Abstract
Time reversal of ultrasonic fields 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 reemitted. 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. The time reversal process is applied to track, in real time a moving gall bladder or kidney stone embedded in its surrounding medium. The feasibility of adaptative beam forming techniques to track the stone during a lithotripsy treatment is investigated. It is shown that TRM allows sharp focusing on one bright point of the stone. Once the bright point is selected, a time of flight profile is determined and used in a least-mean-square method to calculate the spatial coordinates of the stone. Stone trajectories can be tracked by this technique at 30 Hz.
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