Comparison of modelled and observed in vivo temperature elevations inducedby focused ultrasound: implications for treatment planning

Two numerical models for predicting the temperature elevations resulting fr om focused ultrasound heating of muscle tissue were tested against experime ntal data. Both models use the Rayleigh-Sommerfeld integral to calculate th e pressure field from a source distribution. The first method assumes a sou rce distribution derived from a uniformly radiating transducer whereas the second uses a source distribution obtained by numerically projecting pressu re field measurements from an area near the focus backward toward the trans ducer surface. Both of these calculated ultrasound fields were used as heat sources in the bioheat equation to calculate the temperature elevation in vivo. Experimental results were obtained from in vivo rabbit experiments us ing eight-element sector-vortex transducers at 1.61 and 1.7 MHz and noninva sive temperature mapping with MRI. Results showed that the uniformly radiat ing transducer model over-predicted the peak temperature by a factor rangin g from 1.4 to 2.8, depending on the operating mode. Simulations run using t he back-projected sources were much closer to experimental values, ranging from 1.0 to 1.7 times the experimental results, again varying with mode. Th us, a significant improvement in the treatment planning can be obtained by using actual measured ultrasound field distributions in combination with ba ckward projection.