Understanding advantages, limitations of SS-OCT

The history of commercialization of ophthalmic optical coherence tomography (OCT) is a fascinating story considering its evolution and how the technology moved so rapidly from the laboratory into the clinic, according to Jay S. Duker, MD.

Speaking at AAO 2018 during a symposium on imaging structure, Dr. Duker brought attendees up to date on swept-source (SS-OCT), the latest generation of OCT devices.

He said that SS-OCT first became commercially available in 2013, and he noted that it has significant theoretical advantages and promise for having clinical advantages over the previous generation spectral domain (SD) systems. However, Dr. Duker also pointed out that SS-OCT has some disadvantages, of which higher cost relative to SD-OCT is a major issue that is hampering its uptake.

“The big caveat right now is that SS-OCT devices are not widely available because of their increased cost, said Dr. Duker, professor and chair, Department of Ophthalmology, Tufts University School of Medicine, Boston.

Pros and cons

Compared with SD-OCT, SS-OCT generally has faster image acquisition, which leads to less tradeoff between image size and resolution, and also allows wider scanning ranges (12 to 16 mm scan lengths are commercially available now; longer ranges can be gotten with specific lenses and mosaic programs).

In addition, there is no sensitivity roll-off with SS-OCT. Therefore, vitreous and choroid can be imaged well simultaneously. Because SS OCT uses a longer wavelength of light than SD-OCT, SS-OCT also provides deeper penetration.

“SS-OCT can penetrate better through lens opacity, choroid, pigment, blood, and intraocular gas than SD-OCT, and based on two published papers, it appears that SS-OCT will allow better imaging of choroidal neovascularization,” said Dr. Duker. “This all sounds wonderful, and so you might wonder why swept source has not swept the market.” 

Outlining its disadvantages, Dr. Duker noted that the higher cost of SS-OCT comes from the light source that is used.

“SS OCT requires narrow linewidth, high speed, frequency swept lasers that are very pricey,” he said.

Other disadvantages of SS OCT compared with SD OCT include lower axial resolution, worse signal-to-noise ratio, worse motion artifact, and absence of normative databases, which is important for use in glaucoma imaging.

“Worse axial resolution with SS-OCT can be overcome with some image averaging, but in general, the SS-OCT images may not be quite as sharp as what we are used to seeing with SD-OCT,” Dr. Duker said.

He also noted that what he refers to as “commercial pressures” may also be contributing to the limited availability of SS-OCT in clinical practice.

“The OCT companies are doing very well selling SD devices. They have to retool to make SS-OCT and perhaps they do not want to cut into their own market,” Dr. Duker explained.

Current commercial status

Two SS-OCT systems for posterior segment imaging are available in the United States and elsewhere, the Triton DRI (Topcon Medical Systems) and PLEX Elite 9000 (Carl Zeiss Meditec). In addition, a new optical biometer (IOLMaster 700, Carl Zeiss Meditec) features SS-OCT.

“The company chose to use SS-OCT because of its ability to penetrate denser cataracts more easily and its faster speed of image acquisition,” he explained.

Dr. Duker reviewed the specifications and capabilities of the posterior segment SS-OCT imaging systems. He said the Triton DRI is a second generation machine from Topcon. It has a scanning rate of 100,000 A scans/sec and has built-in cameras for color fundus photography and fundus autofluorescence.

In addition, it can be used for fluorescein angiography and OCT angiography, although the latter application is not FDA approved.

The PLEX Elite 9000 also does 100,000 A scans/sec. An upgrade that doubles the scan rate is expected to be forthcoming pending FDA approval. Showing images from a normal eye taken with the currently available and faster scan rates, Dr. Duker pointed out how the upgrade will clearly provide better detail.

The PLEX Elite 9000 features OCT angiography and has FDA approval for that application. It does not have the other capabilities found on the Triton DRI, and access to the PLEX Elite 9000 requires joining the company’s research network.

Both systems feature tracking and have a 12 mm scan length. Although not FDA approved, the PLEX Elite 9000 can do up to a 23 mm cube scan with a lens.

Dr. Duker presented images acquired using the two SS-OCT devices that illustrated the wide scanning length, detailed choroidal images, and better OCT angiography image quality compared with SD-OCT angiography.

He also noted that the readout on the Triton DRI can show multiple images, which he said is a “handy feature” for clinicians.

Disclosures:

Dr. Duker receives research support from Carl Zeiss Meditec and Topcon Medical Systems.