maximum amplitude (FWHM) and is given by the equation:

FWHM 5 cf=ðy lÞ 5 l ðf - numberÞ

where c is the speed of sound, f is the focal length of the transducer, u is the frequency of ultrasound, d is the diameter of the focused transducer, l is the wave length, and f-number is the ratio of the focal length to the diameter of the transducer. It is evident from the equation that high resolution can simply be achieved by selecting the appropriate frequency. For example, if one desires 60 mm resolution, an operating frequency of 60 MHz and an f-number of 2 could be chosen. This resolution is approximately ten times that obtained using conventional clinical instrumentation.

The penalty to be paid for this increase in resolution is loss of penetration. All human tissues exhibit ultrasound attenuation coefficients that increase with frequency. If we assume an attenuation coefficient of 0.5 dB/mm MHz, which is consistent with typical soft tissues, the maximum penetration that could be achieved for a 10 MHz system is approximately 50 mm for an 80 dB dynamic range. For a 60 MHz system, penetration is only 5 mm, which reinforces the need to optimize transducer sensitivity and performance in systems development of ultrasound biomicroscopy.

Transducers

The development of high-frequency transducers has been central to the progress achieved in ultrasound biomicroscopy. The piezoelectric polymer polyvinylidene difluoride and co-polymer polyvinylidene difluoride/trifluoroethylene have been used for external imaging applications such as the eye. Such transducers produce short, wide bandwidth pulses with good sensitivity. The highest resolution, defined as FWHM in the previous equation, is only achieved at the focus. The depth of field (DOF), which is defined as the range of depth over which the beam remains well focused, must also be considered. In general, the DOF increases with the square of the f-number. An f-number ranging from 2 to 4 represents a good compromise between DOF and resolution. Typical ultrasound biomicroscopy transducers have an f-number of 2.0 to 3.0. From the previous discussion, it is clear that the choice of transducer parameters will have a significant impact on image quality; therefore, it is necessary to optimize transducer characteristics for each application to obtain the best compromise for resolution, contrast, and DOF.

CLINICAL USE OF ULTRASOUND BIOMICROSCOPY

Technique

The technique of eye examination using ultrasound biomicroscopy is similar to conventional B-scan examination of the anterior segment. A fluid immersion technique is required to provide an adequate standoff from the structures being examined. This standoff is necessary to avoid distortion of the image close to the transducer and to prevent contact of the transducer and the eye. The authors have designed a series of eye cups that hold the eyelids open and allow more rapid patient preparation. These eye cups resemble those used in conventional ultrasound biometry, with a lip that slides under the eyelids and holds the cup in place. They differ from biometry eye cups in being shallower and having a distinct flair, which allows a good view of precisely where the scanning head is being placed. Fig. 1 shows an examination being performed with one of these eye cups. One percent methyl cellulose is an excellent coupling medium with sufficient viscosity to prevent fluid loss during examination. Saline can also be used. Some ultrasound biomicroscope designs have a longer focal length necessitating a deeper cup. Air bubbles must be carefully avoided in the fluid and on the concave surface of the transducer.

Unlike in conventional 10 MHz B-scanning, high-frequency transducers are frequently not covered by a membrane. Because the transducer

Fig. 1. An ultrasound biomicroscopic examination performed using a small eye cup.

is moving, contact with the eye and resulting corneal abrasion must be carefully avoided. The presence of an articulated arm is helpful in improving control of the scanning head. Careful attention must be paid to the screen image to prevent the scanning head from getting too close to the eye. In the authors’ practice, contact with the eye has been an extremely rare occurrence. Various designs have also incorporated a fluid-filled membrane over the transducer with moderate attenuation of the signal.

Any part of the eye that can be approached directly over the surface can be examined. The cornea and anterior segment structures are easily examined in any meridian. The conjunctiva, underlying sclera, and peripheral retina can be examined by rotating the eye as far as possible away from the region being examined. This positioning allows examination past the muscle insertions with a somewhat greater range temporally because of anatomic considerations. Any adnexal structures that can have their surfaces exposed can be examined. Skin penetration is not as good as that through conjunctiva because of attenuation by keratinized epithelium. Examination of structures such as the posterior pole of the eye is not possible at the present time.

