End-element anomalies in medical ultrasonic piezo-composite arrays

Piezoelectric composites are commonly used in medical diagnostic ultrasonic imaging arrays. The performance of the array elements at either end of the array can differ from that of array elements away from the ends. There is some general understanding about the origin of these effects (such as different acoustic impedance), and some standard compensatory designs exist (such as adding unused array elements further on the end than the last used array element). This work seeks to elucidate the origins of these end-element anomalies and to propose corresponding design changes. A commercially produced array with notable end-element anomalies is examined as a case study. Results from experiments and finite element analysis indicate that, in the presence of a stiffness discontinuity within the composite (such as poled elements adjacent to unpoled ones), a secondary wave that propagates laterally through the composite may be generated during the receive transduction. This wave appears to cause the anomalous behavior observed in the signals and metrics of the end elements. Changing the electrical loading of an element and poling the unused, previously unpoled elements are explored as anomaly-mitigating design alterations. The latter of these 2 initially appears to be the more effective solution.