Q:Can you provide details on swept-source imaging technology for the cornea? What are the advantages to having this technology in a refractive surgery practice?
A:“Swept-source optical coherence tomography (SS-OCT) is the latest technology in the evolution of OCT,” explains Karen Yeung, OD, senior optometrist at the Arthur Ashe Student Health & Wellness Center at the University of California Los Angeles. In this technology, “a photodetector detects wavelength-resolved interference signals from a swept-source monochromatic laser (with wavelength of 1,310µm). Commercially available SS-OCTs scan up to 30,000 A-scans per second, with a longitudinal and transverse resolution of 10µm and 30µm, respectively.”1,2 Typically, she adds, the technology creates a 360-degree scan of the anterior segment, which is divided into 128 cross-sections comprised of 512 A-scans each. These cross-section images are processed through a computer analysis program to create three-dimensional representations of the patient’s cornea, including both the anterior and posterior portions; the anterior chamber; and angle; bleb segments of the sclera and iris thickness; cornea curvature and surface area.
But is this technology beneficial for a refractive surgery practice? Yes, says Dr. Yeung, citing its accuracy with provision of reliable measurements of corneal curvature, thickness and elevation prior to and following a refractive surgery procedure.3 “Precise evaluation of the cornea is important in refractive surgery to preclude complications such as post-surgical corneal ectasia,” she explains, adding that SS-OCTs may provide the highest accuracy in screening for subclinical keratoconus and measuring corneal thickness in keratoconic eyes.4-6 A nomogram developed for use in SS-OCT machines can also be used to differentiate normal, forme fruste and early keratoconus, further determining whether a patient is eligible for refractive surgery.7
Dr. Yeung points out that SS-OCT technology could be useful for measuring corneal thickness in patients with corneal dystrophies, as its three-dimensional mapping capabilities can record the size, depth and location of granular corneal dystrophy deposits to guide phototherapeutic keratectomy and monitor for corneal changes.8,9 Additional research indicates the combination of posterior corneal measurements from the SS-OCT device and anterior autokeratometry measurements may allow for accurate prediction of residual astigmatism following surgery.10,11
SS-OCT technology can also help with evaluating the ocular surface for pre- and postoperative signs of dry eye. Research has indicated a high correlation in tear meniscus measurements between OCT, vital staining scores, Schirmer test values and tear film breakup time.12
In addition to imaging the cornea, Dr. Yeung notes, SS-OCT technology may also help with the improvement of accommodative technologies in pseudophakes and in scleral contact lens fittings.13
1. SH Yun, GJ Tearney, JF De Boer, et al. High-speed optical frequency-domain imaging. Optics Express. 2003;11(22):2953-63.
2. Yasuno Y, Madjarova VD, Makita S, et al. Three-dimensional and high-speed swept-source optical coherence tomography for in vivo investigation of human anterior eye segments. Optics Express. 2005;13(26):10652-80.
3. Lee YW, Choi CY, Yoon GY. Comparison of dual rotating Scheimpflug-Placido, swept-source optical coherence tomography and Placido-scanning-slit systems. J Cataract Refract Surg. 2015;41;1018-29.
4. Steinburg J, Casagrande MK, Frings A, et al. Screening for subclinical keratoconus using swept-source fourier domain anterior segment optical coherence tomography. Cornea 2015 Nov;34(11):1413-9.
5. Jhanji V, Yang B, Yu M, et al. Corneal thickness and elevation measurements using swept-source optical coherence tomography and slit scanning topography in normal and keratoconic eyes. Clin Experiment Ophthalmol. 2013 Nov;41(8):735-45.
6. Szalai E, Berta A, Hassan Z, et al. Reliability and repeatability of swept-source Fourier domain optical coherence tomography and Scheimpflug imaging in keratoconus. J Cataract Ref Surg. 2012 Mar;38(3):485-94.
7. Rabinowitz YS, Li X, Canedo A, et al. Optical coherence tomography (OCT) combined with videokeratography to differentiate mile keratoconus subtypes. J Refrat Surg. 2014;30(2):80-7.
8. Nowinska AK, Teper SJ, Janiszewska DA, et al. Comparative study of anterior eye segment measurements with spectral swept-source and time-domain optical coherence tomography in eyes with corneal dystrophies. BioMed Research International. 2015 Sept. [Epub].
9. Mori H, Mirua M, Iwasaki T, et al. Three-dimensional optical coherence tomography-guided phototherapeutic keratectomy for granular corneal dystrophy. Cornea. 2009;28(8):944-7.
10. Hoffman PC, Abraham M, Hirnschall N, et al. Prediction of residual astigmatism after cataract surgery using swept source fourier domain optical coherene tomography. Current Eye Research. 2014 Dec:39(12):1178-86.
11. Hoffman PC, Jutz WW, Analysis of biometry and prevalence data for corneal astigmatism in 23,239 eyes. J Cataract Surgery. 2010;36:1479-85.
12. Akiyama-Fukuda R, Usui T, Yoshida T, et al. Evaluation of tear meniscus dynamics using anterior segment swept-source optical coherence tomography after topical solution instillation for dry eye. Cornea 2016;35:654-8.
13. Neri A, Ruggeri M, Protti A, et al. Dynamic imaging of accommodation by swept-source anterior-segment optical coherence tomography. J Cataract Refract Surg. 2015 Mar;41(3):501-510.