REVIEW URRENT C OPINION
Development in anterior segment imaging for glaucoma Sunita Radhakrishnan a,b and Dmitry Yarovoy b
Purpose of review Recent developments in anterior segment imaging enable high-resolution imaging with higher speeds than previous systems. This review will focus on advances in optical coherence tomography (OCT) imaging of the anterior segment with an emphasis on angle evaluation. Recent findings Anterior segment imaging with OCT is possible with both time domain and Fourier domain devices, and with light sources of different wavelengths. Recent studies have identified new risk factors for angle closure and prediction models incorporating quantitative angle width parameters have been developed for detection of gonioscopically defined angle closure. Swept source OCT is a variation of Fourier domain OCT that allows three-dimensional imaging of the angle analogous to gonioscopy and novel quantitative parameters have been described for angle measurement with this technology. Fourier domain OCT devices have higher resolution and sampling density than time domain OCT and enable visualization of more angle structures such as the Schwalbe’s line and Schlemm’s canal. Summary OCT technology is expanding our understanding of angle closure disease. Although it does not replace gonioscopy in angle evaluation, it may help us identify predictors of progression along the angle closure disease spectrum such that treatment may be better targeted. Keywords angle closure, anterior segment OCT, swept source OCT, time domain anterior segment OCT
INTRODUCTION Cross-sectional imaging of the anterior segment is a useful adjunct to gonioscopy. Ultrasound biomicroscopy was the first anterior segment imaging technology to be developed and allows detailed visualization of all angle structures including the ciliary body; however, it is cumbersome to perform on a routine basis. In contrast, anterior segment optical coherence tomography (ASOCT) is a noncontact technology that is easy to use and allows visualization of the entire anterior chamber in one scan such that the inter-relationship between various ocular structures may be evaluated. Although ASOCT has been used for imaging of trabeculectomy blebs, aqueous drainage devices and newer surgical devices, its main application in glaucoma has been to evaluate the anterior chamber angle. In recent years, there have been significant advances in OCT technology, and this article will focus on developments in OCT imaging of the anterior segment with an emphasis on angle closure disease. www.co-ophthalmology.com
Time domain anterior segment OCT imaging at 1310 nm Anterior segment OCT imaging was first reported with the same wavelength of light used for retinal imaging (830 nm) [1]. The subsequent use of a longer wavelength light at 1310 nm allowed better visualization of the angle region due to increased penetration of scattering tissue like the sclera. The absorption characteristics of 1310 nm light also enabled the use of higher power with resultant increase in scanning speed such that time domain anterior segment OCT (TD-ASOCT) imaging could
a Glaucoma Center of San Francisco and bGlaucoma Research and Education Group, San Francisco, California, USA
Correspondence to Sunita Radhakrishnan, MD, Glaucoma Center of San Francisco, 55 Stevenson Street, San Francisco, CA 94105, USA. Tel: +1 415 981 2020; e-mail: [email protected] Curr Opin Ophthalmol 2014, 25:98–103 DOI:10.1097/ICU.0000000000000026 Volume 25 Number 2 March 2014
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Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy
KEY POINTS Imaging of the ASOCT can be performed with time domain and FDOCT devices. Imaging with 1310 nm ASOCT allows visualization of the entire anterior chamber in a single scan and circumferential angle imaging is possible with SSOCT at this wavelength. FDOCT devices at 830–870 nm are primarily used for retinal imaging, but can also provide high-resolution images of angle structures such as the Schwalbe’s line and the Schlemm’s canal; the visibility of these structures is highly variable.
be performed in real time [2] when time domain retinal OCT still required 1–2 s per scan. Numerous TD-ASOCT-based studies have now expanded our knowledge base about angle closure, which causes a disproportionately high visual morbidity when compared with primary open angle glaucoma (POAG). Although a shallow central anterior chamber has been considered the most important factor associated with angle closure, most persons with a shallow anterior chamber do not develop angle closure disease as represented by primary angle closure [PAC, including acute PAC (APAC)], and PAC glaucoma (PACG). The importance of other risk factors in the development of angle closure disease is also highlighted by the finding that the distribution of anterior chamber depth in Chinese eyes is not different from other ethnic groups [3], even though the prevalence of angle closure is higher in the Chinese population. Several new risk factors for angle closure have recently been identified and these have been discussed in detail in a recent Current Opinion article [4]. In brief, the new factors can be classified as anatomical and dynamic. The former include anterior chamber parameters (anterior chamber width, area and volume), iris parameters (area, thickness and curvature) and the lens vault, which represents the portion of the lens anterior to the plane of the scleral spurs (Fig. 1). Dynamic factors that have been associated with angle closure include changes in iris volume with mydriasis, and choroidal expansion. In a series of studies comparing ASOCT parameters in Chinese and Caucasian patients, Wang et al. [5–7] demonstrated that Chinese eyes had smaller anterior chamber area/volume and showed greater iris thickening with light to dark adaptation. Quantitative ASOCT parameters in the different subtypes of angle closure have also been compared. Two clinic-based populations from Iran [8] and Singapore [9] showed that eyes with APAC had the
Lens vault
FIGURE 1. Anterior segment OCT image of an eye with PACG obtained using the Visante anterior segment OCT device (Carl Zeiss Meditec, Dublin California, USA). The lens vault (LV) is shown; this parameter is defined as the perpendicular distance between the anterior pole of the lens and the horizontal line connecting the two scleral spurs. PACG, primary angle closure glaucoma.
