Energy-Based Adaptive Focusing of waves: Application to Ultrasonic Transcranial Therapy

Abstract

We propose a general concept of adaptive focusing through complex media based on the estimation or measurement of the wave energy density at the desired focal spot. As it does not require the knowledge of phase information, this technique has many potential applications in acoustics and optics for light focusing through diffusive media. We present here the application of this technique to the problem of ultrasonic aberration correction for HIFU treatments. The estimation of wave energy density is based on the maximization of the ultrasound radiation force, using a multi‐elements (64) array. A spatial coded excitation method is developed by using ad‐hoc virtual transducers that include all the elements for each emission. The radiation force is maximized by optimizing the displacement of a small target at the focus. We measured the target displacement using ultrasound pulse echo on the same elements. A method using spatial coded excitation is developed in order to estimate the phase and amplitude aberration based on the target displacement. We validated this method using phase aberration up to 2π. The phase correction is achieved and the pressure field is measured using a needle hydrophone. The acoustic intensity at the focus is restored through very large aberrations. Basic experiments for brain HIFU treatment are presented. Optimal transcranial adaptive focusing is performed using a limited number of short ultrasonic radiation force pushes.