Experimental evaluation of some possible nonlinearity indicators

Due to regulatory reasons, the output levels of ultrasound sources for use in medical applications must be known. Currently, this is done by making focal pressure measurements for a very large number of drive voltage amplitudes and then linearly derating the measured pressure levels. In order to reduce calibration times, some have proposed to linearly extrapolate pressures rather than perform direct measurements. However, nonlinear propagation effects corrupt the linear extrapolation and derating procedure. Thus, a reliable indicator of nonlinearity is needed to identify when linear extrapolation/derating would be valid. In this study, eight different nonlinearity indicators were evaluated experimentally in terms of their correspondence to the linear extrapolation error. Spherically focused ultrasound transducers were selected and excited to test the indicators sensitivity to frequency (3 - 8 MHz), f/# (1 and 2), transducer diameter (1.905 and 5.08 cm), pulse duration (1 and 3 cycles), and pulse phase (0° and 180°). None of the eight nonlinearity indicators yielded consistent results. The lack of consistency resulted from the competing effects of nonlinear absorption and asymmetric distortion, which have yet to be combined into a unified theory.