smallest anterior segment dimensions, narrowest angles, thickest iris and largest lens vault when compared with PACS, PAC and PACG eyes. In addition to providing valuable data on the pathogenesis of angle closure, initial OCT studies have also highlighted several issues that impact its use. Firstly, when using a qualitative definition of angle closure, ASOCT detects more closed angles than gonioscopy [10,11], but the long-term implications of this finding are not yet known. The disparity is partly because of differing definitions of angle closure and partly because gonioscopy may fail to identify iridotrabecular contact (ITC) due to illumination-induced widening of the angle. Angle closure detected by ASOCT cannot be solely used to recommend laser iridotomy since the majority of patients with even gonioscopically defined angle closure do not develop APAC. Secondly, the combined sensitivity and specificity profile of individual quantitative ASOCT parameters is not sufficiently high to recommend population-based screening for angle closure [12,13]. Thirdly, quantitative ASOCT measurements rely on identification of the scleral spur, which is not visible in 25–30% of images [14,15]. Higher sampling density and image averaging can improve scleral spur visibility, but 100% visualization may be limited by inherent tissue reflectivity characteristics that we do not fully understand; scleral spur visibility in narrow angles may also be affected by the juxtaposition of highly reflective structures with resulting loss of anatomical detail. Finally, TD-ASOCT only provides one or two cross-sectional images at a time, and 3608 information must either be extrapolated or obtained by a time-consuming and impractical method of scanning meridian by meridian. More recent research has focused on ways to overcome some of these limitations. To improve the
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diagnostic performance of ASOCT in the detection of angle closure, Foo et al. [16] incorporated multiple biometric variables into a mathematical model to predict angle width as defined by the quantitative measurements TISA 750 (trabecular iris space area, a trapezoidal area bounded by the AOD 750, the anterior iris surface, the inner corneo-scleral wall and the perpendicular distance between the scleral spur and the opposing iris) and AOD 750 (angle opening distance at 750 mm, the perpendicular distance between a point 750 mm anterior to the scleral spur and the opposing iris). Data for the predictive model was derived from 1067 Singapore Chinese individuals recruited from a population-based study and the model was validated in a separate sample of 1293 Chinese individuals recruited from a community clinic. The anterior chamber volume (ACV), anterior chamber area (ACA) and lens vault were reported to be the three most important predictors of angle width, with ACV being the major contributor. A predictive model consisting of six ASOCT parameters (anterior chamber width, ACA, ACV, lens vault, iris thickness at 750 mm from scleral spur, and iris area) explained more than 80% of variability in angle width. In a subsequent study, Nongpiur et al. [17 ] reported that a stepwise logistic regression-based classification algorithm which used a combination of these six ASOCT parameters identified patients with gonioscopic angle closure more than 95% of the time. Of note, all six factors could be obtained from a single horizontal scan. These results are encouraging for the use of a simple ASOCT image-based screening tool for angle closure. &
Anterior segment imaging with swept source OCT at 1310 nm The development of swept source ASOCT has enabled three-dimensional (3D) imaging of the anterior chamber angle. Swept source OCT (SSOCT) is a variant of Fourier domain OCT (FDOCT) that uses a wavelength scanning light source with a center wavelength of 1310 nm, the scanning speed is 30 000 A scans per second and the axial resolution is 10 mm. The sampling density with SSOCT is much higher than TD-ASOCT; the commercially available CASIA SS-1000 (Tomey Corporation, Nagoya, Japan) has scan protocols with the number of A scans per image ranging from 256 to 2048. In contrast, scan protocols in the commercially available TD-ASOCT (Visante, Carl Zeiss Meditec, Dublin, California, USA) have an A scan sampling range of 128–512. The CASIA SSOCT device allows rapid circumferential imaging of the anterior chamber angle analogous to gonioscopy; in the ‘3D angle analysis’ protocol, 128 radial scans are acquired in 2.4 s. 100
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The eyelids have to be held open by an operator for the duration of the scan. High reproducibility for quantitative angle width measures with this device has been reported [18]. Scleral spur visibility with SSOCT is, in general, higher than with TD-ASOCT and varies with scan protocol and imaged quadrant. With the 3D angle analysis protocol which performs 32 A scans/mm (512 A scans over a 16 mm scan length), the scleral spur visibility in open angles was reported as 90% by Liu et al. [18] and 50–95% by McKee et al. [19 ]. In the latter study, scleral spur visibility was lowest in the inferior quadrant at 50% and highest in the nasal quadrant at 95%. In the same study, patients were also imaged with a higherdensity raster scan which performs 64 A scans/mm (512 A scans over an 8 mm scan length) and the scleral spur visibility was higher, ranging from 95% in the inferior quadrant to 100% in the temporal and superior quadrants. Tun et al. [20] also reported excellent scleral spur visibility of 99.7% with the high-density scan protocol in a group of patients which included 23% with angle closure. For longitudinal analysis, identifying the exact location of the scleral spur is perhaps not as important as reproducibly identifying the same landmark over time so that changes can be measured. Cumba et al. [21] described the use of a scleral spur landmark (SSL) in their study of 31 eyes (39% with open angles and 61% with Shaffer grade 0–1 angles) imaged with the CASIA SS-1000 using the two-dimensional (2D) angle analysis mode. This scan protocol has an even higher sampling density than the 3D mode at 128 A scans/mm (2048 A scans over a scan length of 16 mm). By using predetermined criteria, the SSL could be identified in all eyes and the intraobserver and interobserver reproducibility of angle width measurements was excellent. The agreement between observers in placing the SSL was similar in the open and narrow angle groups. The visibility of other angle landmarks apart from the scleral spur has also been reported with SSOCT. In the study by Tun et al. [20], Schwalbe’s line could be visualized in 99.7% of eyes. The authors also calculated trabecular meshwork (TM) width as the distance between scleral spur and Schwalbe’s line, and reported that the mean width varied from 710 to 890 mm in the different quadrants of the eye with the inferior quadrant having the widest TM. In the study by McKee et al. [19 ], Schwalbe’s line was visible in 68–98% of highdensity scans, but only in 0–10% of low-density images (Fig. 2). The visibility of Schlemm’s canal was low in both scan protocols – 12–42% with highdensity scans and 0% in the low-density scans. The inbuilt software in the CASIA SS-1000 device allows quantification of the extent of angle &
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Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy
in this study and on average, it took approximately 15–20 min to analyze each eye. Baskaran et al. [23 ] used the same device to report the diagnostic performance of a novel parameter, the ITC index, when compared with gonioscopy. The ITC index is the ratio of positive ITC (angle closure) in degrees to the total angle with visible scleral spur and end point in degrees; this essentially represents the extent of angle closure in percentage. The scan protocol included 16 meridians and 8% of eyes had to be excluded due to inability to identify the scleral spur. The ITC index had good diagnostic performance for angle closure when compared with gonioscopy as the reference standard area under the curve (AUC) between 0.83 and 0.91 for various definitions of gonioscopic angle closure). Analysis of iris volume with SSOCT has the advantage of providing a more complete representation of iris data when compared with TD-ASOCT. Quigley et al. [24] first proposed that a smaller change in iris cross-sectional area after dilation is a potential risk factor for angle closure. Aptel and Denis [25], and Aptel et al. [26] demonstrated that calculated iris volume increased after physiological and pharmacological mydriasis in fellow eyes of APAC, whereas all POAG and most PAC eyes had a decrease. In contrast, Narayanaswamy et al. [27] reported that fellow eyes of APAC showed no change in iris volume with mydriasis, whereas eyes with PAC and PACG showed a decrease. These studies utilized relatively few TD-ASOCT images (2–4 scans) to calculate dynamic iris changes. Mak et al. [28] reported iris volume changes with SSOCT in a group of individuals including normal, POAG and angle closure (PAC or PACS) groups. The scan protocol included 64 B scans (128 meridians) and iris volume was calculated as a summation of pixel volume derived from individual B scans. The mean iris volume decreased after pupil dilation in all three groups, and unlike previous TD-ASOCT-based studies, fellow eyes of APAC also showed a decrease in iris volume in 75% (six of eight eyes) after physiological mydriasis and 100% (eight of eight eyes) after pharmacological dilation. Of note, eyes with angle closure had a smaller ACV, and the degree of iris volume reduction was also less in eyes with a smaller ACV. This finding suggests that the change in iris volume would have a greater impact on angle width in eyes with smaller ACV which might explain the increased predisposition of these eyes in developing angle closure. &
(a)
SS
SL
(b)
SS SS
FIGURE 2. (a) A high-density scan image obtained with swept source optical coherence tomography showing the scleral spur (SS), Schwalbe’s line (SL), and Schlemm’s canal (SC). (b) A low-density scan image on the same eye reveals only the SS. Reproduced with permission [19 ]. &
closure, but requires a time-consuming manual identification of the scleral spur and the ITC end point in each meridian. Lai et al. [22 ] reported visualization (Fig. 3) and reproducible measurements of the area and degree of PAS with this device, and there was good agreement between gonioscopy and OCT. The scan protocol include 128 meridians &
FIGURE 3. Three-dimensional reconstruction of OCT images depicting the morphology of peripheral anterior synechiae. Images were obtained with a swept source OCT, the CASIA SS-1000 (Tomey Corporation, Nagoya, Japan). Reproduced with permission [22 ]. &
Anterior segment imaging with Fourier domain OCT at 830–870 nm Although FDOCT systems designed for retinal imaging (830–870 nm wavelength) are similar to time
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Schlemm’s canal Schwalbe’s line
allows analysis of the inter-relationship between various anterior segment structures and measurement of global parameters such as anterior chamber volume. In addition, FDOCT images are not corrected for distortion due to refraction and measurements are not interchangeable with other devices. Lastly, unlike dedicated anterior segment imaging devices such as the Visante and the CASIA SS-1000, the FDOCT devices are primarily designed for retinal imaging and require either additional hardware or modifications to the internal optical system in order to image the anterior segment.
