MR-guided adaptive focusing of ultrasound.

Adaptive focusing of ultrasonic waves under the guidance of a magnetic resonance (MR) system is demonstrated for medical applications. This technique is based on the maximization of the ultrasonic wave intensity at one targeted point in space. The wave intensity is indirectly estimated from the local tissue displacement induced at the chosen focus by the acoustic radiation force of ultrasonic beams. Coded ultrasonic waves are transmitted by an ultrasonic array and an MRI scanner is used to measure the resulting local displacements through a motion-sensitive MR sequence. After the transmission of a set of spatially encoded ultrasonic waves, a non-iterative inversion process is employed to accurately estimate the spatial-temporal aberration induced by the propagation medium and to maximize the acoustical intensity at the target.Both programmable and physical aberrating layers introducing strong distortions (up to 2pi radians) were recovered within acceptable errors (<0.8 rad). This noninvasive technique is shown to accurately correct phase aberrations in a phantom gel with negligible heat deposition and limited acquisition time. These refocusing performances demonstrate a major potential in the field of MR-guided ultrasound therapy in particular for transcranial brain high-intensity focused ultrasound.