The most prevalent use of ocular ultrasonography is to obtain globe length in order to calculate corrective intraocular lens power requirements. The data collected by the transducer produces a corresponding image. In B-scan, or brightness amplitude scan, sound waves are generated at 10 MHz. In A-scan, or time-amplitude scan, sound waves are generated at 8 MHz and converted into spikes that correspond with tissue interface zones. There are two main types of ultrasound used in ophthalmologic practice currently, A-Scan and B-scan. Shadowing can occur distal to a very dense lesion, resulting in an anechoic region. Sound waves that return to the transducer are called echoes, and ultrasound imaging zones can be hyperechoic, hypoechoic, or anechoic. Some sound is absorbed by tissue as well. When sound waves travel between tissue interfaces with different acoustic impedance, or densities, they can either scatter, reflect, or refract.
For instance, sound waves have higher velocity when traveling through solids than through liquids. Ultrasound waves, like other waves, have predictive behaviors based on properties of the medium they travel through. In contrast, lower frequency waves penetrate more deeply but have worse resolution. Higher frequency waves penetrate less into tissue but have better resolution. Ultrasound b-scan exam being performed on a patient.