CONCLUSION
FIGURE 4. Fourier domain OCT image obtained with the Cirrus (Carl Zeiss Meditec, Dublin, California, USA) shows Schlemm’s canal and Schwalbe’s line which is the termination of the Descemet’s membrane. The device utilizes an 830 nm light source.
domain 830 nm OCT in that penetration through scattering tissue is relatively less, the much higher resolution of FDOCT allows visualization of more detail in the angle region (Fig. 4). The Schwalbe’s line can be visualized with FDOCT, but not with TD-ASOCT [29]. The visibility of Schwalbe’s line ranges from 44 to 100% and that of scleral spur, from 27 to 100%, with various commercially available FDOCT devices that are primarily used for retinal imaging [29–33]. Quantitative angle measurements using Schwalbe’s line as the landmark have shown good reproducibility and good correlation with gonioscopy [31,32]. Visibility of the Schlemm’s canal with retinal wavelength FDOCT devices varies from 10 to 87% [33–35] and one study reported decreased Schlemm’s canal area in eyes with POAG when compared with normal eyes [35]. Kagemann et al. [36] used a combination of FDOCT images and Doppler signals to image Schlemm’s canal and collector channels in healthy eyes. The ability to assess aqueous outflow pathways can have significant clinical impact, especially in the field of minimally invasive surgeries, as placement of devices could be targeted based on the status of the outflow system. Whereas more anatomical detail in the angle region can be seen with retinal FDOCT devices, it must be kept in mind that they do not provide the scan length and depth combination to enable imaging of the entire anterior chamber as is possible with OCT at 1310 nm wavelength. Currently, only the latter 102
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OCT imaging of the anterior segment is possible with several devices, using time domain and Fourier domain technology, and with light sources of different wavelengths. Whereas imaging at 1310 nm has the advantages of visualizing the entire anterior chamber in one scan, as well as circumferential 3D imaging analogous to gonioscopy, imaging at the shorter wavelength typical of retinal devices provides more anatomical detail. Further development of imaging with a 1050 nm light source could simultaneously provide the advantages of the currently available devices [37]. Recent ASOCT studies have improved our understanding of angle closure, in particular, and it is hoped that in the future, we will not only be able to accurately identify patients with angle closure but also identify predictors for the development of angle closure disease so that laser iridotomy can be targeted to those patients. Acknowledgements None. Conflicts of interest Conflict of interest and Financial Disclosure: Sunita Radhakrishnan – Consultant, Netra Systems Inc. S.R. and D.Y. – The Glaucoma Research and Education Group receives funding from the Glaucoma Research Foundation.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112:1584–1589. 2. Radhakrishnan S, Rollins AM, Roth JE, et al. Real-time optical coherence tomography of the anterior segment at 1310 nm. Arch Ophthalmol 2001; 119:1179–1185. 3. Congdon NG, Youlin Q, Quigley H, et al. Biometry and primary angle-closure glaucoma among Chinese, white, and black populations. Ophthalmology 1997; 104:1489–1495.
