Principles of Ultrasound

Abstract

The normal range of sound that human beings can perceive is 20–20 000 Hz. A sound wave with a frequency higher than 20000 Hz is called ultrasound. The ultrasound used in abdominal imaging has a frequency of 3.5 or 5 MHz (1 MHz = 10⁶ Hz). Ultrasound of such high frequency is barely transmitted in air but is transmitted well in solid or fluid materials (Fig. 1.1). In the human body, ultrasound is transmitted well in the abdominal organs and soft tissues but is not transmitted in air-containing organs such as the lungs or the gastrointestinal tract. Since bones do not transmit ultrasound, organs surrounded by bones cannot be examined.