Diagnostic Ultrasound Safety 2: Physical Property of Diagnostic Ultrasound

Abstract

Ultrasound is a mechanical vibration of its propagating medium and it is a kind of sound but not such ionizing radiation as X-ray. It is inaudible by human ear, due its very high frequency, higher than 20,000 Hz, and usually several 1,000,000 Hz (MHz) in medical diagnosis. There are two kinds of diagnostic ultrasound. It is common to use regularly oscillating very short ultrasound pulses in ultrasonic imaging device, to measure the reflection time of ultrasound pulses from an interface to form image of interface. Since oscillation time is very short and the interval is long, the time averaged intensity of pulse wave ultrasound is as weak as some mW/cm2, while its instantaneous intensity is as large as several 10 W/cm2. Continuous wave, however, used in Doppler fetal heart detector and fetal heart rate monitor has no peak and only continuous and weak vibration, i.e. time average and instantaneous intensities are the same in continuous wave. Since heating effect of ultrasound is measured by time average intensity, pulse wave and continuous wave have similar heating (thermal) effect, while mechanical effect of pulse wave ultrasound is large, and it is small in continuous wave”.