Résumé:
Harmonic imaging coupled to the use of contrast agents has become a topic of wide interest in ultrasonic medical imaging. Instead of achieving images at the fundamental frequency of the emission signals, images are build at twice the excitation frequency that corresponds to the resonance frequency of the bubbles embedded in the medium. Consequently, the contrast becomes important between areas of low and high concentration of bubbles. However, at high mechanical index, the harmonic components of backscattered echoes depends on the intrinsic nonlinear properties of tissues as well as the bubbles resonance. The focused beam is generating harmonics during its nonlinear propagation. It results in a degradation of the harmonic image contrast. Time reversal is an elegant way to find the emission codes allowing to cancel the harmonic components due to nonlinear propagation of the ultrasonic beam and so to recover an optimal image contrast. Indeed, The wave equation in a nonlinear regime remains time reversal invariant : in the absence of bubbles, if the backscattered echoes are time-reversed and reemitted by the array, the harmonic components of the resulting wavefront are transferred back to the fundamental frequency during propagation. Experiments conducted with 1D-linear arrays illustrate this cancellation technique.

Mots-clés: nonlinearity, time reversal, harmonic imaging