**3. Anterior-segment optical coherence tomography (AS-OCT)**

AS-OCT is a non-contact imaging device allowing the visualization and measurement of the anterior ocular structures [11]. The Visante AS-OCT (Carl Zeiss Meditec Inc., Dublin, CA, USA) and the slit-lamp OCT (SL-OCT) (Heildelberg Engineering, Heildelberg, Germany) are the commercially available AS-OCT devices [11]. Compared with the OCT, the SL-OCT has a lower axial and transverse resolution of <25 µm and 20–100 µm, respectively. A major difference between the two devices is their scan speed, which is 2000 A-scans per s for Visante OCT, and 200 A-scans per s for SL-OCT. With a line scan of 256 and 215 A-scans, each image frame takes 0.13 and 1.08s for Visante OCT and SL-OCT, respectively [11]. Furthermore, the SL-OCT requires manual rotation of the scanning beam.

segment. Instead of using a spectrometer as in spectral-domain OCT, swept-source OCT uses a monochromatic tunable fast scanning laser source and a photodetector to detect wavelengthresolved interference signal [17]. The iris profiles and the angle configurations can be visual‐ ized three dimensionally and evaluated for 360° [16]. There might be apposition of the peripheral iris to the cornea that would be identified as a closed angle. SS-OCT imaging of the anterior segment could be useful to improve detection of angle closure, while the high cost of

Screening for Narrow Angles in the Japanese Population Using Scanning Peripheral Anterior Chamber Depth Analyzer

http://dx.doi.org/10.5772/54556

255

(a) (b)

**Figure 2. a and b.** Transectional images of normal anterior segment **(a)** and plateau iris configuration **(b)** obtained using Visante AS-OCT (Carl Zeiss Meditec Inc., Dublin, CA, USA). Note the shallowperipheral anterior chamber depth of the plateau iris configuration compared with the normal. **c.** Transectional image of the conjunctival bleb after trabecu‐

The scanning peripheral anterior chamber depth analyzer (SPAC) is a non-invasive device that objectively and quantitatively assesses the anterior ocular segment by employing the Scheimp‐ flug camera principle [18]. The light from the slit lamp is in the visible spectrum and is projected from the temporal side at an angle of 60° from the optical axis. A camera records cross sectional slit images from the anterior cornea to the anterior iris, and does not rotate as Pentacam-Scheimpflug. The SPAC measures the peripheral ACD and converts the measurements into numerical and categorical grades by comparison with a normative database. SPAC quantita‐ tively measures ACD in a noncontact fashion from the optical axis to the limbus in approxi‐ mately 0.66 second and takes 21 consecutive slit-lamp images at 0.4 mm intervals. SPAC measurements ranged from 1 to 12, with 1 representing the shallowest anterior chamber. SPAC is equipped with an autofocusing system and a program for the detection of eyes with narrow angle, and usually completes measurement within 15 seconds for a pair of eyes by pressing

these devices may be a limiting factor for their use in screening examination.

(c)

**4. Scanning peripheral anterior chamber depth analyzer**

lectomy using Visante AS-OCT.

The advantages of the AS-OCT devices are non-contact, easy operation and a rapid im‐ age acquisition. The incorporation of automated analysis software allows for rapid esti‐ mation of the various anterior segment parameters, including corneal thickness, anterior chamber depth, etc.

Precise location of the scleral spur is a pre-requisite for reliable measurement of the angle. Limited by a relatively low-image resolution, the scleral spur may not always be visible even with the anterior segment OCT. Currently available software analysis programs require the manual localization of the scleral spur, which can at times be difficult, especially in closed angles or where there is a smooth transition from cornea to sclera [14]. Sakata et al. found that the sclera spur could not be detected in approximately 30% of the quadrants, this problem being worse in the superior and inferior quadrants [14].

It has been reported that AS-OCT is highly sensitive in detecting angle closure when compared with gonioscopy. Using gonioscopy as a reference standard results in AS-OCT having a sensitivity of 98.0% [15]. Several explanations have been suggested for the disparate findings between gonioscopy and AS-OCT [11]. The structures of the angle cannot be directly viewed by other techniques than gonioscopy (and may be SS-OCT in future), and therefore, cannot be identified. However, inadvertent pressure on the globe during gonioscopy may alter the configuration of the angle, leading to artificial widening of the angle. Another reason could be a difference in the definition angle closure. On gonioscopy, angle closure was defined as the apposition between the iris and the posterior trabecular meshwork, whereas on the AS-OCT, it was defined as any contact between the iris and the angle structures anterior to the sclera spur in 2-dimensional cross sections obtained by AS-OCT.

