Time reversal and phase conjugation with acoustic waves: industrial and medical applications

Time-reversal invariance, a fundamental symmetry that holds nearly everywhere in microscopic physics, can be exploited in a unique way in acoustics, to create a variety of useful instruments. In the past years, taking advantage of the reversibility of the acoustic propagation that holds in many situations (to the extent that everything is adiabatic) time reversal mirrors (TRMs) have been developed that create time reversed waves. Applications of time reversal to medical therapy are also under study. The focal spot of a TRM can track a kidney stone during a lithotripsy treatment. New improvement of this technique have recently be obtained due to time reversal pulse amplification in solid waveguide. Taking advantage of wave dispersion in the waveguide, a time-reversal operation made on a small number of transducers transforms the long-lasting low-level dispersed wave forms into a sharp intense pulse that can easily destroy kidney stones. Others applications of TRMs to brain therapy are also under study and first in vivo tissue necrosis of brain tissue have been obtained through skull on sheep with ultrasonic time reversal mirrors