A first transducer (20) transmits a first acoustic field (22) at a first frequency into a region (24) of a medium (26), generating oscillatory motion of scatterers (28) disposed in the region. A second transducer (30) transmits acoustic pulses (32, 34) into the region, and receives respective echoes of each pulse scattering off an oscillating scatterer in the region. The pulses are synchronized with the first acoustic field such that a first pulse scatters off the oscillating scatterer when the scatterer is at a first displacement extremum (36), and a second pulse scatters off the oscillating scatterer when the scatterer is at a second displacement extremum (38) that is opposite the first displacement extremum. A computer processor (29) extracts a time shift between the received echoes, calculates a displacement amplitude of the scatterer, and outputs an indication of the displacement amplitude of the scatterer. Other applications are also described.
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