Focusing with ultrasonic phased arrays-comparison between time-delay laws and time-reversal

In this paper, we present a concrete situation for which the computation of a time-delay law may yield some difficulties, particularly in the computation of the resulting focal pattern. These difficulties are directly related to the finite aperture of the elements of the array, that are generally of large size. We also prove that a time-reversal approach provides an elegant and efficient solution to these difficulties. Finally, the temporal signal obtained by time-reversal may be used to extract an adequate time-delay law; these delays do not result from an a priori algorithm (average, minimum delays), but are the consequence of an objective approach to the problem