At the time of writing this letter, the RTVue-100 (Optovue, Inc.) used by Wylęgala et al.1 is the only commercially available Fourier-domain optical coherence tomography (OCT) device that can scan the anterior segment without “personal” modifications. Similar to all commercially available Fourier-domain OCT devices, it uses shorter wavelength light than the Visante-OCT (Carl Zeiss Meditec) (840 nm versus 1310 nm). It scans at a faster rate (26 000 A-scans/s) than time-domain OCT devices (2048 A-scans/s for Visante-OCT). This improves image quality by achieving higher resolution and also reducing motion artifact.
We agree with the authors that Fourier-domain OCT can image Bowman layer as a distinct entity in a healthy cornea (Figure 1, A, in article). However, Figures 3, A, and 4, A, suggest that visualization of Bowman layer may be limited in the presence of corneal pathology. The use of short wavelength light may be a contributing factor. Using a modified Fourier-domain OCT, the Cirrus high-definition OCT (Carl Zeiss Meditec), Wong et al.2 imaged the termination of the Descemet/endothelium layer at the Schwalbe line. As this may potentially be important in analyzing the angle, do the authors have any information regarding the capabilities of the RTVue-100 in imaging this landmark?
Wavelength light of 1310 nm has a good depth of penetration, including through nontransparent structures such as limbus and corneal opacity.3,4 Figure 6 illustrates the poor penetration of shorter wavelength light. Fourier-domain OCT visualization of the Ahmed valve and ocular structures deeper than this is inferior to that with the Visante OCT. Similarly, in Figure 2, the root of the angle is not imaged adequately with the Fourier-domain OCT. We would not necessarily agree with the authors' statement that “all angle structures can be visualized” with the Fourier-domain OCT. Wong et al.2 have recently highlighted this limitation of Fourier-domain OCT.
It is well recognized that the scleral spur can be visualized in 70% to 80% of cases with the Visante OCT.2,5 In Figure 2, the scleral spur is identified with the Visante OCT, but not with the RTVue-100. It would be interesting to know in what percentage of cases the scleral spur was visualized with the RTVue-100. In addition, how did the authors define and measure the angle-opening distance?
1. Wylęgala E, Teper S, Nowińska AK, Milka M, Dobrowolski D. Anterior segment imaging: Fourier-domain optical coherence tomography versus time-domain optical coherence tomography. J Cataract Refract Surg. 2009;35:1410-1414.
2. Wong H-T, Lim MC, Sakata LM, Aung HT, Amerasinghe N, Friedman DS, Aung T. High-definition optical coherence tomography imaging of the iridocorneal angle of the eye. Arch Ophthalmol. 2009;127:256-260.
3. Radhakrishnan S, Rollins AM, Roth JE, Yazdanfar S, Westphal V, Bardenstein DS, Izatt JA. Real-time optical coherence tomography of the anterior segment at 1310 nm. Arch Ophthalmol. 2001;119:1179-1185.
4. Konstantopoulos A, Kuo J, Anderson D, Hossain P. Assessment of the use of anterior segment optical coherence tomography in microbial keratitis. Am J Ophthalmol. 2008;146:534-542.
5. Sakata LM, Lavanya R, Friedman DS, Aung HT, Seah SK, Foster PJ, Aung T. Assessment of the scleral spur in anterior segment optical coherence tomography images. Arch Ophthalmol. 2008;126:181-185.