The contribution of shear wave absorption to ultrasound heating in bones: Coupled elastic and thermal modeling

Precisely quantifying the heating of bone is important in many diagnostic and therapeutic applications of ultrasound. Here, the heating of bone by ultrasound is investigated using an elastic wave model coupled with the bioheat transfer equation. The volume rate of heat deposition is calculated separately for the compressional and shear waves by splitting the acoustic particle velocity during the simulation using a k-space dyadic. Four bone geometries of increasing complexity are studied for both plane piston and focused bowl transducers. Close to normal incidence, the heating is dominated by the absorption of compressional waves. Under these conditions, both fluid and elastic models give similar predicted temperature rises. As the angle of incidence is increased beyond 10 degrees, the contribution from shear wave absorption becomes significant. For the plane piston transducer, there is a sharp drop in compressional wave heating around the critical angle. In contrast, using a focused transducer, the reduction in compressional wave heating with angle is much smoother, extending well beyond the critical angle.