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 diff erent rates of speed. The point at which adjacent tissues with differe nt speeds of sound meet is referred to as an acoustic interface. When sound hits an acoustic interface, an echo is created. Medical ultrasou nd is essentially a means of producing visual images based on echoes t hat occur at such acoustic interfaces. Crystals within the ultrasound transducer are capable of generating and receiving sound waves based o n 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 br east has been greatly improved by computer-enhancement capabilities an d the availability of high-frequency transducers. Although-the detaile d imaging of modern ultrasound allows for satisfactory evaluation of m ost breast lesions, there are a variety of artifacts inherent to breas t ultrasound of which one must be cognizant, so as to avoid misinterpr etation. This article will address a number of these issues, thereby p resenting an introduction to the basic physics and principles relevant to breast ultrasound.