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Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy 4. Nongpiur ME, Ku JY, Aung T. Angle closure glaucoma: a mechanistic review. Curr Opin Ophthalmol 2011; 22:96–101. 5. Wang D, Chiu C, He M, et al. Differences in baseline dark and the dark-to-light changes in anterior chamber angle parameters in whites and ethnic Chinese. Invest Ophthalmol Vis Sci 2011; 52:9404–9410. 6. Wang D, Qi M, He M, et al. Ethnic difference of the anterior chamber area and volume and its association with angle width. Invest Ophthalmol Vis Sci 2012; 53:3139–3144. 7. Wang D, He M, Wu L, et al. Differences in iris structural measurements among American Caucasians, American Chinese and mainland Chinese. Clin Experiment Ophthalmol 2012; 40:162–169. 8. Moghimi S, Vahedian Z, Fakhraie G, et al. Ocular biometry in the subtypes of angle closure: an anterior segment optical coherence tomography study. Am J Ophthalmol 2013; 155:664–673. 9. Guzman CP, Gong T, Nongpiur ME, et al. Anterior segment optical coherence tomography parameters in subtypes of primary angle closure. Invest Ophthalmol Vis Sci 2013; 54:5281–5286. 10. Nolan WP, See JL, Chew PT, et al. Detection of primary angle closure using anterior segment OCT in Asian eyes. Ophthalmology 2007; 114:33– 39. 11. Sakata LM, Lavanya R, Friedman DS, et al. Comparison of gonioscopy and anterior segment ocular coherence tomography in detecting angle closure in different quadrants of the anterior chamber angle. Ophthalmology 2008; 115:769–774. 12. Lavanya R, Foster PJ, Sakata LM, et al. Screening for narrow angles in the Singapore population: evaluation of new noncontact screening methods. Ophthalmology 2008; 115:1720–1727. 13. Pekmezci M, Porco TC, Lin SC. Anterior segment optical coherence tomography as a screening tool for the assessment of the anterior segment angle. Ophthalmic Surg Lasers Imaging 2009; 40:389–398. 14. Sakata LM, Lavanya R, Friedman DS, et al. Assessment of the scleral spur in anterior segment optical coherence tomography images. Arch Ophthalmol 2008; 126:181–185. 15. Narayanaswamy A, Sakata LM, He MG, et al. Diagnostic performance of anterior chamber angle measurements for detecting eyes with narrow angles: an anterior segment OCT study. Arch Ophthalmol 2010; 128:1321– 1327. 16. Foo LL, Nongpiur ME, Allen JC, et al. Determinants of angle width in Chinese Singaporeans. Ophthalmology 2012; 119:278–282. 17. Nongpiur ME, Haaland BA, Friedman DS, et al. Classification algorithms & based on anterior segment optical coherence tomography measurements for detection of angle closure. Ophthalmology 2013; 120:48–54. In this study, six classification algorithms based on ASOCT measurements were evaluated for detection of gonioscopic angle closure. An algorithm based on stepwise logistic regression that used a combination of six parameters obtained from a single horizontal ASOCT scan identified patients with gonioscopic angle closure for more than 95% of the time. The algorithm was validated in an independent second sample. These results are promising for a simple imagingbased screening test for angle closure. 18. Liu S, Yu M, Ye C, et al. Anterior chamber angle imaging with swept-source optical coherence tomography: an investigation on variability of angle measurement. Invest Ophthalmol Vis Sci 2011; 52:8598–8603. 19. McKee H, Ye C, Yu M, et al. Anterior chamber angle imaging with swept& source optical coherence tomography: detecting the scleral spur, Schwalbe’s line, and Schlemm’s canal. J Glaucoma 2013; 22:468–472. In this study, a SSOCT device was used to image normal and POAG eyes using two scan protocols with differing A scan sampling densities. The visibility of scleral spur, Schwalbe’s line and Schlemm’s canal was reduced in the low-density scan protocol, especially in the inferior quadrant. The results highlight variations in visibility of angle structures with scan type and imaged quadrant. 20. Tun TA, Baskaran M, Zheng C, et al. Assessment of trabecular meshwork width using swept source optical coherence tomography. Graefes Arch Clin Exp Ophthalmol 2013; 251:1587–1592.
21. Cumba RJ, Radhakrishnan S, Bell NP, et al. Reproducibility of scleral spur identification and angle measurements using Fourier domain anterior segment optical coherence tomography. J Ophthalmol 2012. [Epub 1 Nov 2012; Article ID 487309] 22. Lai I, Mak H, Lai G, et al. Anterior chamber angle imaging with swept-source & optical coherence tomography: measuring peripheral anterior synechia in glaucoma. Ophthalmology 2013; 120:1144–1149. Swept source OCT was used for circumferential imaging of the angle and enabled visualization and reproducible measurement of PAS. This imaging technology allows angle assessment analogous to gonioscopy and has implications in imagebased evaluation of progression of angle closure disease. 23. Baskaran M, Ho SW, Tun TA, et al. Assessment of circumferential angle& closure by the iris-trabecular contact index with swept-source optical coherence tomography. Ophthalmology 2013; 120:2226–2231. In this study, swept source anterior segment OCT was used to assess circumferential angle closure and the diagnostic performance of a new parameter, the ITC index. The index was calculated as a percentage of the angle that was closed on SSOCT images and it showed good diagnostic performance for angle closure. Three dimensional imaging with SSOCT provides more complete angle data than time domain ASOCT. 24. Quigley HA, Silver DM, Friedman DS, et al. Iris cross-sectional area decreases with pupil dilation and its dynamic behavior is a risk factor in angle closure. J Glaucoma 2009; 18:173–179. 25. Aptel F, Denis P. Optical coherence tomography quantitative analysis of iris volume changes after pharmacological mydriasis. Ophthalmology 2010; 117:3–10. 26. Aptel F, Chiquet C, Beccat S, Denis P. Biometric evaluation of anterior chamber changes after physiologic pupil dilation using Pentacam and anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci 2012; 53:4005–4010. 27. Narayanaswamy A, Zheng C, Perera SA, et al. Variations in iris volume with physiologic mydriasis in subtypes of primary angle closure glaucoma. Invest Ophthalmol Vis Sci 2013; 54:708–713. 28. Mak H, Xu G, Leung CK. Imaging the iris with swept-source optical coherence tomography: relationship between iris volume and primary angle closure. Ophthalmology 2013; 120:2517–2524. 29. Perera SA, HoCL. Aung T, et al. Imaging of the iridocorneal angle with the RTVue spectral domain OCT. Invest Ophthalmol Vis Sci 2012; 53:1710–1713. 30. Wong HT, Lim MC, Sakata LM, et al. High-definition optical coherence tomography imaging of the iridocorneal angle of the eye. Arch Ophthalmol 2009; 127:256–260. 31. Cheung CY, Zheng C, Ho CL, et al. Novel anterior-chamber angle measurements by high-definition optical coherence tomography using the Schwalbe line as the landmark. Br J Ophthalmol 2011; 95:955–959. 32. Qin B, Francis BA, Li Y, et al. Anterior chamber angle measurements using Schwalbe’s line with high-resolution Fourier-domain optical coherence tomography. J Glaucoma 2012; 53:5131–5136. 33. Quek DT, Narayanaswamy AK, Tun TA, et al. Comparison of two spectral domain optical tomography devices for angle-closure assessment. Invest Ophthalmol Vis Sci 2012; 53:1710–1713; 5131–5136. 34. Day AC, Garway-Heath DF, Broadway DC, et al. Spectral domain optical coherence tomography imaging of the aqueous outflow structures in normal participants of the EPIC-Norfolk Eye Study. Br J Ophthalmol 2013; 97:189– 195. 35. Hong J, Xu J, Wei A, et al. Spectral-domain optical coherence tomographic assessment of Schlemm’s canal in Chinese subjects with primary open-angle glaucoma. Ophthalmology 2013; 120:709–715. 36. Kagemann L, Wollstein G, Ishikawa H, et al. Identification and assessment of Schlemm’s canal by spectral-domain optical coherence tomography. Invest Ophthalmol Vis Sci 2010; 51:4054–4059. 37. Potsaid B, Baumann B, Huang D, et al. Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100 000 to 400 000 axial scans per second. Opt Express 2010; 18:20029–20048.
