**2. Ultrasound biomicroscopy (UBM) (Fig. 1)**

UBM, which originally was used in ophthalmology to image the posterior segment (B-scan ultrasonography), is an objective alternative for anterior chamber angle assessment. Although ultrasound and UBM are based on the same principle, the frequencies are different. Objective and reproducible measurements of the anterior chamber structures can be obtained with crosssectional imaging by UBM. Electric signals are converted by a radiofrequency signal generator coupled to a piezoelectric transducer into 50 MHz frequency ultrasonic sound waves, which are transmitted to the eye via saline solution that is held in a cup reservoir [11]. The examination may be performed through a viscous material such as sodium hyaloronate. UBM generates high-resolution images of the angle, which can be used in quantitative analysis, and it adds useful information regarding mechanisms of angle closure [11]. Although angle dimensions measured by UBM correlated significantly with gonioscopy in general [12], gonioscopic assessment sometimes resulted in an overestimation of the angle width in eyes with occludable angles [13]. Gonioscopy is the gold standard examination, because it allows direct viewing of the angle. Nevertheless, it may induce changes in the apposition of the iris depending on the technique and the lens.

gonioscopy. However, it is limited by its dependency on subjective interpretation and difficulties in manipulation techniques. Ultrasound biomicroscopy (UBM) generates highresolution images of the angle, which can be used in quantitative analysis, and it adds useful information regarding causal mechanisms of angle closure. However, this method also requires trained and experienced technicians and is time consuming. Both gonioscopy and UBM require contact with the globe, and as a result, they can be unpleasant for the patient and

New devices for evaluating the anterior ocular segment in a more objective and quantita‐ tive manner have been introduced. Anterior-segment optical coherence tomography (AS-OCT) is a noninvasive technique allowing the measurement of the anterior ocular structures. A new generation of OCT, swept-source OCT (SS-OCT), has been recently in‐ troduced for the measurement of the anterior ocular segment. The SS-OCT is over ten‐ fold faster than the time-domain OCT and gives a three-dimensional (3D) observation of the anterior ocular segment. The SS-OCT employs 1,310 nmin the nearinfrared light

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. The SPAC measures the peripheral ACD and converts the measure‐ ments into numerical and categorical grades by comparison with a normative database. The

In the study reported here, we review the advantages and limitations of newer anterior chamber imaging technologies, namely ultrasound biomicroscopy (UBM), anterior segment optical coherence tomography (AS-OCT), and scanning peripheral anterior chamber depth analyzer (SPAC). Additionally, the present study assessed the effectiveness and possibility of

UBM, which originally was used in ophthalmology to image the posterior segment (B-scan ultrasonography), is an objective alternative for anterior chamber angle assessment. Although ultrasound and UBM are based on the same principle, the frequencies are different. Objective and reproducible measurements of the anterior chamber structures can be obtained with crosssectional imaging by UBM. Electric signals are converted by a radiofrequency signal generator coupled to a piezoelectric transducer into 50 MHz frequency ultrasonic sound waves, which are transmitted to the eye via saline solution that is held in a cup reservoir [11]. The examination may be performed through a viscous material such as sodium hyaloronate. UBM generates high-resolution images of the angle, which can be used in quantitative analysis, and it adds useful information regarding mechanisms of angle closure [11]. Although angle dimensions measured by UBM correlated significantly with gonioscopy in general [12], gonioscopic assessment sometimes resulted in an overestimation of the angle width in eyes with occludable angles [13]. Gonioscopy is the gold standard examination, because it allows direct viewing of

SPAC has been proposed as a clinician-independent screening tool for angle closure.

can induce artifacts.

252 Glaucoma - Basic and Clinical Aspects

source and its scan rate is 30,000 A scan/s.

the SPAC in the glaucoma screening.

**2. Ultrasound biomicroscopy (UBM) (Fig. 1)**

The UBM measurement requires trained and experienced technicians and is time consuming. In addition, UBM require contact with the globe, and as a result, UBM can induce artifacts by inadvertent compression of the globe. Consequently, UBM is not suitable for glaucoma screening examination.

**Figure 1. (a)** UBM image of the normal anterior segment. This scan demonstrates all anterior segment structures, in‐ cluding anterior lens surface, iris, and ciliary body. In UBM, frequencies of 35-50 MHz and above provide over a three‐ fold improvement in resolution compared with conventional ophthalmic ultrasound systems **(b)**. **b.** Conventional Bmode ultrasound image of the posterior segment. **c and d.** UBM image of the normal **(c)** and the PAC anterior segment **(d)**. Note the shallow anterior chamber depth of the PAC compared with the normal. **e and f.** UBM image of the anterior segment of the PACG patient before **(e)** and after laser iridotomy **(f)**. Note the increase of anterior cham‐ ber depth after laser iridotomy (LI). Arrow indicates the portion of the LI. **g and h.** UBM image of the anterior segment of the PACG patient before **(g)** and after cataract surgery (phacoemulsification and intraocular lens implantation) **(h)**. Note the increase of anterior chamber depth after cataract surgery.
