Aberration correction in ultrasonic medical imaging with time-reversal techniques

Abstract

Degradation of image quality is currently observed with ultrasonic scanners owing to distortion of the ultrasonic beams through inhomogeneous tissue layers. Adaptive time-delay focusing techniques allow an efficient correction of these effects when the inhomogeneous layer is close to the transducer array. If the aberrating layers are far from the array, these techniques are no longer appropriate to correct the propagation effects between the layer and the transducer array. In this article we show that time-reversal mirrors can solve this problem. In the matched-filter approach that extends the concept of time reversal mirrors, the Green's function of a dominant scatterer available in the medium is recorded in digital memory and used to focus on the scatterer in both transmit and receive modes. An extension of this technique is also presented to focus, in the presence of an aberrating layer, not only on the dominant scatterer, but also around it to image the surrounding zone. From the knowledge of the Green's function needed to focus on the initial scatterer, new Green's functions matched to the new point of interest are calculated. The algorithm uses the concept of time-reversal propagation, and theoretical and experimental results obtained with this technique are presented. The calculation of each Green's function matched to each new desired focal point allows us to realize a B-scan image of the zone surrounding the reflector. © 1997 John Wiley & Sons, Inc. Int J Imaging Syst Technol, 8, 110–125, 1997