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Development in anterior segment imaging for glaucoma Sunita Radhakrishnan a,b and Dmitry Yarovoy b
Purpose of review Recent developments in anterior segment imaging enable high-resolution imaging with higher speeds than previous systems. This review will focus on advances in optical coherence tomography (OCT) imaging of the anterior segment with an emphasis on angle evaluation. Recent findings Anterior segment imaging with OCT is possible with both time domain and Fourier domain devices, and with light sources of different wavelengths. Recent studies have identified new risk factors for angle closure and prediction models incorporating quantitative angle width parameters have been developed for detection of gonioscopically defined angle closure. Swept source OCT is a variation of Fourier domain OCT that allows three-dimensional imaging of the angle analogous to gonioscopy and novel quantitative parameters have been described for angle measurement with this technology. Fourier domain OCT devices have higher resolution and sampling density than time domain OCT and enable visualization of more angle structures such as the Schwalbe’s line and Schlemm’s canal. Summary OCT technology is expanding our understanding of angle closure disease. Although it does not replace gonioscopy in angle evaluation, it may help us identify predictors of progression along the angle closure disease spectrum such that treatment may be better targeted. Keywords angle closure, anterior segment OCT, swept source OCT, time domain anterior segment OCT
INTRODUCTION Cross-sectional imaging of the anterior segment is a useful adjunct to gonioscopy. Ultrasound biomicroscopy was the first anterior segment imaging technology to be developed and allows detailed visualization of all angle structures including the ciliary body; however, it is cumbersome to perform on a routine basis. In contrast, anterior segment optical coherence tomography (ASOCT) is a noncontact technology that is easy to use and allows visualization of the entire anterior chamber in one scan such that the inter-relationship between various ocular structures may be evaluated. Although ASOCT has been used for imaging of trabeculectomy blebs, aqueous drainage devices and newer surgical devices, its main application in glaucoma has been to evaluate the anterior chamber angle. In recent years, there have been significant advances in OCT technology, and this article will focus on developments in OCT imaging of the anterior segment with an emphasis on angle closure disease. www.co-ophthalmology.com
Time domain anterior segment OCT imaging at 1310 nm Anterior segment OCT imaging was first reported with the same wavelength of light used for retinal imaging (830 nm) [1]. The subsequent use of a longer wavelength light at 1310 nm allowed better visualization of the angle region due to increased penetration of scattering tissue like the sclera. The absorption characteristics of 1310 nm light also enabled the use of higher power with resultant increase in scanning speed such that time domain anterior segment OCT (TD-ASOCT) imaging could
a Glaucoma Center of San Francisco and bGlaucoma Research and Education Group, San Francisco, California, USA
Correspondence to Sunita Radhakrishnan, MD, Glaucoma Center of San Francisco, 55 Stevenson Street, San Francisco, CA 94105, USA. Tel: +1 415 981 2020; e-mail: [email protected] Curr Opin Ophthalmol 2014, 25:98–103 DOI:10.1097/ICU.0000000000000026 Volume 25 Number 2 March 2014
Copyright © Lippincott Williams & Wilkins. Unauthorized reproduction of this article is prohibited.
Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy
KEY POINTS Imaging of the ASOCT can be performed with time domain and FDOCT devices. Imaging with 1310 nm ASOCT allows visualization of the entire anterior chamber in a single scan and circumferential angle imaging is possible with SSOCT at this wavelength. FDOCT devices at 830–870 nm are primarily used for retinal imaging, but can also provide high-resolution images of angle structures such as the Schwalbe’s line and the Schlemm’s canal; the visibility of these structures is highly variable.
be performed in real time [2] when time domain retinal OCT still required 1–2 s per scan. Numerous TD-ASOCT-based studies have now expanded our knowledge base about angle closure, which causes a disproportionately high visual morbidity when compared with primary open angle glaucoma (POAG). Although a shallow central anterior chamber has been considered the most important factor associated with angle closure, most persons with a shallow anterior chamber do not develop angle closure disease as represented by primary angle closure [PAC, including acute PAC (APAC)], and PAC glaucoma (PACG). The importance of other risk factors in the development of angle closure disease is also highlighted by the finding that the distribution of anterior chamber depth in Chinese eyes is not different from other ethnic groups [3], even though the prevalence of angle closure is higher in the Chinese population. Several new risk factors for angle closure have recently been identified and these have been discussed in detail in a recent Current Opinion article [4]. In brief, the new factors can be classified as anatomical and dynamic. The former include anterior chamber parameters (anterior chamber width, area and volume), iris parameters (area, thickness and curvature) and the lens vault, which represents the portion of the lens anterior to the plane of the scleral spurs (Fig. 1). Dynamic factors that have been associated with angle closure include changes in iris volume with mydriasis, and choroidal expansion. In a series of studies comparing ASOCT parameters in Chinese and Caucasian patients, Wang et al. [5–7] demonstrated that Chinese eyes had smaller anterior chamber area/volume and showed greater iris thickening with light to dark adaptation. Quantitative ASOCT parameters in the different subtypes of angle closure have also been compared. Two clinic-based populations from Iran [8] and Singapore [9] showed that eyes with APAC had the
Lens vault
FIGURE 1. Anterior segment OCT image of an eye with PACG obtained using the Visante anterior segment OCT device (Carl Zeiss Meditec, Dublin California, USA). The lens vault (LV) is shown; this parameter is defined as the perpendicular distance between the anterior pole of the lens and the horizontal line connecting the two scleral spurs. PACG, primary angle closure glaucoma.
