since its initial development approximately two decades ago , optical coherence tomography ( oct ) has become an indispensable tool in ophthalmic imaging . although it was initially designed to image the posterior segment and retina [ 2 , 3 ] , oct 's capability to generate cross - sectional images of internal biological structures without contacting the ocular surface anterior segment oct technology has been improved a great deal since izatt et al . first reported using oct to image the anterior segment of the eye in 1994 . of the many uses that have come out of those improvements , one of the most commonly cited is the evaluation of the anterior chamber angle for narrow - angle glaucoma . 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 . 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 ) . therefore , fd - oct instruments can provide scan speeds 10100 times faster than td - oct instruments . 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 1 , 2 , 3 , 4 , 5 , and 6 ) . anterior segment oct systems can also be differentiated by the central wavelengths they utilize for scans . current oct instruments commonly use 840  nm , 1050  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 , 12 ] , 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 [ 1315 ] . as a result , past studies used 1310  nm oct systems and developed quantitative angle parameters using scleral spur as the anatomical landmark [ 1619 ] . the axial resolutions of the earlier oct systems used in the abovementioned studies were limited to 1520  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 15  m are now available [ 20 , 21 ] . this marked increase in resolution is due to the combination of broader bandwidth and shorter wavelength . the 840  nm fd - oct systems discussed in this paper had an axial resolution of 5  m and were able to visualize details of the anterior segment that could not be resolved with previous oct systems [ 14 , 16 ] . these details include the schwalbe 's line , schlemm 's canal , trabecular meshwork , and aqueous collector veins ( figures 16 ) . 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 . 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 ( figure 1 ) , 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 . 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 , wylegaa 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 . schwalbe 's line represents the termination of descemet 's membrane and is approximately 500750  m anterior to the scleral spur . the distance between schwalbe 's line and the scleral spur represents the trabecular meshwork and the aqueous humor filtration distance . schwalbe 's line was proposed as an anatomical landmark because it could be consistently identified in 840  nm fd - oct images . several research groups recommended measuring angle opening distance at the schwalbe 's line ( aod - sl ) for quantitative angle assessment [ 2325 ] . further , aod - sl has demonstrated good repeatability and strong correlation with gonioscopic grading . each participant underwent dark - room gonioscopy , and the angle was graded in four quadrants ( superior , inferior , nasal , and temporal ) using the modified shaffer grading system . oct imaging was performed on the nasal and temporal quadrants of each eye using the cirrus hd - oct model 4000 ( software version 3.0 , carl zeiss meditec , dublin , ca , usa ) . an external fixation light was used to guide each patient 's fixation to the side of the oct instrument during imaging . the aod - sl measurements showed consistent interobserver and intraobserver reliability with intraclass correlation coefficients of 0.979 and 0.988 , respectively . there was also strong correlation between aod - sl measurements and gonioscopic grading , with a spearman correlation coefficient of 0.709 . the occludable angle aod - sl measurements ranged from 94  m to 172  m , and the nonoccludable angle aod - sl measurements ranged from 286  m to 347  m . although cheung et al.did not report a diagnostic cutoff for occludable angle , based on these results one could be estimated at approximately 230  m .   in a similar study , qin et al . recruited 35 glaucoma patients from the doheny eye institute at the university of southern california , los angeles , california , usa . each subject underwent dark - room gonioscopic examination and a modified shaffer grade was recorded for each of four quadrants . angle scans were performed under standardized dim illumination conditions ( 6.6 foot candles ) using the rtvue fd - oct ( optovue , inc . , qin et al . also reported good repeatability for aod - sl measurements with intraobserver coefficients of variation ranging from 9.8% to 12.0% and interobserver coefficients of variation ranging from 10.1% to 13.3% . spearman 's rho analysis showed strong correlation between aod - sl measurements and gonioscopic grading with correlation coefficients of 0.80 for nasal angles and 0.81 for temporal angles . the occludable angle aod - sl measurements ranged from 0  m to 350  m , and the nonoccludable angle measurements ranged from 192  m to 922  m . qin et al . determined a diagnostic cutoff value for occludable angle of 290  m by performing receiver operating characteristic ( roc ) analyses . pre- and post - operative angle imaging with fd - oct is useful for documenting and evaluating surgery outcomes . for example , using fd - oct to image the angle both before and after laser peripheral iridotomy made it possible to document the resulting angle morphological changes ( figure 5 ) . this technology was also used to evaluate filtering blebs and demonstrate subconjunctival filtration after trabeculotomy , and to confirm the excision of trabecular tissue after trabectome treatment ( figure 6 ) . oct is also used to examine iridotrabecular contact as a peripheral anterior synechia after penetrating keratoplasty . high - resolution 840  nm fd - oct is capable of providing angle images with fine anatomical structures . studies showed that the schwalbe 's line could be identified in over 95% of fd - oct angle scans . in addition , measurements of the angle opening distance at the schwalbe 's line were shown to have good repeatability and strong correlation with gonioscopic assessments . quantitative angle measurements using schwalbe 's line as the anatomical landmark may help clinicians to better assess and manage narrow - angle glaucoma . based on gonioscopic correlation , studies so far indicate the aod - sl diagnostic cutoff for occludable angle is between 230  m and 290  m . therefore , measurement of angle opening distance with fd - oct may be useful in the assessment of angle closure risk . after filtration surgery , fd - oct is also a useful tool for postsurgical evaluation of angle structures .