An imaging system for an object such as human or animal tissue applies acoustic vibrations localized in two or three dimensions and simultaneously illuminates the object with an illuminating electromagnetic wave. The acoustic vibration comprises a carrier wave that is amplitude modulated by an AM waveform. The carrier wave is selected to provide the localization of the acoustic vibration, whereas the AM waveform includes a frequency component selected to provide a vibration of the object of greater magnitude than the carrier wave. To detect a Doppler component shifted by the frequency of said frequency component of the AM waveform, a signal processing apparatus comprises a phase locked-loop locked to the EM frequency, producing a frequency-demodulated signal comprising the set of the Doppler components, and a lock-in amplifier configured to extract a signal at a reference frequency equal to the frequency of a frequency component of the AM waveform.
