  Heat enhancement at the target in a High Intensity Focused Ultrasound (HIFU)
field is investigated by considering the effects of the injection of
microbubbles in the vicinity of the tumor to be ablated. The interaction
between the bubble cloud and the HIFU field is investigated using a
three-dimensional numerical model. The propagation of non-linear ultrasonic
waves in the tissue or in a phantom medium is modeled using the compressible
Navier-Stokes equations on a fixed grid, while the microbubbles dynamics and
motion are modeled as discrete singularities, which are tracked in a Lagrangian
framework. These two models are coupled to each other such that both the
acoustic field and the bubbles influence each other. The temperature rise in
the field is calculated by solving a heat transfer equation applied over a much
longer time scale. The compressible continuum part of the model is validated by
conducting HIFU simulation without microbubbles and comparing the pressure and
temperature fields against available experiments. The coupled
Eulerian-Lagrangian approach is then validated against existing experiments
with a phantom tissue. The various mechanisms through which microbubbles
enhance heat deposition are then examined in detail. The effects of the initial
void fraction in the cloud are then sought by considering the changes in the
attenuation of the primary ultrasonic wave and the modifications of the
enhanced heat deposition in the focal region. Finally, the effects of the
microbubble cloud size and its localization in the focal region are shown and
the effects of these parameters in altering the temperature rise and the
location of the temperature peak are discussed.
