Time reversal acoustics

An overview of time reversal techniques is presented in this paper. In the first section, we focus on the ability of using reflecting targets embedded in the body as sources of time reversal waves. Real time tracking and destruction of moving kidney stones are demonstrated in the case of lithotripsy application. We show the strong potential of iterative time reversal techniques in multiple target environments to select and focus in real time on each target of a medium. The ability of iterative time reversal to improve the detection of microcalcifications in a random scattering media (speckle noise) is also presented. We show that distortions induced by sound velocity heterogeneities are compensated by the iterative time reversal technique guaranteeing the maximum pressure to be reached at the target position. In the second section of this paper, we discuss the time reversal focusing properties observed in dissipative media like the skull. We show that the time reversal focal spot can be strongly degraded as, in such medium, we can no more rely on the time reversal invariance of the wave equation. Important sidelobes appear around the main focus. However, combining time reversal with amplitude compensation techniques allows correction of absorption effects and decreasing of these sidelobes. Application of this coupled technique to high precision brain hyperthermia through the skull is demonstrated. Beyond these straightforward applications of time reversal to spatial focusing of waves through aberrating medium, we show that time reversal techniques allow us also to revisit the concept of piezoelectric transducer designing. Contrary to conventional transducer technology avoiding unwanted reverberations in piezoelectric elements, time reversal can take benefit of strongly reverberating media to increase the transducer efficiency. We demonstrate that very high pressure fields (1000 Atm.) can be obtained with a few transducers connected to reverberating media such as solid waveguides. The dispersive property of waveguides; is compensated by time reversal allowing very long coded excitations to be recompressed in very short high amplitude pulses. It leads to a new generation of ultra-compact shock wave lithotripters that use a very small transducer number a- nd to time reversal kaleidoscopes that can replace the 2D array.