Time-reversal mirrors

Time-reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through inhomogeneous media. Time-reversal mirrors (TRMS) are made of large transducer arrays, allowing the incident acoustic field to be sampled, time-reversed and re-emitted. The paper introduces the time-reversal approach in a discussion of the techniques used in optics for focusing through inhomogeneous media. The discussion is extended to ultrasonic pulsed fields where adaptive time delay focusing and TRM focusing are the main techniques of interest. Time reversal focusing of a pulsed wave is optimal in comparison with time delay focusing in the sense that it realizes the spatiotemporally matched filter to the inhomogeneous propagation transfer function between the array and the target. Time-reversal processing also permits choice of any temporal window to be time-reversed, allowing operation in an iterative mode. In multi-target media, such a process converges on the most reflective target. In the case of an extended target, automatic resonances can be achieved. Applications of TRM are numerous and include medical applications (imaging, lithotripsy and hyperthermia) as well as non-destructive testing (NDT) and underwater acoustics. Experimental results obtained with 64 and 128 channel TRMS demonstrate the efficiency of this technique.