Apparatus for use while a focal region of high intensity focused ultrasound (HIFU) energy is applied, the apparatus comprising: a first ultrasound transducer configured to transmit a first acoustic field by emitting the HIFU energy into a region of a medium at a first frequency, the first acoustic field generating oscillatory motion at the first frequency of scatterers disposed in the region, each scatterer oscillating around a respective equilibrium position; 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 the group consisting of the first ultrasound transducer, a second ultrasound transducer, and the acoustic probe is configured to (i) transmit first and second acoustic pulses (32,34) into the region, each pulse having a center frequency that is higher than the first frequency, and the time interval between the pulses being n+0.5 times the period of the first acoustic field, n being a positive integer, and (ii) receive respective echoes of each pulse scattering off an oscillating scatterer in the region, the first and second pulses (32,34) being synchronized with the first acoustic field such that the first pulse scatters off the oscillating scatterer when the oscillating scatterer is at a first displacement extremum with respect to the equilibrium position, and the second pulse scatters off the oscillating scatterer when the 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 a time shift between the received echoes that is due to motion of the oscillating scatterer, (c) based on the extracted time shift, calculate a displacement amplitude of the oscillating scatterer, and (d) generate a map of displacement amplitudes on a portion of the sonogram corresponding to the region. a first ultrasound transducer configured to transmit a first acoustic field by emitting the HIFU energy into a region of a medium at a first frequency, the first acoustic field generating oscillatory motion at the first frequency of scatterers disposed in the region, each scatterer oscillating around a respective equilibrium position; 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 the group consisting of the first ultrasound transducer, a second ultrasound transducer, and the acoustic probe is configured to (i) transmit first and second acoustic pulses (32,34) into the region, each pulse having a center frequency that is higher than the first frequency, and the time interval between the pulses being n+0.5 times the period of the first acoustic field, n being a positive integer, and (ii) receive respective echoes of each pulse scattering off an oscillating scatterer in the region, the first and second pulses (32,34) being synchronized with the first acoustic field such that the first pulse scatters off the oscillating scatterer when the oscillating scatterer is at a first displacement extremum with respect to the equilibrium position, and the second pulse scatters off the oscillating scatterer when the oscillating scatterer is at a second displacement extremum that is opposite the first displacement extremum with respect to the equilibrium position; and 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 the group consisting of the first ultrasound transducer, a second ultrasound transducer, and the acoustic probe is configured to (i) transmit first and second acoustic pulses (32,34) into the region, each pulse having a center frequency that is higher than the first frequency, and the time interval between the pulses being n+0.5 times the period of the first acoustic field, n being a positive integer, and (ii) receive respective echoes of each pulse scattering off an oscillating scatterer in the region, the first and second pulses (32,34) being synchronized with the first acoustic field such that the first pulse scatters off the oscillating scatterer when the oscillating scatterer is at a first displacement extremum with respect to the equilibrium position, and the second pulse scatters off the oscillating scatterer when the 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 a time shift between the received echoes that is due to motion of the oscillating scatterer, (c) based on the extracted time shift, calculate a displacement amplitude of the oscillating scatterer, and (d) generate a map of displacement amplitudes on a portion of the sonogram corresponding to the region.