Boundaries to effective acoustic output regulation from acoustic saturation

Abstract

Acoustic saturation limits the pressure amplitude which may be reached at any position in an ultrasonic beam. The saturation pressure, psat, is a function of acoustic frequency, distance from the source, and the acoustic nonlinearity parameter. It is independent of the source pressure. An approximate expression has been used [Naugol’nykh and Romanenko, Sov. Phys. Acoust. 5, 191–195 (1959)] to predict psat at the focus of spherically focused medical diagnostic beams in a lossless medium. psat typically lies in the range 1–10 MPa, lower values being associated with higher frequencies and longer focal depths. Mechanical index and spatial-peak intensities are also subject to saturation effects, being calculated from acoustic pressure. A significant range of conditions exist for which acoustic saturation prevents MI, and derated I(spta) and I(sppa), from exceeding the regulatory limits set by the Food and Drug Administration in the USA. Action limits set by IEC 61157 are sufficiently low to be unaffected in the majority of cases. It is concluded that the existence of acoustic saturation presents a strong argument to review present procedures for regulating acoustic output from diagnostic ultrasound equipment.