The effect of nonlinear propagation on heating of tissue: numerical modelling and experimental measurement

A combination of finite difference and finite element models is used to model the nonlinear propagation of medical ultrasound fields and the resulting temperature rises generated in a tissue phantom. A non-axisymmetric finite difference model is used to solve the KZK equation, and so model the beam of diagnostic scanners in pulsed Doppler mode. The predictions are made for both water and a fluid-tissue path. The initial conditions used in the model were obtained from measurements made close to the scan heads of medical systems. The resulting losses from the beam intensity are then used as the source term for a finite element model of tissue heating. The resulting predictions are compared with experimental measurements of the temperature increases that were produced in a tissue mimicking gel by the diagnostic fields and found to be in excellent agreement. The results show that nonlinear propagation can increase the temperature rise significantly (50%) when there is a fluid path overlying tissue, but that this is not always the case, as saturation effects can become dominant.