OPTICAL COHERENCE TOMOGRAPHY

Overview
Since its commercial introduction in 1996, the clinical applications of optical coherence tomography (OCT) have expanded continuously. After nearly a decade of exploration of OCT applications for use in the posterior segment, the first dedicated anterior segment OCT (AS-OCT) system debuted in 2005 (Visante OCT; Carl Zeiss Meditec, Inc.). AS-OCT uses a longer wavelength (approximately 1,300 nm) than posterior segment OCT (approximately 840 nm), which permits deeper penetration and can image from the cornea to the iris in one scan.

Preparation
Because AS-OCT is noncontact, the only preparation required for using the device is to clean the chin and forehead rests with alcohol prep pads and to enter patients' information.

Assessment of the Cornea and Anterior Chamber
The basic type of scan is low-resolution sulcus-to-sulcus, which provides clinicians with a comprehensive view of the entire anterior chamber. I recommend performing not only a single scan of the anterior segment (along one horizontal axis) but also dual (two axes) and quad (four axes) scans. High-resolution scans of the central cornea can also be performed, especially for quantitative assessments such as thickness measurements for LASIK flaps.

The OCT's signal is strongest when the scanning beam hits its target perpendicularly. Look for the center of the corneal surface, where the bright white lines appear horizontally together with a vertical white line (Figure 1). If these lines are not visible—even when the overall image is well aligned—adjust the scanner vertically by moving it slightly higher or lower as the scanning plane slices the target in an oblique fashion.

Assessment of the Angle
I recommend the high-resolution scan for optimal visualization of the anterior segment angle. Ask the patient to look sideways so that the nasal or temporal side of the limbus is fully exposed to the scanner. Because it is nearly impossible to install an effective external fixation light on AS-OCT scanners, you must work with patients to maximize the effect. The goal is to visualize the interface between the sclera and ciliary body as horizontally as possible (Figure 2). Because the scleral spur defines one end of the trabecular meshwork, the subjective assessment of the angle's opening totally relies on its location. To maximize the visibility of the scleral spur, ensure that the scanning beam hits the limbus' surface as perpendicularly as possible.

Sometimes, the Visante's image preview screen shows a distorted cornea no matter how you adjust the scanning location and angle (Figure 3). This is because the device's image-visualization engine is optimized for scanning the cornea instead of the angle, but you can still subjectively assess narrow-angle conditions. If angle occludability is the only clinical question, I suggest ignoring the distortion, but I do not recommend making a quantitative assessment of distorted images.

Spectral-Domain OCT
The software of two commercial posterior segment spectral-domain OCT (SD-OCT) devices (RTVue [Optovue Inc.] and Cirrus HD-OCT [Carl Zeiss Meditec, Inc.]) was recently upgraded to include imaging of the anterior segment. These all-in-one OCT systems image both the anterior and posterior segments. Moreover, they bring the benefits of SD-OCT technology to anterior segment imaging, namely faster scanning (26 kHz) and a higher axial resolution (approximately 5 μm). High resolution makes it possible for you to visualize not only the trabecular meshwork but also Schlemm canal (Figure 4). Despite the differences in OCT engines, you can directly apply the scanning techniques described earlier to scanning the anterior segment with SD-OCT. Because the wavelength of SD-OCT's light source is optimized for scanning the posterior segment, however, only the surface of the iris can be visualized (Figure 4).

ULTRASOUND BIOMICROSCOPY

Overview
In 1991, Pavlin et al introduced ultrasound biomicroscopy (UBM), a clinical application of highfrequency ultrasound. UBM uses a higher range of frequency (50-100 MHz) than conventional ultrasound (5-20 MHz), achieving a higher axial resolution (approximately 25 μm). Since its inception, UBM dominated the field of anterior segment imaging until 2005, when the first commercial AS-OCT unit was introduced.

Preparation
Most UBM devices use balloon-like disposable probe covers, which contain distilled water that serves as a coupling medium between a UBM probe and the eye (Figure 5A). Unlike the conventional plastic eyecup system, the probe cover system allows patients to be scanned in a variety of postures—supine, sitting, and even prone. For maximum maneuverability and probe stability, choose the supine position.

Do not completely fill the probe cover with distilled water. The balloon should look a bit saggy so that you can minimize the pressure on the eye when applying the probe to the corneal surface (Figure 5B and 5C).

After instilling an anesthetic drop, place the little finger of the hand holding the probe on the lower eyelid, and place the thumb or middle finger of your other hand on the upper eyelid (Figure 6). With one smooth synchronized rotating motion of both hands, gently place the probe (covered by a water-filled balloon) on the cornea while applying gentle but firm tension on both the upper and lower eyelids. Finally, place the index finger of the hand holding the upper eyelid to the side of the probe above the balloon. This way, the hand that is holding the probe rests rigidly on the patient's cheek, and you can control the probe with a delicate but precise motion with support from the index finger of the other hand. This method also gives you a sense of the probe's location in threedimensional space, which is useful when you are performing UBM in a dark room (dark room provocative test for angle-closure glaucoma).

Probe Handling
The UBM probe is held freely in three-dimensional space, which makes it challenging for you to master 6 degrees of freedom (Figure 7). I always ask beginners to scan a plastic model eye to get a feel of each degree of freedom one by one. Use forward-back and left-right movements for coarse registration of the region of interest (ROI) in the center of the viewing screen. Adjust up-down for the vertical location of the ROI within the viewing screen. Up-down is the key movement to keep the ROI in focus. The focal plane is perpendicular to the scanning beam. Think of this as a horizontal band in the viewing screen. After registering the ROI to the optimal scanning location, use the other 3 degrees of freedom (pitch, yaw, and roll) to fine-tune the angle to be scanned to the ROI.

Scanning for Glaucoma Assessment
It is essential to provide clear images of the anterior chamber angle at any location in a uniform way so that the physician can easily compare one image to another. I recommend placing the scleral side on the left of the screen and the corneal side on the right (Figure 8). Ask the patient to look away from the ROI so that the angle in question is fully exposed to the probe. For example, if you want to scan the temporal angle (9 o'clock for the right eye), ask the patient to look to the nasal side (look to the left when the right eye is scanned). Alternatively, to scan the 1-o'clock region, ask the patient to look in the direction of 7 o'clock.

Scanning Tips
As with OCT, UBM also generates the strongest signal when the scanning beam hits its target perpendicularly. You therefore must carefully choose an incident angle depending on the structure of focus. For narrow angle cases, focus on the angle's structure, especially on the trabecular meshwork. Try imaging the cornea as horizontally as possible so that the scleral spur and Schwalbe line are visible (Figure 8B). On the other hand, for a case in which a tube is implanted behind the iris, image the iris as horizontally as possible so that the tube can be seen in full length within the scanning window (Figure 8C).

Scanning for Other Pathologies
UBM can also be used to assess tumors and cysts. Most cases require size and extent measurements. To demonstrate three-dimensional extent, image a longitudinal (parallel to the limbus) slice through the lesion in addition to a regular transverse (radial) slice. On healthy eyes, a group of ciliary processes can be visualized through the width of the scanning window on successful longitudinal scans (Figure 8D).