smallest anterior segment dimensions, narrowest angles, thickest iris and largest lens vault when compared with PACS, PAC and PACG eyes. In addition to providing valuable data on the pathogenesis of angle closure, initial OCT studies have also highlighted several issues that impact its use. Firstly, when using a qualitative definition of angle closure, ASOCT detects more closed angles than gonioscopy [10,11], but the long-term implications of this finding are not yet known. The disparity is partly because of differing definitions of angle closure and partly because gonioscopy may fail to identify iridotrabecular contact (ITC) due to illumination-induced widening of the angle. Angle closure detected by ASOCT cannot be solely used to recommend laser iridotomy since the majority of patients with even gonioscopically defined angle closure do not develop APAC. Secondly, the combined sensitivity and specificity profile of individual quantitative ASOCT parameters is not sufficiently high to recommend population-based screening for angle closure [12,13]. Thirdly, quantitative ASOCT measurements rely on identification of the scleral spur, which is not visible in 25–30% of images [14,15]. Higher sampling density and image averaging can improve scleral spur visibility, but 100% visualization may be limited by inherent tissue reflectivity characteristics that we do not fully understand; scleral spur visibility in narrow angles may also be affected by the juxtaposition of highly reflective structures with resulting loss of anatomical detail. Finally, TD-ASOCT only provides one or two cross-sectional images at a time, and 3608 information must either be extrapolated or obtained by a time-consuming and impractical method of scanning meridian by meridian. More recent research has focused on ways to overcome some of these limitations. To improve the
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diagnostic performance of ASOCT in the detection of angle closure, Foo et al. [16] incorporated multiple biometric variables into a mathematical model to predict angle width as defined by the quantitative measurements TISA 750 (trabecular iris space area, a trapezoidal area bounded by the AOD 750, the anterior iris surface, the inner corneo-scleral wall and the perpendicular distance between the scleral spur and the opposing iris) and AOD 750 (angle opening distance at 750 mm, the perpendicular distance between a point 750 mm anterior to the scleral spur and the opposing iris). Data for the predictive model was derived from 1067 Singapore Chinese individuals recruited from a population-based study and the model was validated in a separate sample of 1293 Chinese individuals recruited from a community clinic. The anterior chamber volume (ACV), anterior chamber area (ACA) and lens vault were reported to be the three most important predictors of angle width, with ACV being the major contributor. A predictive model consisting of six ASOCT parameters (anterior chamber width, ACA, ACV, lens vault, iris thickness at 750 mm from scleral spur, and iris area) explained more than 80% of variability in angle width. In a subsequent study, Nongpiur et al. [17 ] reported that a stepwise logistic regression-based classification algorithm which used a combination of these six ASOCT parameters identified patients with gonioscopic angle closure more than 95% of the time. Of note, all six factors could be obtained from a single horizontal scan. These results are encouraging for the use of a simple ASOCT image-based screening tool for angle closure. &
Anterior segment imaging with swept source OCT at 1310 nm The development of swept source ASOCT has enabled three-dimensional (3D) imaging of the anterior chamber angle. Swept source OCT (SSOCT) is a variant of Fourier domain OCT (FDOCT) that uses a wavelength scanning light source with a center wavelength of 1310 nm, the scanning speed is 30 000 A scans per second and the axial resolution is 10 mm. The sampling density with SSOCT is much higher than TD-ASOCT; the commercially available CASIA SS-1000 (Tomey Corporation, Nagoya, Japan) has scan protocols with the number of A scans per image ranging from 256 to 2048. In contrast, scan protocols in the commercially available TD-ASOCT (Visante, Carl Zeiss Meditec, Dublin, California, USA) have an A scan sampling range of 128–512. The CASIA SSOCT device allows rapid circumferential imaging of the anterior chamber angle analogous to gonioscopy; in the ‘3D angle analysis’ protocol, 128 radial scans are acquired in 2.4 s. 100
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The eyelids have to be held open by an operator for the duration of the scan. High reproducibility for quantitative angle width measures with this device has been reported [18]. Scleral spur visibility with SSOCT is, in general, higher than with TD-ASOCT and varies with scan protocol and imaged quadrant. With the 3D angle analysis protocol which performs 32 A scans/mm (512 A scans over a 16 mm scan length), the scleral spur visibility in open angles was reported as 90% by Liu et al. [18] and 50–95% by McKee et al. [19 ]. In the latter study, scleral spur visibility was lowest in the inferior quadrant at 50% and highest in the nasal quadrant at 95%. In the same study, patients were also imaged with a higherdensity raster scan which performs 64 A scans/mm (512 A scans over an 8 mm scan length) and the scleral spur visibility was higher, ranging from 95% in the inferior quadrant to 100% in the temporal and superior quadrants. Tun et al. [20] also reported excellent scleral spur visibility of 99.7% with the high-density scan protocol in a group of patients which included 23% with angle closure. For longitudinal analysis, identifying the exact location of the scleral spur is perhaps not as important as reproducibly identifying the same landmark over time so that changes can be measured. Cumba et al. [21] described the use of a scleral spur landmark (SSL) in their study of 31 eyes (39% with open angles and 61% with Shaffer grade 0–1 angles) imaged with the CASIA SS-1000 using the two-dimensional (2D) angle analysis mode. This scan protocol has an even higher sampling density than the 3D mode at 128 A scans/mm (2048 A scans over a scan length of 16 mm). By using predetermined criteria, the SSL could be identified in all eyes and the intraobserver and interobserver reproducibility of angle width measurements was excellent. The agreement between observers in placing the SSL was similar in the open and narrow angle groups. The visibility of other angle landmarks apart from the scleral spur has also been reported with SSOCT. In the study by Tun et al. [20], Schwalbe’s line could be visualized in 99.7% of eyes. The authors also calculated trabecular meshwork (TM) width as the distance between scleral spur and Schwalbe’s line, and reported that the mean width varied from 710 to 890 mm in the different quadrants of the eye with the inferior quadrant having the widest TM. In the study by McKee et al. [19 ], Schwalbe’s line was visible in 68–98% of highdensity scans, but only in 0–10% of low-density images (Fig. 2). The visibility of Schlemm’s canal was low in both scan protocols – 12–42% with highdensity scans and 0% in the low-density scans. The inbuilt software in the CASIA SS-1000 device allows quantification of the extent of angle &
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Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy
in this study and on average, it took approximately 15–20 min to analyze each eye. Baskaran et al. [23 ] used the same device to report the diagnostic performance of a novel parameter, the ITC index, when compared with gonioscopy. The ITC index is the ratio of positive ITC (angle closure) in degrees to the total angle with visible scleral spur and end point in degrees; this essentially represents the extent of angle closure in percentage. The scan protocol included 16 meridians and 8% of eyes had to be excluded due to inability to identify the scleral spur. The ITC index had good diagnostic performance for angle closure when compared with gonioscopy as the reference standard area under the curve (AUC) between 0.83 and 0.91 for various definitions of gonioscopic angle closure). Analysis of iris volume with SSOCT has the advantage of providing a more complete representation of iris data when compared with TD-ASOCT. Quigley et al. [24] first proposed that a smaller change in iris cross-sectional area after dilation is a potential risk factor for angle closure. Aptel and Denis [25], and Aptel et al. [26] demonstrated that calculated iris volume increased after physiological and pharmacological mydriasis in fellow eyes of APAC, whereas all POAG and most PAC eyes had a decrease. In contrast, Narayanaswamy et al. [27] reported that fellow eyes of APAC showed no change in iris volume with mydriasis, whereas eyes with PAC and PACG showed a decrease. These studies utilized relatively few TD-ASOCT images (2–4 scans) to calculate dynamic iris changes. Mak et al. [28] reported iris volume changes with SSOCT in a group of individuals including normal, POAG and angle closure (PAC or PACS) groups. The scan protocol included 64 B scans (128 meridians) and iris volume was calculated as a summation of pixel volume derived from individual B scans. The mean iris volume decreased after pupil dilation in all three groups, and unlike previous TD-ASOCT-based studies, fellow eyes of APAC also showed a decrease in iris volume in 75% (six of eight eyes) after physiological mydriasis and 100% (eight of eight eyes) after pharmacological dilation. Of note, eyes with angle closure had a smaller ACV, and the degree of iris volume reduction was also less in eyes with a smaller ACV. This finding suggests that the change in iris volume would have a greater impact on angle width in eyes with smaller ACV which might explain the increased predisposition of these eyes in developing angle closure. &
(a)
SS
SL
(b)
SS SS
FIGURE 2. (a) A high-density scan image obtained with swept source optical coherence tomography showing the scleral spur (SS), Schwalbe’s line (SL), and Schlemm’s canal (SC). (b) A low-density scan image on the same eye reveals only the SS. Reproduced with permission [19 ]. &
closure, but requires a time-consuming manual identification of the scleral spur and the ITC end point in each meridian. Lai et al. [22 ] reported visualization (Fig. 3) and reproducible measurements of the area and degree of PAS with this device, and there was good agreement between gonioscopy and OCT. The scan protocol include 128 meridians &
FIGURE 3. Three-dimensional reconstruction of OCT images depicting the morphology of peripheral anterior synechiae. Images were obtained with a swept source OCT, the CASIA SS-1000 (Tomey Corporation, Nagoya, Japan). Reproduced with permission [22 ]. &
Anterior segment imaging with Fourier domain OCT at 830–870 nm Although FDOCT systems designed for retinal imaging (830–870 nm wavelength) are similar to time
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Schlemm’s canal Schwalbe’s line
allows analysis of the inter-relationship between various anterior segment structures and measurement of global parameters such as anterior chamber volume. In addition, FDOCT images are not corrected for distortion due to refraction and measurements are not interchangeable with other devices. Lastly, unlike dedicated anterior segment imaging devices such as the Visante and the CASIA SS-1000, the FDOCT devices are primarily designed for retinal imaging and require either additional hardware or modifications to the internal optical system in order to image the anterior segment.
