Acoustic field mapping with ultrasonic particle velocity estimator

An apparatus for use with a focal region of high intensity focused ultrasound (HIFU) energy, the apparatus comprising:

• a first ultrasound transducer configured to transmit a first acoustic field by emitting the HIFU energy into a medium at a first frequency, a propagation of the first acoustic field generating oscillatory motion at the first frequency of scatterers disposed in the medium, (i) each scatterer oscillating around a respective equilibrium position, and (ii) the oscillatory motion of the scatterers being an acoustic particle-velocity of the first acoustic field;
• an acoustic probe,
• wherein the acoustic probe is configured to emit pulse-echo ultrasound energy into the medium at an imaging frequency, and
• wherein an acoustic element selected from a group consisting of the first ultrasound transducer, a second ultrasound transducer, and the acoustic probe is configured to (i) transmit a plurality of pairs of first and second acoustic pulses into the focal region in a plurality of respective directions while the HIFU energy is being emitted, each pulse having a center frequency that is higher than the first frequency, a time interval between the first and second pulses of each respective pair being n+0.5 times a period of the first acoustic field, n being a positive integer, and (ii) receive respective pairs of echoes of each pair of pulses scattering off a respective oscillating scatterer in the focal region,
• the plurality of pairs of first and second pulses being synchronized with the first acoustic field such that, for each pair, the first pulse scatters off the respective oscillating scatterer when the respective oscillating scatterer is at a first displacement extremum with respect to the equilibrium position, and the second pulse scatters off the respective oscillating scatterer when the respective oscillating scatterer is at a second displacement extremum that is opposite the first displacement extremum with respect to the equilibrium position;
• and
• a computer processor configured to (a) generate a real-time sonogram of the medium based on reflections of the pulse-echo ultrasound energy that is transmitted by the acoustic probe, (b) extract respective time shifts, each respective time shift being between the received echoes of a respective pair of echoes that are due to motion of the respective oscillating scatterer, (c) based on the extracted respective time shifts, calculate displacement amplitudes of the respective oscillating scatterers, and (d) generate a map of the displacement amplitudes on a portion of the sonogram corresponding to the focal region.