Modeling HMI Measurement of HIFU Lesion Formation with Temperature-Dependent Tissue Properties

Monitoring tissue stiffness changes during HIFU lesion formation may be possible by measuring the motion induced by the amplitude-modulation of the HIFU beam. As a lesion forms both the stiffness and the acoustic absorption increase. Nominally the increase in tissue stiffness results in less motion, whilst the increase in acoustic absorption results in more forcing and hence more displacement. To investigate whether these two effects cancel out, a finite element model was developed which allowed the temperature dependence of absorption and shear modulus to affect the simultaneous evolution of the acoustic, thermal, and displacement fields. For the displacement the tissue surface was assumed to be in contact with water and was therefore free to move. Measurements of bovine liver and chicken breast indicated that after lesion formation attenuation increased by 200-500% and stiffness increased by 230% for chicken, and by a factor of 40 for liver. The model results showed that, until the lesion grew well outside of the focal region, the increase in attenuation dominated and the displacement increased by 30% after lesion formation. Experimental measurement of displacement vs depth in excised bovine liver and chicken breast subject to 1.1 MHz HIFU were consistent with the predictions of the model. However, sample-to-sample variation in displacement was greater than the predicted change due to lesion formation, suggesting that changes may be difficult to detect in practice. [Work supported in part by NSF through award EEC-9986821]