Finite difference modelling of the temperature rise in non-linear medical ultrasound fields

Non-linear propagation of ultrasound can lead to increased heat generation in medical diagnostic imaging due to the preferential absorption of harmoni cs of the original frequency. A numerical model has been developed and test ed that is capable of predicting the temperature rise due to a high amplitu de ultrasound field. The acoustic held is modelled using a numerical soluti on to the Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation, known as the Berg en Code, which is implemented in cylindrical symmetric form. A finite diffe rence representation of the thermal equations is used to calculate the resu lting temperature rises. The model allows for the inclusion of a number of layers of tissue with different acoustic and thermal properties and account s for the effects of non-linear propagation, direct heating by the transduc er, thermal diffusion and perfusion in different tissues; The effect of tem perature-dependent skin perfusion and variation in background temperature b etween the skin and deeper layers of the -body are included. The model has been tested against analytic solutions for simple configurations and then u sed to estimate temperature rises in realistic obstetric situations. A puls ed 3 MHz transducer operating with an average acoustic power of 200 mW lead s to a maximum steady state temperature rise inside the foetus of 1.25 degr ees C compared with a 0.6 degrees C rise for the same transmitted power und er linear propagation conditions. The largest temperature rise occurs at th e skin surface, with the temperature rise at the foetus limited to less tha n 2 degrees C for the range of conditions considered. (C) 2000 Elsevier Sci ence B.V. All rights reserved.