Physics and principles of breast ultrasound

Ultrasound refers to sound of any frequency greater than 20 kilohertz; that is, above the frequency for which humans can normally hear. All sound, including ultrasound, travels through different tissues at different rates of speed. The point at which adjacent tissues with different speeds of sound meet is referred to as an acoustic interface. When sound hits an acoustic interface, an echo is created. Medical ultrasound is essentially a means of producing visual images based on echoes that occur at such acoustic interfaces. Crystals within the ultrasound transducer are capable of generating and receiving sound waves based on the "piezo-electric" effect. By this effect, the mechanical energy of the echo is converted into electrical energy that can be imaged on the ultrasound monitor. The resolution of ultrasound images of the breast has been greatly improved by computer-enhancement capabilities and the availability of high-frequency transducers. Although the detailed imaging of modern ultrasound allows for satisfactory evaluation of most breast lesions, there are a variety of artifacts inherent to breast ultrasound of which one must be cognizant, so as to avoid misinterpretation. This article will address a number of these issues, thereby presenting an introduction to the basic physics and principles relevant to breast ultrasound.