In the 19th century, the collaboration between engineering and medicine revolutionized the field of ophthalmology with the invention of ophthalmoscopy, and later fundus photography, that enabled lasting visualization and characterization of the posterior pole. Ophthalmologists relied solely on subjective observation of the optic nerve and retinal nerve fiber layer to guide the management of glaucoma until the advent of digital imaging technologies in the 20th century.

Various imaging modalities have fluctuated in popularity and utility for the diagnosis and progression monitoring of glaucoma, but quantification of optic nerve data has remained elusive. OCT has become the technology of choice among the available scanning computerized ophthalmic diagnostic imaging tests.

Since the introduction of the prototype in 1991,¹ OCT evolved rapidly with several device iterations from time-domain (TD) to spectral-domain (SD) OCT that exponentially improved scanning speed, acquisition time, and image resolution. TD-OCT was limited by slow scan acquisition times and only allowed for two-dimensional imaging. With SD-OCT came significantly faster scanning speeds, which resulted in improved resolution and even more dramatic improvements in software, allowing for rapid three-dimensional data acquisition.

Most recently, the FDA cleared swept-source (SS) OCT, the newest imaging platform that generates unprecedented high-resolution images that are deeper and wider than those acquired with previous generations of OCT devices.

Here, I will provide an overview of SS-OCT technology, summarize clinical applications of imaging the anterior and posterior segment in glaucoma, highlight available commercial platforms, and explain why cutting-edge practitioners may want to adapt SS-OCT for the management of glaucoma patients.

SS-OCT TECHNOLOGY

Although SD-OCT is nearly ubiquitous in the ophthalmic market, it still has several limitations. SD-OCT can result in low image quality due to media opacity, and although image acquisition is fast (40,000 A-scans/second), patient movement can still introduce artifacts.

Furthermore, similar to predecessor generations, SD-OCT (850nm wavelength) only penetrates deep enough to offer reliable segmentation of the neuroretinal rim, peripapillary retinal nerve fiber layer, and macular ganglion cell complex.² The choroid and ciliary body cannot be imaged reliably.

The newest generation SS-OCT offers longer wavelengths, faster scanning speeds, along with a deeper and wider imaging range that allows for acquisition of the highest-resolution images to date.³ Specifically, SS-OCT uses a short cavity swept laser with a wavelength of 1,050nm and sweeping range of approximately 100nm that allows for imaging of deeper ocular structures and more efficient signal penetration of opaque media. Using two parallel photodetectors, SS-OCT can achieve a scanning speed of 100,000 A-scans per second with an axial resolution of 5µm that allows for improved image acquisition with less chance of movement artifact. In addition to quicker image acquisition, SS-OCT doesn’t emit visible light, which makes the experience less distracting to patients because they won’t unintentionally follow the scanning laser light rather than holding target fixation.

Finally, unlike the narrow imaging depth of focus with SD-OCT, SS-OCT can image a large axial window, capturing vitreous and retina simultaneously, while imaging as deep as the choroid and lamina cribrosa (LC). A wide-field imaging protocol also provides the capability of scanning the optic nerve and macula simultaneously. These unique features of SS-OCT facilitate broad clinical applications for imaging the anterior and posterior segment in glaucoma patients.

SS-OCT CLINICAL APPLICATION

Anterior segment SS-OCT imaging protocols provide improved visualization and objective quantification of anterior chamber structures, as well as a comprehensive three-dimensional assessment of aqueous humor drainage angle. Studies have demonstrated reproducible measurements for assessing the risk of angle closure and longitudinal changes pre- and post-laser interventions in patients with occludable narrow angles.⁴

Furthermore, SS-OCT imaging of the anterior segment facilitates evaluation of bleb morphology, tube shunt insertion, and patency, as well as confirming accurate microincisional glaucoma surgery device placement. In the posterior segment, SS-OCT has demonstrated that the diagnostic accuracy of both the wide-field and peripapillary RNFL thickness is similar to that of peripapillary RNFL thickness obtained with SD-OCT.⁵ Furthermore, SS-OCT and SD-OCT have similar diagnostic accuracy using the macular ganglion cell complex.⁶

The longer wavelength of SS-OCT allows for improved visualization of deeper ocular structures in the glaucomatous eye, specifically the choroid and LC.⁷ Choroidal thickness has been implicated to have a role in the glaucomatous disease process. Choroidal thickness in highly myopic eyes with normal-tension glaucoma is significantly thinner than in myopic eyes without glaucoma.⁸ Although the precise pathogenesis of glaucoma remains unknown, the LC is the presumed site of axonal injury in glaucoma. SS-OCT imaging of LC allows for identification of focal defects and changes in the overall microarchitecture (thinner LC and LC displacement) that are associated with rapid glaucomatous progression.^9,10

