Nonlinear propagation in ultrasonic fields: measurements, modelling and harmonic imaging

In high amplitude ultrasonic fields, such as those used in medical ultrasou nd, nonlinear propagation can result in waveform distortion and the generat ion of harmonics of the initial frequency. In the nearfield of a transducer this process is complicated by diffraction effects associated with the sou rce. The results of a programme to study the nonlinear propagation in the f ields of circular, focused and rectangular transducers are described, and c omparisons made with numerical predictions obtained using a finite differen ce solution to the Khokhlov-Zabolotskaya-Kuznetsov (or KZK) equation. These results are extended to consider nonlinear propagation in tissue-like medi a and the implications for ultrasonic measurements and ultrasonic heating a re discussed. The narrower beamwidths and reduced side-lobe levels of the h armonic beams are illustrated and the use of harmonics to form diagnostic i mages with improved resolution is described. (C) 2000 Elsevier Science B.V. All rights reserved.