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For more than ten years, time reversal techniques have been developed in many different fields of applications including detection of defects in solids, underwater acoustics, room acoustics and also ultrasound medical imaging and therapy. The essential property that makes time reversed acoustics possible is that the underlying physical process of wave propagation would be unchanged if time were reversed. In a non dissipative medium, the equations governing the waves guarantee that for every burst of sound that diverges from a source there exists in theory a set of waves that would precisely retrace the path of the sound back to the source. If the source is pointlike, this allows focusing back on the source whatever the medium complexity. For this reason, time reversal represents a very powerful adaptive focusing technique for complex media. The generation of this reconverging wave can be achieved by using Time Reversal Mirrors (TRM). It is made of arrays of ultrasonic reversible piezoelectric transducers that can record the wavefield coming from the sources and send back its time‐reversed version in the medium. It relies on the use of fully programmable multi‐channel electronics. In this paper we present some applications of iterative time reversal mirrors to target detection in medical applications.

A method for tracking locally the 3D motion of biological tissues is developed and applied to the correction of motion during high intensity focused ultrasound (HIFU) therapy. The motion estimation technique is based on an accurate ultrasonic speckle tracking method. A pulse-echo sequence is performed for a subset of the transducers of a phased array. For each of these sub-apertures, the displacement is estimated by computing the 1D cross-correlation of the backscattered signals acquired at two consecutive times. The local 3D motion vector is then computed using a inversion algorithm. This technique is experimentally validated in vivo on anesthetized pigs. The 3D motion of liver tissues is tracked in real-time. The technique is combined with HIFU sequences and a real-time feedback correction of the HIFU beam is achieved by adjusting the delays of each channel. The sonications "locked on target" are interleaved with very motion estimation sequences.

Brain therapy using focused ultrasound remains very limited due to the strong aberrations induced by the skull. A technique using time-reversal was validated recently in-vivo on 20 sheep. The principal handicap of this technique is the need of an hydrophone at the focal point for the first step of the time-reversal procedure, which is minimally invasive but lightly traumatizing. A completely noninvasive therapy requires a reliable model of the acoustical properties of the skull in order to simulate this first step. 3-D simulations based on high-resolution CT images of a skull have been successfully performed with a finite differences code developed in our Laboratory. Thanks to the skull porosity, directly extracted from the CT images, we reconstructed acoustic speed, density and absorption maps and performed the computation. Computed wavefronts are in good agreement with experimental wavefronts acquired through the same part of the skull and this technic was validated in-vitro in the laboratory. A stereotactic frame has been designed and built in order to perform non invasive transcranial focusing. Here we will describe all the steps of our new protocol, from the CT-scans to the therapy treatment and the first in vivo results on monkeys will be presented. This protocol is based on protocols already existing in radiotherapy.