EXPERIMENTAL AND COMMERCIAL DEVICES

As of this publication, the Zeiss PLEX Elite 9000 (Zeiss) is the only SS-OCT device that is FDA-approved (November 2016) and commercially available in the U.S. The anterior segment SS-OCT device, SS-1000 Casia (Tomey), is pending FDA approval. The Topcon DRI OCT1 Atlantis (Topcon) launched in 2012, and although approved in Europe (CE mark), it is not approved for use in the U.S. Germany-based Heidelberg Engineering has entered into a patent license agreement with Massachusetts General Hospital in Boston for the development of SS-OCT technology for use in ophthalmology.

So, although there is currently only device available in the United States, many companies are in a race to obtain approval for their SS-OCT platforms.

ADVANTAGES OF SS-OCT

Although the diagnostic capability of SS-OCT wide-field and peripapillary RNFL thickness is similar to that of peripapillary RNFL thickness obtained with SD-OCT, the SS-OCT platform may offer some advantages. The SS-OCT wide-field scan allows for data acquisition from the macula and optic disc to be captured in a single scan, and although not currently available, a segmentation algorithm that combines peripapillary RNFL and macular ganglion cell complex for structure-structure analysis has the potential to yield higher glaucoma diagnostic accuracy and earlier detection of glaucomatous progression.

The wide-field SS-OCT scan is superior in dealing with atypical optic disc configurations (tilted optic discs and extensive peripapillary atrophy) and provides better visualization of the posterior pole in highly myopic eyes.¹¹ Additionally, the wide-field scan is potentially less susceptible to media artifacts that may affect image acquisition and measurements of ocular structures.

CONCLUSION

OCT has arguably been the greatest ophthalmic imaging innovation in history, and now the recent advancements of the SS-OCT platform allow for rapid acquisition of high-quality images with unprecedented depth, width, and signal quality. SS-OCT has immense potential to revolutionize the imaging of the anterior and posterior segments to identify novel individual or combination structural parameters for glaucoma diagnosis and monitoring of disease progression. GP

References

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2. Dong ZM, Wollstein G, Schuman JS. Clinical utility of optical coherence tomography in glaucoma. Invest Ophthalmol Vis Sci. 2016;57(9):OCT556-567.
3. Grulkowski I, Liu JJ, Potsaid B, et al. Retinal, anterior segment and full eye imaging using ultrahigh speed swept source OCT with vertical-cavity surface emitting lasers. Biomed Opt Express. 2012;3(11):2733-2751.
4. Angmo D, Nongpiur ME, Sharma R, Sidhu T, Sihota R, Dada T. Clinical utility of anterior segment swept-source optical coherence tomography in glaucoma. Oman J Ophthalmol. 2016;9:3-10.
5. Yang Z, Tatham AJ, Zangwill LM, et al. Diagnostic ability of retinal nerve fiber layer imaging by swept-source optical coherence tomography in glaucoma. Am J Ophthalmol. 2015;159(1):193-201.
6. Lee KM, Lee EJ, Kim TW, Kim H. Comparison of the abilities of SD-OCT and SS-OCT in evaluating the thickness of the macular inner retinal layer for glaucoma diagnosis. PLoS One Jan. 26, 2016; available at: http://dx.doi.org/10.1371/journal.pone.0147964 . Last accessed Feb. 9, 2017.
7. Mansouri K, Nuyen B, N Weinreb R. Improved visualization of deep ocular structures in glaucoma using high penetration optical coherence tomography. Expert Rev Med Devices. 2013;10(5):621-628.
8. Usui S, Ikuno Y, Miki A, Matsushita K, Yasuno Y, Nishida K. Evaluation of the choroidal thickness using high-penetration optical coherence tomography with long wavelength in highly myopic normal-tension glaucoma. Am J Ophthalmol. 2012;153(1):10-16.e1.
9. Faridi OS, Park SC, Kabadi R, et al. Effect of focal lamina cribrosa defect on glaucomatous visual field progression. Ophthalmology. 2014;121(8):1524-1530.
10. Lee EJ, Kim TW, Kim M, Kim H. Influence of lamina cribrosa thickness and depth on the rate of progressive retinal nerve fiber layer thinning. Ophthalmology. 2015;122(4):721-729.
11. Lim LS, Cheung G, Lee SY. Comparison of spectral domain and swept-source optical coherence tomography in pathological myopia. Eye (Lond). 2014;28(4):488-491.