Acoustic saturation and output regulation.

Acoustic saturation pressures are predicted for ultrasonic beams of a range of frequencies and focal depths. Using reasonable approximations, saturation values for mechanical index (MI) and derated spatial-peak, time-average and pulse-average intensities are calculated. These are compared with thresholds set for regulatory purposes by the U. S. Food and Drug Administration (FDA), and by the International Electrotechnical Commission (IEC). It is concluded that there are many conditions for which acoustic saturation in water prevents the values of MI and regulated intensities from exceeding thresholds set by the FDA. These conditions are particularly associated with higher frequencies and deeper focal lengths. The thresholds for action set by IEC 61157 are sufficiently low that similar problems do not arise. It is concluded that present regulations are not fully effective in limiting the output from diagnostic ultrasound equipment, and that some conditions exist that are not subject to output control.