Ultrasonic beam focusing through tissue inhomogeneities with a time reversal mirror: application to transskull therapy

TLDR

Time reversal of ultrasonic fields efficiently focuses pulsed waves through lossless inhomogeneous media by correcting phase, amplitude, and shape aberrations, outperforming time‑shift compensation techniques. The study aims to correct aberrations for ultrasonic hyperthermia by combining a time‑reversal mirror with an implanted acoustic source or sensor, and to propose an amplitude‑correction method for focusing through nearby attenuating layers. The authors use large transducer arrays to form a time‑reversal mirror that samples, time‑reverses, and re‑emits the incident field, requiring knowledge of the medium’s Green’s function; they further introduce an amplitude‑inversion technique followed by time reversal and numerical back‑propagation to compensate for attenuating layers near or distant from the array. Experimentally, time reversal was applied to focus through the skull, revealing that skull‑induced attenuation severely reduces the method’s efficiency.

Abstract

Time reversal of ultrasonic fields allows a very efficient approach to focus pulsed ultrasonic waves through lossless inhomogeneous media. Time reversal mirrors (TRMs) are made of large transducer arrays, allowing the incident field to be sampled, time reversed, and reemitted. Time reversal method corrects for phase, amplitude, and even shape aberration and thus, is more efficient than time shift compensation techniques. However, this technique needs the knowledge of the Green's function of a dominant scatterer available in the medium. Aberration correction for ultrasonic hyperthermia could be achieved by combining TRM with an artificial acoustic source or sensor implanted inside the treatment volume. In this paper, time reversal method has been experimentally applied to the focusing through the skull bone. It is shown that the skull induces severe attenuation of ultrasound and reduces the efficiency of the time reversal approach. Then, an amplitude correction method is proposed to focus through an attenuating layer located close to the array. This method consists in inversing the amplitude modulation and then time reversing these signals. Finally, this method is combined with numerical backpropagation to compensate for an attenuating layer located some distance away from the transducer array.