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 .
 new advances in anterior segment optical coherence tomography ( oct ) technology development allow visualizing the anterior chamber angle of the eye with high speed and high resolution . 
 fourier - domain ( fd ) oct instruments working at 840 nm can reliably identify fine angle structures such as the schwalbe 's line . 
 this paper demonstrates quantitative angle assessment with 840  nm fd - oct and provides diagnostic cutoff values for occludable angle detection . 
 this is useful for angle closure glaucoma diagnosis and management . 
 moreover , 840  nm fd - oct is a useful tool for postsurgical evaluation in glaucoma patients . 
1. Introduction
2. Anterior Segment OCT 
3. Schwalbe's Line-Based Quantitative Angle Assessment with High-Resolution FD-OCT
4. Postoperative Angle Imaging with FD-OCT
5. Conclusion