CONCLUSION
FIGURE 4. Fourier domain OCT image obtained with the Cirrus (Carl Zeiss Meditec, Dublin, California, USA) shows Schlemm’s canal and Schwalbe’s line which is the termination of the Descemet’s membrane. The device utilizes an 830 nm light source.
domain 830 nm OCT in that penetration through scattering tissue is relatively less, the much higher resolution of FDOCT allows visualization of more detail in the angle region (Fig. 4). The Schwalbe’s line can be visualized with FDOCT, but not with TD-ASOCT [29]. The visibility of Schwalbe’s line ranges from 44 to 100% and that of scleral spur, from 27 to 100%, with various commercially available FDOCT devices that are primarily used for retinal imaging [29–33]. Quantitative angle measurements using Schwalbe’s line as the landmark have shown good reproducibility and good correlation with gonioscopy [31,32]. Visibility of the Schlemm’s canal with retinal wavelength FDOCT devices varies from 10 to 87% [33–35] and one study reported decreased Schlemm’s canal area in eyes with POAG when compared with normal eyes [35]. Kagemann et al. [36] used a combination of FDOCT images and Doppler signals to image Schlemm’s canal and collector channels in healthy eyes. The ability to assess aqueous outflow pathways can have significant clinical impact, especially in the field of minimally invasive surgeries, as placement of devices could be targeted based on the status of the outflow system. Whereas more anatomical detail in the angle region can be seen with retinal FDOCT devices, it must be kept in mind that they do not provide the scan length and depth combination to enable imaging of the entire anterior chamber as is possible with OCT at 1310 nm wavelength. Currently, only the latter 102
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OCT imaging of the anterior segment is possible with several devices, using time domain and Fourier domain technology, and with light sources of different wavelengths. Whereas imaging at 1310 nm has the advantages of visualizing the entire anterior chamber in one scan, as well as circumferential 3D imaging analogous to gonioscopy, imaging at the shorter wavelength typical of retinal devices provides more anatomical detail. Further development of imaging with a 1050 nm light source could simultaneously provide the advantages of the currently available devices [37]. Recent ASOCT studies have improved our understanding of angle closure, in particular, and it is hoped that in the future, we will not only be able to accurately identify patients with angle closure but also identify predictors for the development of angle closure disease so that laser iridotomy can be targeted to those patients. Acknowledgements None. Conflicts of interest Conflict of interest and Financial Disclosure: Sunita Radhakrishnan – Consultant, Netra Systems Inc. S.R. and D.Y. – The Glaucoma Research and Education Group receives funding from the Glaucoma Research Foundation.
REFERENCES AND RECOMMENDED READING Papers of particular interest, published within the annual period of review, have been highlighted as: & of special interest && of outstanding interest 1. Izatt JA, Hee MR, Swanson EA, et al. Micrometer-scale resolution imaging of the anterior eye in vivo with optical coherence tomography. Arch Ophthalmol 1994; 112:1584–1589. 2. Radhakrishnan S, Rollins AM, Roth JE, et al. Real-time optical coherence tomography of the anterior segment at 1310 nm. Arch Ophthalmol 2001; 119:1179–1185. 3. Congdon NG, Youlin Q, Quigley H, et al. Biometry and primary angle-closure glaucoma among Chinese, white, and black populations. Ophthalmology 1997; 104:1489–1495.
