ABERRATION CORRECTION IN ULTRASONIC MEDICAL IMAGING WITH TIME-REVERSAL TECHNIQUES

Degradation of image quality is currently observed with ultrasonic sca nners owing to distortion of the ultrasonic beams through inhomogeneou s tissue layers. Adaptive time-delay focusing techniques allow an effi cient correction of these effects when the inhomogeneous layer is clos e to the transducer array. If the aberrating layers are far from the a rray, these techniques are no longer appropriate to correct the propag ation effects between the layer and the transducer array. In this arti cle we show that time-reversal mirrors can solve this problem. In the matched-filter approach that extends the concept of time reversal mirr ors, the Green's function of a dominant scatterer available in the med ium is recorded in digital memory and used to focus on the scatterer i n both transmit and receive modes. An extension of this technique is a lso presented to focus, in the presence of an aberrating layer, not on ly on the dominant seatterer, but also around it to image the surround ing zone. From the knowledge of the Green's function needed to focus o n the initial scatterer, new Green's functions matched to the new poin t of interest are calculated. The algorithm uses the concept of time-r eversal propagation, and theoretical and experimental results obtained with this technique are presented. The calculation of each Green's fu nction matched to each new desired focal point allows us to realize a B-scan image of the zone surrounding the reflector. (C) 1997 John Wile y & Sons, Inc.