Measuring Ocular Structures

Ultrasound biomicroscopy expands the ability to accurately measure ocular structures. The improved measurement accuracy of ultrasound biomicroscopy has an axial resolution five to ten times that of conventional 10 MHz ultrasound. The authors perform measurements on the screen during examination using electronic calipers. Stored images can be transferred to a computer via an image capture board and measured using imaging software. Measuring a structure accurately with ultrasound requires knowledge of the speed of sound in the structure being examined. Little is known of the true speed of sound in the structures that are capable of being measured, particularly at these frequencies. The authors have used a speed of sound of 1540 m/s to make the majority of measurements. This speed is used in conventional ultrasound scanning to measure distances in most tissue. A speed of 1640 m/s is generally used for the cornea and has been used for sclera as well, which is similar in structure.

Conventional ultrasound is capable of measuring relatively large distances such as anterior chamber depth. Ultrasound biomicroscopy increases the measurement accuracy of such structures because the shorter wavelength allows

a finer positioning of end points, and the exact measurement position can be defined more precisely.6,7 Ocular structures such as the ciliary body, sclera, and iris cannot be measured by other techniques because of inadequate resolution and the inability to differentiate these structures from adjacent tissue. Ultrasound biomicroscopy allows clarification of tissue borders and accurate measurement.

Normal Ocular Structures

Anterior chamber

Anterior chamber depth is easily measured using ultrasound biomicroscopy (Fig. 2). Measuring the axial distance from the internal corneal surface to the lens surface is facilitated by the ease of distinguishing the iris from the lens surface. This differentiation can be difficult in eyes with small pupils using conventional ultrasound. Measurement of anterior chamber depth is not confined to the axial position but can be taken from any point on the endothelial surface to either the iris or lens surface, providing an improved ability to define the profile of the entire anterior chamber.

The cornea

All corneal layers can be differentiated in a cross section (Fig. 3). The first highly reflective line is the surface of the corneal epithelium. The epithelium can be differentiated from Bowman’s membrane, which forms a highly reflective line just below this. The distance between these two lines is the epithelial thickness. The corneal stroma reveals a low internal reflectivity that is lower than that found in the more irregular collagen distribution of the sclera. This difference allows definition of the corneoscleral junction.

Fig. 2. Ultrasound biomicroscopic view of anterior chamber. Arrows, anterior chamber depth measurement; I, iris.

Fig. 3. Ultrasound biomicroscopy of the cornea showing corneal layers.

The endothelium cannot be differentiated from Descemet’s membrane, but, together, they form a single highly reflective line at the posterior corneal margin. Greater definition of corneal structure can be achieved with higher frequency transducers.

Anterior chamber angle region

The corneoscleral junction and scleral spur can be distinguished consistently with ultrasound biomicroscopy (Fig. 4). These structures are important in maintaining orientation in the angle region. The scleral spur in a particularly useful landmark presenting a constant reference point for measurement in the angle region.

Fig. 4. Ultrasound biomicroscopy of the angle region. The scleral spur (arrow) is used as a stable landmark. The ciliary processes (cp), zonule (Z), lens (L), and iris

(I) are clearly imaged. The highly reflective sclera (S) is distinguished from the lower reflective cornea (C). The angle is narrow in this patient with plateau iris syndrome.

The iris

The iris normally shows variations in thickness. Histologic studies show that it is generally thinnest at the iris root and thickest near the pupillary margin. In addition, there are variations in thickness depending on the presence of crypts and the state of dilation or constriction of the pupil. The iris epithelium forms a constant highly reflective layer on the posterior iris surface. This highly reflective line defines the posterior iris border and can be useful when one is differentiating intra-iris lesions from lesions behind the iris.

The ciliary body

The ciliary body is well defined by ultrasound biomicroscopy. The ciliary processes can be variable in configuration and length. The appearance of the ciliary body will vary depending on whether one is passing through a process or a valley between processes. During the examination, one can produce either of these views as desired.

The zonule

The anterior zonule and lens surface can be consistently visualized in all eyes (see Fig. 4). The zonule inserts smoothly into the surface of the lens but is occasionally more irregular.

