Energy-based adaptive focusing of waves: Application to ultrasonic imaging and therapy

A novel aberration correction method based on the maximization of the acoustic intensity at the focus of a transducer array is presented. The potential of this new adaptive focusing technique is investigated in for ultrasonic focusing in biological tissues. The acoustic intensity is maximized non-invasively through the measurement or indirect estimation of the beam energy at focus for a series of spatial coded emissions. The acoustic energy at the desired focus location can be indirectly, estimated from the local displacements induced by the ultrasonic radiation force of the beam. Based on these displacement estimates, this method allow the precise estimation of the phase and amplitude aberrations and consequently the corrections of aberrations along the beam travel path. The proof of concept is first performed experimentally using a large therapeutic array with strong simulated phase aberrations (up to 2pi). Displacements induced by the ultrasonic radiation force at the desired focus are indirectly estimated using the time shifts of backscattered echoes recorded on the array. The phase estimation is deduced accurately using a direct inversion algorithm. The corrected beam focusing quality, is controlled using the needle hydrophone. A sharp focus is restored and a strong increase of the acoustic intensity at focus is obtained. Secondly, the technique is tested experimentally using a linear imaging array through a real aberrating layer. Acoustic intensity is indirectly estimated using focal displacements measurements obtained using conventional speckle tracking of backscattered echoes. A strong improvement of both spatial resolution and sidelobes level is found.