Time reversal of ultrasonic fields. Il. Experimental results

For pt.I see ibid., vol.39, no.5, p.555-66 (1992). 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 TRM experiments using a 64-channel prototype are described, and the results are given. Focusing experiments conducted on point targets through different aberrating media are described. 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, the authors show that the time-reversal process can be iterated in order to focus on the most reflective one. Lithotripsy applications are also discussed. Kidney stones are spatially extended targets and TRM experiments 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.