ULTRASOUND BIOMICROSCOPY

IN OCULAR DISEASE

Because ultrasound biomicroscopy is a nonspecific imaging tool, it is suitable for examination of a large range of diseases that fall within the penetration limits of this technique. It is particularly useful in conditions in which structural abnormalities are present, that is, conditions that produce rearrangement of normal anatomy.

Glaucoma

Several types of glaucoma are caused by structural abnormalities of the anterior segment of the globe. This observation is particularly true of angle-closure glaucoma and infantile glaucoma. The ability of ultrasound biomicroscopy to image structural abnormalities on a much finer scale than has previously been possible provides a new quantitative tool for research and clinical assessment of glaucomatous disease. Ultrasound biomicroscopy has been helpful in elucidating the mechanisms of angle-closure glaucoma,8–13 pigmentary glaucoma,14–16 malignant glau- coma,17–20 and many other glaucoma entities. The method is also helpful in defining the mechanisms and results of various types of glaucoma surgery (see the article by Rockwood and colleagues, elsewhere in this issue).21,22

Corneal and Scleral Disease

Ultrasound biomicroscopy can be helpful in patients with opaque corneas before transplantation.23 Anterior segment details such as the depth of the anterior chamber, state of the angle, presence of anterior synechiae, and intraocular lens positioning can be determined preoperatively (see the article by Heur and Jeng, elsewhere in this issue). Intracorneal abnormalities can also be imaged. An arc scanner has recently been developed that uses a transducer path which follows the corneal curvature, allowing imaging of the entire cornea in one sweep.24 This instrumentation has allowed construction of three-dimensional depth maps of corneal thickness, epithelial thickness, and the depth of intracorneal incisions. This instrumentation is helpful in assessing the results and complications of refractive surgery. Ultrasound biomicroscopy is also useful in scleritis,25 allowing differentiation between extrascleral and intrascleral disease and assessment of the degree of scleral thinning (see the article by Ventura and colleagues, elsewhere in this issue).

Intraocular Lens Complications

Ultrasound biomicroscopy can easily assess the position of intraocular lens haptics. This information is useful in assessing malpositioned lenses,26 assessing the source of intraocular bleeding, and determining haptic freedom if removal or repositioning is required (see the article by Heur and Jeng, elsewhere in this issue).

Trauma

Ultrasound biomicroscopy can image cyclodialysis clefts even when the anterior chamber is shallow and the cyclodialysis cleft is not obvious with gonioscopy (see the article by Heur and Jeng, elsewhere in this issue).27 There is always 360 degrees of supraciliary fluid present in these cases. The region of the cleft is usually obvious from the displacement of the iris root from the scleral spur. Other causes of hypotony in which ultrasound biomicroscopy can provide useful information include occult wound leaks and ciliary body membranes. In other trauma problems, ultrasound biomicroscopy can image the state of the anterior chamber under traumatic opacities28 and detect small foreign bodies that are difficult to image using conventional techniques.29

Conjunctival and Adnexal Disease

Ultrasound biomicroscopy can provide valuable information in the differential diagnoses of tumors, judging the depth of conjunctival and limbal

Fig. 5. Iris tumor. Tumor thickness (arrow) can be measured.

lesions and imaging canalicular conditions.30 Conjunctival tumors can be assessed for underlying scleral integrity and intraocular involvement.

Anterior Segment Tumors

Ultrasound biomicroscopy is an important adjunct in the management of anterior segment tumors.31–36 It provides a clear image of even the smallest anterior segment lesions. The ability to measure these lesions accurately adds the dimension of depth to criteria for demonstrating growth. Generally, a radial cross-section image is obtained through the thickest part of the tumor as determined by careful scanning (Figs. 5 and 6). This imaged is stored. The tumor is measured on the B-scan by placing electronic calipers gating the thickest part of the lesion. This method is reproducible and allows accurate serial measurements.

The ability to determine the underlying structure of the tumor allows improved classification and the ability to determine ciliary body involvement.

Fig. 6. Ciliary body tumor thickness can be measured from the inner sclera (arrow).