Ultrasound refers to sound waves with frequencies above the audible range. Ultrasonography uses echoes, much like light-based optical coherence tomography uses reflections, to image and differentiate tissues. During ultrasonography, electrical energy is converted into sound waves by means of a piezoelectric crystal. The resultant waves are emitted by the ultrasound probe, which is placed as close as possible to the tissue being studied. When the sound waves encounter tissues, their speed changes depending on the density of the surface/ interface, and some of the waves bounce back to the probe; on the basis of their amplitude, frequency, and travel time, these echoes are then converted into a signal. For example, during ultrasonography, a sound wave traversing the cornea encounters the aqueous of the anterior chamber and then the lens–iris diaphragm. As the tissue density changes at the posterior cornea and then again at the lens–iris diaphragm, signals are generated. The distance between the 2 signal spikes is then used to determine the anterior chamber depth.
The frequency of the ultrasound determines the depth of penetration and the resolution. These 2 variables are inversely related. High-frequency ultrasound, which provides greater detail, is used to evaluate smaller objects such as the eye. Low-frequency ultrasound provides less resolution but can penetrate deeper. For example, it is useful in obstetrics to traverse through the abdominal wall and uterus to image a fetus.
Ophthalmic ultrasonography utilizes high-frequency sound waves (8–80 MHz) for safe, effective, noninvasive imaging of the anterior and posterior segments of the eye and orbit using equipment routinely found in most practices. Indications for ophthalmic ultrasonography include biometry and evaluation of the following structures and conditions:
Three main ultrasound devices are used to evaluate the eye: the A-scan probe, the B-scan probe, and the ultrasound biomicroscopy (UBM) probe (Fig 17-3).
Figure 17-3 Ophthalmic ultrasound probes. A, A-scan probe. B, B-scan probe. C, Ultrasound biomicroscopy (UBM) probe.
(Courtesy of Vikram S. Brar, MD.)
Table 17-4 A-Scan: Quantification of Reflectivity
Biometry of the eye with an A-scan probe (eg, for measuring axial length) uses frequencies between 8 and 12 MHz. After a topical anesthetic agent is applied, the probe makes direct contact with the cornea, or it can be applied via immersion. The latter eliminates the possibility of altering the measurement with compression of the cornea.
When operating at 8 MHz, the A-scan probe can also enable demonstration of intralesional characteristics within the eye and orbit, known as internal reflectivity. Reflectivity within a lesion may be low, medium, or high depending on the relative percentage of the internal spike compared with that of the initial spike of the lesion (Table 17-4). The reflectivity within a tissue is inversely proportional to its homogeneity. Less-organized tissue, as in a vascular lesion (ie, a choroidal hemangioma), will demonstrate high internal reflectivity compared with homogenous tissue (ie, a choroidal melanoma).
Excerpted from BCSC 2020-2021 series: Section 2 - Fundamentals and Principles of Ophthalmology. For more information and to purchase the entire series, please visit https://www.aao.org/bcsc.