Acoustic saturation and output regulation

Acoustic saturation pressures are predicted for ultrasonic beams of a range of frequencies and focal depths, Using reasonable approximations, saturati on values for mechanical index (MI) and derated spatial-peak, time-average and pulse-average intensities are calculated. These are compared with thres holds set for regulatory purposes by the U. S. Food and Drug Administration (FDA), and by the International Electrotechnical Commission (IEC). It is c oncluded that there are many conditions for which acoustic saturation in wa ter prevents the values of MI and regulated intensities from exceeding thre sholds set by the FDA, These conditions are particularly associated with hi gher frequencies and deeper focal lengths. The thresholds for action set by IEC 61157 are sufficiently low that similar problems do not arise. It is c oncluded that present regulations are not fully effective in limiting the o utput from diagnostic ultrasound equipment, and that some conditions exist that are not subject to output control. (C) 1999 World Federation for Ultra sound in Medicine & Biology.