A comparison of theoretical and experimental ultrasound field distributions in canine muscle tissue in vivo

Abstract

Relative ultrasound field distributions were measured using thermal techniques in canine thighs in vivo and in water. The experimental results were compared with distributions obtained from a numerical model based on the one-dimensional integration of the Rayleigh-Sommerfeld diffraction integral. The comparisons showed that the theoretical model is a good approximation to the distributions measured in water, with the agreement decreasing for regions in front of the acoustic focus. The main lobe profiles obtained in the muscle tissue also agreed well with both theoretical results and results measured in water (focussing was not lost). However, these in vivo distributions showed enlargement of the side lobes indicating scattering of the waves. It was also found that the interfaces between muscle groups produced considerable beam distortions as well as increased side lobe levels. Scattering of energy from the main lobe to the side lobes was verified by measurements of the peak intensity and the total acoustic power attenuation coefficients for passage of the beams through the thighs which showed that the former was about 40% higher than the latter. Also, absolute intensity values at the acoustic focus were measured in water using a hydrophone (0.5 mm active diameter) for 11 transducers ranging in frequency from 0.246 to 3.54 MHz. When these absolute values were compared with the model predictions, it was found that the model consistently overestimated the experimental data by a factor of less than 2. That is, the model can also be used to obtain upper bounds for absolute intensity values. Consequences of these results on ultrasound hyperthermia treatments are discussed.