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Anterior segment imaging for glaucoma Radhakrishnan and Yarovoy 4. Nongpiur ME, Ku JY, Aung T. Angle closure glaucoma: a mechanistic review. Curr Opin Ophthalmol 2011; 22:96–101. 5. Wang D, Chiu C, He M, et al. Differences in baseline dark and the dark-to-light changes in anterior chamber angle parameters in whites and ethnic Chinese. Invest Ophthalmol Vis Sci 2011; 52:9404–9410. 6. Wang D, Qi M, He M, et al. Ethnic difference of the anterior chamber area and volume and its association with angle width. Invest Ophthalmol Vis Sci 2012; 53:3139–3144. 7. Wang D, He M, Wu L, et al. Differences in iris structural measurements among American Caucasians, American Chinese and mainland Chinese. Clin Experiment Ophthalmol 2012; 40:162–169. 8. Moghimi S, Vahedian Z, Fakhraie G, et al. Ocular biometry in the subtypes of angle closure: an anterior segment optical coherence tomography study. Am J Ophthalmol 2013; 155:664–673. 9. Guzman CP, Gong T, Nongpiur ME, et al. Anterior segment optical coherence tomography parameters in subtypes of primary angle closure. Invest Ophthalmol Vis Sci 2013; 54:5281–5286. 10. Nolan WP, See JL, Chew PT, et al. Detection of primary angle closure using anterior segment OCT in Asian eyes. Ophthalmology 2007; 114:33– 39. 11. Sakata LM, Lavanya R, Friedman DS, et al. Comparison of gonioscopy and anterior segment ocular coherence tomography in detecting angle closure in different quadrants of the anterior chamber angle. Ophthalmology 2008; 115:769–774. 12. Lavanya R, Foster PJ, Sakata LM, et al. Screening for narrow angles in the Singapore population: evaluation of new noncontact screening methods. Ophthalmology 2008; 115:1720–1727. 13. Pekmezci M, Porco TC, Lin SC. Anterior segment optical coherence tomography as a screening tool for the assessment of the anterior segment angle. Ophthalmic Surg Lasers Imaging 2009; 40:389–398. 14. Sakata LM, Lavanya R, Friedman DS, et al. Assessment of the scleral spur in anterior segment optical coherence tomography images. Arch Ophthalmol 2008; 126:181–185. 15. Narayanaswamy A, Sakata LM, He MG, et al. Diagnostic performance of anterior chamber angle measurements for detecting eyes with narrow angles: an anterior segment OCT study. Arch Ophthalmol 2010; 128:1321– 1327. 16. Foo LL, Nongpiur ME, Allen JC, et al. Determinants of angle width in Chinese Singaporeans. Ophthalmology 2012; 119:278–282. 17. Nongpiur ME, Haaland BA, Friedman DS, et al. Classification algorithms & based on anterior segment optical coherence tomography measurements for detection of angle closure. Ophthalmology 2013; 120:48–54. In this study, six classification algorithms based on ASOCT measurements were evaluated for detection of gonioscopic angle closure. An algorithm based on stepwise logistic regression that used a combination of six parameters obtained from a single horizontal ASOCT scan identified patients with gonioscopic angle closure for more than 95% of the time. The algorithm was validated in an independent second sample. These results are promising for a simple imagingbased screening test for angle closure. 18. Liu S, Yu M, Ye C, et al. Anterior chamber angle imaging with swept-source optical coherence tomography: an investigation on variability of angle measurement. Invest Ophthalmol Vis Sci 2011; 52:8598–8603. 19. McKee H, Ye C, Yu M, et al. Anterior chamber angle imaging with swept& source optical coherence tomography: detecting the scleral spur, Schwalbe’s line, and Schlemm’s canal. J Glaucoma 2013; 22:468–472. In this study, a SSOCT device was used to image normal and POAG eyes using two scan protocols with differing A scan sampling densities. The visibility of scleral spur, Schwalbe’s line and Schlemm’s canal was reduced in the low-density scan protocol, especially in the inferior quadrant. The results highlight variations in visibility of angle structures with scan type and imaged quadrant. 20. Tun TA, Baskaran M, Zheng C, et al. Assessment of trabecular meshwork width using swept source optical coherence tomography. Graefes Arch Clin Exp Ophthalmol 2013; 251:1587–1592.
21. Cumba RJ, Radhakrishnan S, Bell NP, et al. Reproducibility of scleral spur identification and angle measurements using Fourier domain anterior segment optical coherence tomography. J Ophthalmol 2012. [Epub 1 Nov 2012; Article ID 487309] 22. Lai I, Mak H, Lai G, et al. Anterior chamber angle imaging with swept-source & optical coherence tomography: measuring peripheral anterior synechia in glaucoma. Ophthalmology 2013; 120:1144–1149. Swept source OCT was used for circumferential imaging of the angle and enabled visualization and reproducible measurement of PAS. This imaging technology allows angle assessment analogous to gonioscopy and has implications in imagebased evaluation of progression of angle closure disease. 23. Baskaran M, Ho SW, Tun TA, et al. Assessment of circumferential angle& closure by the iris-trabecular contact index with swept-source optical coherence tomography. Ophthalmology 2013; 120:2226–2231. In this study, swept source anterior segment OCT was used to assess circumferential angle closure and the diagnostic performance of a new parameter, the ITC index. The index was calculated as a percentage of the angle that was closed on SSOCT images and it showed good diagnostic performance for angle closure. Three dimensional imaging with SSOCT provides more complete angle data than time domain ASOCT. 24. Quigley HA, Silver DM, Friedman DS, et al. Iris cross-sectional area decreases with pupil dilation and its dynamic behavior is a risk factor in angle closure. J Glaucoma 2009; 18:173–179. 25. Aptel F, Denis P. Optical coherence tomography quantitative analysis of iris volume changes after pharmacological mydriasis. Ophthalmology 2010; 117:3–10. 26. Aptel F, Chiquet C, Beccat S, Denis P. Biometric evaluation of anterior chamber changes after physiologic pupil dilation using Pentacam and anterior segment optical coherence tomography. Invest Ophthalmol Vis Sci 2012; 53:4005–4010. 27. Narayanaswamy A, Zheng C, Perera SA, et al. Variations in iris volume with physiologic mydriasis in subtypes of primary angle closure glaucoma. Invest Ophthalmol Vis Sci 2013; 54:708–713. 28. Mak H, Xu G, Leung CK. Imaging the iris with swept-source optical coherence tomography: relationship between iris volume and primary angle closure. Ophthalmology 2013; 120:2517–2524. 29. Perera SA, HoCL. Aung T, et al. Imaging of the iridocorneal angle with the RTVue spectral domain OCT. Invest Ophthalmol Vis Sci 2012; 53:1710–1713. 30. Wong HT, Lim MC, Sakata LM, et al. High-definition optical coherence tomography imaging of the iridocorneal angle of the eye. Arch Ophthalmol 2009; 127:256–260. 31. Cheung CY, Zheng C, Ho CL, et al. Novel anterior-chamber angle measurements by high-definition optical coherence tomography using the Schwalbe line as the landmark. Br J Ophthalmol 2011; 95:955–959. 32. Qin B, Francis BA, Li Y, et al. Anterior chamber angle measurements using Schwalbe’s line with high-resolution Fourier-domain optical coherence tomography. J Glaucoma 2012; 53:5131–5136. 33. Quek DT, Narayanaswamy AK, Tun TA, et al. Comparison of two spectral domain optical tomography devices for angle-closure assessment. Invest Ophthalmol Vis Sci 2012; 53:1710–1713; 5131–5136. 34. Day AC, Garway-Heath DF, Broadway DC, et al. Spectral domain optical coherence tomography imaging of the aqueous outflow structures in normal participants of the EPIC-Norfolk Eye Study. Br J Ophthalmol 2013; 97:189– 195. 35. Hong J, Xu J, Wei A, et al. Spectral-domain optical coherence tomographic assessment of Schlemm’s canal in Chinese subjects with primary open-angle glaucoma. Ophthalmology 2013; 120:709–715. 36. Kagemann L, Wollstein G, Ishikawa H, et al. Identification and assessment of Schlemm’s canal by spectral-domain optical coherence tomography. Invest Ophthalmol Vis Sci 2010; 51:4054–4059. 37. Potsaid B, Baumann B, Huang D, et al. Ultrahigh speed 1050 nm swept source/Fourier domain OCT retinal and anterior segment imaging at 100 000 to 400 000 axial scans per second. Opt Express 2010; 18:20029–20048.
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