Optimal focusing through aberrating media: a comparison between time reversal mirror and time delay correction techniques

The problem of focusing an ultrasonic beam through an inhomogeneous medium was investigated experimentally for conventional time delay focusing, adaptive time delay focusing, and time reversal focusing. It is shown that neither time delay based technique gives a properly focused beam for all aberrator locations. Only time reversal focusing allows a robust focusing regardless of the aberrator location. The matched filter theory of signal processing allows an interpretation of this result: time reversal focusing provides the optimal input to the linear system consisting of the acousto-electric and diffraction impulse response