FOCUSING AND STEERING THROUGH ABSORBING AND ABERRATING LAYERS - APPLICATION TO ULTRASONIC PROPAGATION THROUGH THE SKULL

The time-reversal process is applied to focus pulsed ultrasonic waves through the human skull bone. The aim here is to treat brain tumors, w hich are difficult to reach with classical surgery means. Such a surgi cal application requires precise control of the size and location of t he therapeutic focal beam. The severe ultrasonic attenuation in the sk ull reduces the efficiency of the time reversal process. Nevertheless, an improvement of the time reversal process in absorbing media has be en investigated and applied to the focusing through the skull [J.-L. T homas and M. Fink, IEEE Trans. Ultrason. Ferroelectr. Freq. Control 43 , 1122-1129 (1996)]. Here an extension of this technique is presented in order to focus on a set of points surrounding an initial artificial source implanted in the tissue volume to treat. From the knowledge of the Green's function matched to this initial source location a new Gr een's function matched to various points of interest is deduced in ord er to treat-the whole volume. In a homogeneous medium, conventional st eering consists of tilting the wave front focused on the acoustical so urce. In a heterogeneous medium, this process is only valid for small angles or when aberrations are located in a layer close to the array. It is shown here how to extend this method to aberrating and absorbing layers, Like the skull bone, located at any distance from the array o f transducers. (C) 1998 Acoustical Society of America.