Ultrasonic Focusing with Time Reversal Mirrors

Abstract

Time reversal invariance is a very rich and powerful concept in physics. In the field of ultrasonic wave propagation, time reversal experiments can be realized easily with large ultrasonic piezoelectric transducer arrays. Applications of these experiments in problems using large ultrasonic transducer arrays are numerous. They concern medical applications (imaging, lithotripsy, and hyperthermia) as well as nondestructive testing. In these applications, which require a very narrow beam, high focusing performances are needed. However, large arrays may suffer from strong geometrical distortions which should be corrected. Besides, the focusing of ultrasonic waves becomes a difficult operation as soon as the medium of propagation contains heterogeneities (spatial variations of the compressibility and/or the density in biological tissues and in sea water, liquid-solid interfaces in nondestructive testing). In such conditions, the acoustic beam can be distorted and redirected, so that optimal focusing cannot be achieved. Compensation for these two kinds of distortions must be self-adaptive because the designer has no a priori knowledge of them. The use of a time reversal mirror (TRM) represents an original solution to this problem.(1-3)