Abstract:

Medical ultrasound scanners use high-energy pulses to probe the human body. It can be shown that the radiation force resulting from the impact of such pulses to the object can vibrate the object, producing a localized high-intensity noise in the audible range. When the scanner is used in a maternal examination, and the beam is directed toward the fetal head, this noise can disturb the fetus and cause it to move vigorously [J. Ultrasound Med. 20, 883--889 (2001)]. Here, a theoretical model for the audio noise generated by ultrasound scanners is presented. This model describes the temporal and spectral characteristics of the noise. It has been shown that the noise has rich frequency components at the pulse repetition frequency and its harmonics. Experiments have been conducted in a water tank to measure the noise generated by a clinical ultrasound scanner. Results indicate that the noise pressure level at the focal point of the ultrasound beam on a reflective object can reach up to 120 dB relative to 20(mu)P. This result is in agreement with the theoretical predictions. The noise was also measured in a human uterus, indicating intensities in the range of 90--100 dB.