vitreous,” he said. “In addition, the retina appears normal and is not overly bright.”
The current SS-OCT model scans at a high rate of speed, i.e., 100,000 A-scans per second.
“This is not important in normal clinical use,” he said. “However, speed is important for OCT angiography.”
The current SD-OCT angiography instrument by Optovue can scan at 70,000 A-scans per second, which requires 3 to 4 seconds to obtain a 3 by 3-mm scan. The patient has to hold the eye steady during this time. The images produced are great, but a larger scan area would be better. If a 6 by 6-millimeter scan is desired at the same resolution, the patient will have to remain still for 12 to 16 seconds, which is almost impossible.
“Scanning at faster scan speeds is the answer to this,” he said.
In the future, greater emphasis will be placed on developing faster scanning speeds in commercial instruments. Current commercial instruments, other than the Topcon DRI-OCT, are all SD-OCT. It is likely some instrument manufacturers will turn to SS-OCT as part of their efforts to get faster scanning speed, Dr. Spaide noted.
Richard F. Spaide, MD
This article was adapted from Dr. Spaide’s presentation during Retina Subspecialty Day at the 2014 meeting of the American Academy of Ophthalmology. Dr. Spaide is a consultant to and receives royalties from Topcon Corp. and holds patents in some optical coherence tomography instruments.