There are currently two types of OCT instruments used for imaging the anterior segment: time domain (TD) and the more recently developed Fourier domain (FD). Relying on the mechanical movement of the reference mirror to produce each axial scan (A-scan), TD-OCT samples the range of depth being imaged one point at a time. This serial sampling limits the speed of image acquisition. In contrast, FD-OCT—also known as spectral-domain (SD) OCT, spectral OCT, high-definition (HD) OCT, and frequency-domain OCT—does not rely on the mechanical movement of a reference mirror: the reflections from the entire depth range being imaged are sampled simultaneously. The interference between the sample and reference beams is detected as a spectral interferogram, which undergoes Fourier transform to produce axial scans (A-scans) [9
]. The parallel detection greatly improves speed without sacrificing signal level. Therefore, FD-OCT instruments can provide scan speeds 10–100 times faster than TD-OCT instruments [10
]. The faster speeds minimize the effect of eye movements during imaging and allow higher-definition imaging due to denser axial scans in the same transverse scan length. The scanning speed of FD-OCT also facilitates the registration and averaging of sequential frames. This frame averaging makes it possible to both increase the signal-to-noise ratio of an image and sharpen the anatomic features contained therein (Figures , , , , , and ).
Figure 1 Two scans of the same nasal angle in a normal subject. (a) Fourier-domain RTVue OCT image using a 6mm CL-Angle scan pattern. Schwalbe's line (SL), trabecular meshwork (TM), Schlemm's canal (SC), and scleral spur (SS) are visible. However, the (more ...)
Figure 2 Frame-averaged cross-sectional OCT image of the nasal angle in a normal subject. The high resolution helps to visualize the termination of the endothelium and Descemet's membrane (Schwalbe's line, SL), which is a useful landmark on these images. Also (more ...)
Figure 3 Frame-averaged cross-sectional OCT image of a closed angle with modified Shaffer grade of 0 by gonioscopy. The high definition of the image allows the visualization of the Schwalbe's line (SL) and the contact between the iris and the trabecular meshwork (more ...)
Figure 4 Frame-averaged cross-sectional OCT image of the nasal angle in an eye with primary narrow angle glaucoma. The AOD-SL (dotted line) was measured at 177 µm, below the diagnostic cutoff value of 230–290μm, indicating a potentially (more ...)
Figure 5 Pre- and postlaser peripheral iridotomy OCT images of an eye with narrow angles. (a) Preoperative image of the nasal angle with frame averaging. The short distance between the trabecular meshwork (TM) and the iris indicates a narrow, potentially occluded (more ...)
Figure 6 A cross-sectional OCT image of the nasal angle following trabectome surgery. This frame-averaged image shows that the posterior trabecular meshwork has been removed, leaving a 374μm wide trabecular cleft (TC) and an anterior trabecular (more ...)
Anterior segment OCT systems can also be differentiated by the central wavelengths they utilize for scans. Current OCT instruments commonly use 840
nm, or 1310
nm wavelength light. The use of longer wavelength light to image the anterior chamber angle does have advantages. 1310
nm and 1050
nm lights show lower scattering and signal loss in turbid media [11
], permitting much better penetration through the limbus and sclera than is possible using shorter wavelengths.
This increased penetration allows 1310
nm wavelength OCT to accurately measure gross angle morphology and visualize angle structures such as the iris root and scleral spur [13
]. As a result, past studies used 1310
nm OCT systems and developed quantitative angle parameters using scleral spur as the anatomical landmark [16
]. The axial resolutions of the earlier OCT systems used in the abovementioned studies were limited to 15–20μ
m and did not allow reliable identification of smaller angle structures such as the trabecular meshwork and Schwalbe's line. Newer OCT systems capable of producing axial resolutions of 1–5μ
m are now available [20
]. This marked increase in resolution is due to the combination of broader bandwidth and shorter wavelength.
nm FD-OCT systems discussed in this paper had an axial resolution of 5μ
m and were able to visualize details of the anterior segment that could not be resolved with previous OCT systems [14
]. These details include the Schwalbe's line, Schlemm's canal, trabecular meshwork, and aqueous collector veins (Figures –). As an example, we used both the 840
nm RTVue FD-OCT (Optovue, Inc. Fremont, CA, USA) and the 1310
nm Visante TD-OCT (Carl Zeiss Meditec, Inc.; Dublin, CA) to image the same anterior chamber angle location of a normal volunteer. We then compared the resulting images (). The 1310
nm Visante OCT had better penetration than the RTVue and could visualize both scleral spur and angle recess. The device's resolution, however, prevented visualization of fine anatomical structures such as Schwalbe's line, Schlemm's canal, and trabecular meshwork. By contrast, the higher resolution 840
nm RTVue OCT image showed Schwalbe's line, trabecular meshwork, Schlemm's canal, and scleral spur (), but provided limited visualization of the angle recess. In summary, the Visante provided a wider field of view and better penetration, but with lower resolution, while the RTVue provided higher-resolution information on a narrower portion of the anterior chamber. Wylegała et al. [22
] reported similar observations in a 2009 study comparing the results of anterior segment imaging with the RTVue and the Visante.
The study examined 54 eyes and compared measurements of central corneal thickness (CCT), trabecular-iris area (TISA), angle opening distance at the scleral spur (AOD-SS), and anterior segment morphology. Upon comparing the mean values of the CCT, TISA, and AOD-SS measurements obtained using the RTVue with those obtained using the Visante, Wylegała et al. found no statistically significant difference between the measurements. There was, however, a difference in the amount of anatomic detail contained in the images. The study found that the RTVue images showed structures, such as Bowman's layer and Schlemm's canal, which could not be visualized with the Visante.