Swept Source OCT
You probably heard about Swept source OCT, the latest technology with the highest definition, a much wider field of view, and the ability to go through most media opacities, not to mention the fact that it can image the choroid with great detail – yet it hasn’t taken over our offices. Have you ever wondered why? Let’s first go back a few steps.
It all started in 1989 with the invention of Time Domain OCT, back then image resolution was low (10-15 microns) but this was a revolution because, for the first time, we were able to see a cross-section of the retina in vivo. Since then, technology has improved with better image quality. Spectral Domain OCT (today’s standard) appeared in 2006 with a definition of 3 microns, assessment of the retina became easier and fewer pathologies were missed.
It was in 2012 that Swept source OCT was invented. Let’s review its characteristics:
- SS-OCT allows imaging of the vitreous, retina & choroid (to its outer border), all 3 in greater detail (see image below).
Note the excellent definition of the vitreous, retina, and choroid, how broad is the scan, and the ability to view the choroid to its outer limit.
- SS-OCT has a better tissue penetration due to:
- Longer wavelength (1050nm Vs 840nm in SD-OCT)
- It sets the choroid to be the “zero-delay line”
- Simplified optical system compared to SD-OCT
=> Allows faster image capture (100,000 scans/sec Vs 20-40,000 for SD-OCT)
=> Less artifacts
=> Wider area of imaging
=> Higher resolution (1 microns Vs 3 microns for SD-OCT)
- Allows visualization of the posterior pole through media opacities such as dense cataracts, vitreous hemorrhage, intraocular oil or gas etc. (see image below)
This patient has brunescent cataracts OS>OD, note how clear the OCT
scan is allowing great assessment of the retinal and choroidal architecture.
This is a 72 y/o patient with a ruptured macroaneurysm and a multilayered hemorrhage, note the difference in penetrance between SD-OCT and SS-OCT.
SS-OCT can scan wider areas, see below.
Photo montage of multiple scans can also be done and allow even wider imaging, see below.
This all sounds wonderful, so what prevents Swept source OCT from taking over our clinics and replacing SD-OCT?
Cost is likely the primary reason. According to Dr. Jay S. Duker, Adjunct Professor of Ophthalmology at the Tufts Medical Center and Tufts University School of Medicine in Boston, Massachusetts, 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”. Other past disadvantages of SS-OCT included lower axial resolution, worse signal-to-noise ratio, and worse motion artifact for which manufacturers worked to improve those issues. Lastly, SS-OCT lacks normative databases, which limits its utility in the glaucoma field.