Acoustic output parameters

Many different parameters have been defined in order to try and characterize medical ultrasound fields. Due to the complexity of these fields, it is not possible to describe the parameters fully and unambiguously here. Definitions of some terms are given; the more interested reader can find further formal definitions and description in IEC 62127–1 (2007a) and AIUM/NEMA (2004). There are five which seem to be most important to safety: peak negative pressure, pulse average intensity, temporal average intensity, total power and acoustic frequency.

Peak negative acoustic pressure

The peak negative acoustic pressure (also called the peak rarefactional pressure and given the symbol pr) is simply the most negative pressure that occurs during the pulse: in Figure 12.1, this is about 3.5 MPa. This is an important parameter because it relates to the occurrence of cavitation. Negative acoustic pressure means that the acoustic wave is trying to pull the water molecules apart; the water molecules resist this separation but if the pressure is sufficiently negative and lasts for long enough, it is possible to produce a small void, a cavitation bubble. If there are pre-existing gas bubbles or dust particles in the water, cavitation occurs more easily. A similar effect can occur in tissue.