Nonlinear modeling of pulsed and CW HIFU beams for dual-mode ultrasound arrays

Nonlinear propagation can have beneficial or detrimental effects on focused ultrasound beams for imaging and therapy. The advent of piezocomposite transducer technology have allowed for the fabrication of new generation of therapeutic arrays with relatively wide bandwidth and low cross coupling between elements (resulting in new dual-mode ultrasound array (DMUA) systems for imaging and therapy). The feedback capabilities of DMUAs offer new opportunities to characterize the HIFU beam in situ, including its spectral components due to harmonic generation. This form of feedback allows for the optimization of the DMUA driving patterns to achieve maximum therapeutic gain, by maximizing the harmonic generation within the focal spot. We present results of full 3D modeling of nonlinear wave propagation from the surface of currently available DMUA prototypes (1 and 3.5 MHz) into the treatment volume, modeled as heterogeneous absorbing medium. For example, a 1 MHz DMUA with f-number of 0.8 when geometrically focused at 100 mm with surface intensity of 2.2 W/cm2 was simulated. For 4-second exposure, accounting for harmonic generation resulted in ∼10% increase in peak temperature at the focus. We will present and discuss examples of complex DMUA focus patterns in the context of therapeutic gain.