Dynamic nonlinear focal shift in amplitude modulated moderately focused acoustic beams

- A dynamic focusing method in ultrasonic beams is proposed for mono-element focused transducers.
- The focusing distance is controlled dynamically by the amplitude of the input signal.
- The dynamic focusing is validated experimentally in water.
- Focal differences on thermo-viscous media and soft-tissues are studied numerically.
- The differences between pressure, intensity and ARF fields at the focus are discussed.

The phenomenon of the displacement of the position of the pressure, intensity and acoustic radiation force maxima along the axis of focused acoustic beams under increasing driving amplitudes (nonlinear focal shift) is studied for the case of a moderately focused beam excited with continuous and 25 kHz amplitude modulated signals, both in water and tissue. We prove that in amplitude modulated beams the linear and nonlinear propagation effects coexist in a semi-period of modulation, giving place to a complex dynamic behavior, where the singular points of the beam (peak pressure, rarefaction, intensity and acoustic radiation force) locate at different points on axis as a function of time. These entire phenomena are explained in terms of harmonic generation and absorption during the propagation in a lossy nonlinear medium both for a continuous and an amplitude modulated beam. One of the possible applications of the acoustic radiation force displacement is the generation of shear waves at different locations by using a focused mono-element transducer excited by an amplitude modulated signal.