New approach toward field and transducer characterization

Abstract

The development of a relatively rapid, but accurate, technique for charactensing pulsed fields from array transducers is reported. The approach enables direct confirmation of the effectiveness of many single element Iarray design, construction and activation procedures. The techniques rests on the employment of a PVDF hydrophone of novel design. Whereas conventional approaches derive field characteristics from point measurements, the new hydrophone allows a direct measurement of the fields 'directivity spectrum' — which efficiently generalises the angular spectrum approach to wideband pulses. The directivity spectrum is shown to encapsulate significant features of both near and far field output characteristics, as well as tightness of focus, even though all measurements are conducted at any convenient distance from the transducer face. The new method is demonstrated in the context of measurements of the fields from typical medical ultrasound transducers. The following field and transducer characteristics are shown : directivity, acoustic axis direction, effective transducer/field coherence, tightness of focus, effective radiating area, effective apodisation, and element uniformity. The relative simplicity of the technique is not compromised when measuring angle-emission characteristics. The theoretical basis for the new field measurement technique is presented, and its advantages over the more usual angular spectrum approach with point measurements, are also discussed. Keywords: Ultrasound; Transducer characterisation; Field characterisation; Directivity spectrum; Large aperture hydrophone.