Optimum Focusing in an Ultrasonic Annular Array Imaging System Using a Nonspherical Lens

Abstract

In this paper, a new focusing method for ultrasonic annular array imaging systems is presented. The technique minimizes the lateral resolution for a specified sidelobe suppression over the depth of interest, using a nonspherical lens for line focusing in transmit in combination with continuous dynamic focusing in receive. For this purpose, an effective analysis model is developed for calculating the impulse response of an annulus with an arbitrarily-shaped lens in the whole imaging plane. Using this analysis method, an iterative technique is then applied to obtain an optimum transmit delay profile when continuous receive focusing is employed at all imaging points along the depth of view. The delay profile is optimum in the sense that the lateral beam width is minimized under a specified sidelobe suppression at three depths of the imaging field. The continuous receive focusing is realized in a new digital focusing system which enables arbitrary control of transmit time delays and dynamic focusing in the receive mode. In the proposed system, focusing is achieved by adjusting the sampling time for each array element according to the arrival time differences among array elements for a given focal point. Experimental results show the validity of the analysis model and the optimum lens design.