Time Reversal of Ultrasonic Fields-Part 11: Experimental Results

A time-reversal mirror (TRM) is made of an array of transmit-receive transducers. The incident pressure field is sampled, digitized, stored, time-reversed and then re-emitted. This process can be used to focus, through inhomogeneous media, on a reflective target that may behave as an acoustic source after being insonified. A 64-channel prototype has been built and TRM experiments demonstrating the TRM performance are described. Present first is focusing experiments conducted on point targets through different aherrating media. The major result shows that the time-reversal focusing technique compensates for all the distortions whatever the TRM-aberrator distance. When the medium contains several targets, we show that the time-reversal process can be iterated in order to focus on the most reflective one. The last part of the paper deals with lithotripsy applications. Kidney stones are spatially extended targets and TRM exper- iments have been conducted on several kidney stones located behind inhomogeneous media. They show that the iterative TRM process is able to select one of the kidney stones and to focus on a small portion of it.