When this device is applied to the prospective observational case series, sensitivity and specificity are calculated as 98% (92.2%–99.6%) and 55.4% (45.2%–65.2%) [15]. The low specificity found with AS-OCT may limit the usefulness of these devices in screening for narrow angle.

A new generation of OCT [CASIA, Tomey, Nagoya, Japan], based on swept-source technology (SS-OCT) methods, has been recently developed for the assessment of the anterior ocular segment [16]. The SS-OCT is a variation of the Fourier-domain OCT, over tenfold faster than the time-domain OCT, and gives a three-dimensional (3D) image of the anterior ocular segment. Instead of using a spectrometer as in spectral-domain OCT, swept-source OCT uses a monochromatic tunable fast scanning laser source and a photodetector to detect wavelengthresolved interference signal [17]. The iris profiles and the angle configurations can be visual‐ ized three dimensionally and evaluated for 360° [16]. There might be apposition of the peripheral iris to the cornea that would be identified as a closed angle. SS-OCT imaging of the anterior segment could be useful to improve detection of angle closure, while the high cost of these devices may be a limiting factor for their use in screening examination.

**3. Anterior-segment optical coherence tomography (AS-OCT)**

requires manual rotation of the scanning beam.

being worse in the superior and inferior quadrants [14].

spur in 2-dimensional cross sections obtained by AS-OCT.

chamber depth, etc.

254 Glaucoma - Basic and Clinical Aspects

narrow angle.

AS-OCT is a non-contact imaging device allowing the visualization and measurement of the anterior ocular structures [11]. The Visante AS-OCT (Carl Zeiss Meditec Inc., Dublin, CA, USA) and the slit-lamp OCT (SL-OCT) (Heildelberg Engineering, Heildelberg, Germany) are the commercially available AS-OCT devices [11]. Compared with the OCT, the SL-OCT has a lower axial and transverse resolution of <25 µm and 20–100 µm, respectively. A major difference between the two devices is their scan speed, which is 2000 A-scans per s for Visante OCT, and 200 A-scans per s for SL-OCT. With a line scan of 256 and 215 A-scans, each image frame takes 0.13 and 1.08s for Visante OCT and SL-OCT, respectively [11]. Furthermore, the SL-OCT

The advantages of the AS-OCT devices are non-contact, easy operation and a rapid im‐ age acquisition. The incorporation of automated analysis software allows for rapid esti‐ mation of the various anterior segment parameters, including corneal thickness, anterior

Precise location of the scleral spur is a pre-requisite for reliable measurement of the angle. Limited by a relatively low-image resolution, the scleral spur may not always be visible even with the anterior segment OCT. Currently available software analysis programs require the manual localization of the scleral spur, which can at times be difficult, especially in closed angles or where there is a smooth transition from cornea to sclera [14]. Sakata et al. found that the sclera spur could not be detected in approximately 30% of the quadrants, this problem

It has been reported that AS-OCT is highly sensitive in detecting angle closure when compared with gonioscopy. Using gonioscopy as a reference standard results in AS-OCT having a sensitivity of 98.0% [15]. Several explanations have been suggested for the disparate findings between gonioscopy and AS-OCT [11]. The structures of the angle cannot be directly viewed by other techniques than gonioscopy (and may be SS-OCT in future), and therefore, cannot be identified. However, inadvertent pressure on the globe during gonioscopy may alter the configuration of the angle, leading to artificial widening of the angle. Another reason could be a difference in the definition angle closure. On gonioscopy, angle closure was defined as the apposition between the iris and the posterior trabecular meshwork, whereas on the AS-OCT, it was defined as any contact between the iris and the angle structures anterior to the sclera

When this device is applied to the prospective observational case series, sensitivity and specificity are calculated as 98% (92.2%–99.6%) and 55.4% (45.2%–65.2%) [15]. The low specificity found with AS-OCT may limit the usefulness of these devices in screening for

A new generation of OCT [CASIA, Tomey, Nagoya, Japan], based on swept-source technology (SS-OCT) methods, has been recently developed for the assessment of the anterior ocular segment [16]. The SS-OCT is a variation of the Fourier-domain OCT, over tenfold faster than the time-domain OCT, and gives a three-dimensional (3D) image of the anterior ocular
