Time Reversal Focusing Applied to Lithotripsy

Time reversal of ultrasonic field allows a very efficient approach to focusing pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRM) are made of large transducer arrays, allowing the incident acoustic field to be sampled, time reversed and re-emitted. Time reversal processing permits a choice of any temporal window to be time reversed, allowing operation in an iterative mode. In multitarget media, this process converges on the most reflective target, i.e., the dominant scatterer. In this paper, the time reversal process is applied to track, in real time, a moving gall bladder or kidney stone embedded in its surrounding medium. We investigate the feasibility of a piezoelectric shock wave generator in which the focal zone is moved electronically to track the stone during a lithotripsy treatment. We show that TRM allows us to obtain sharp focusing on one bright point of the stone. The time of flight profile is then determined and used in a least-mean-square method to calculate the spatial coordinates of the stone.