Time apodization of ultrasonic transducer arrays

A new apodization technique to enhance the contrast resolution of medical ultrasonic imaging systems is presented. It can easily be implemented on any commercial transducer array. The array aperture windowing in the transmitting mode is performed without modifying the driver voltage over the whole array, but only varying the driving pulse length from an element to another. A design curve has been determined which transforms any conventional amplitude apodization into a `time apodization'. Experimental and numerical results are given for a 3.5 MHz convex array and for a 2.5 MHz phased array. They demonstrate the effectiveness of this method in reducing the off-axis energy in the radiated pattern. Moreover, they point out that `time apodization' improves also the range resolution, since the overall length of the ultrasonic pulse transmitted by the probe is reduced.