Wideband and Efficient Ultrasonic Transducers Using Multiple Piezoelectric Polymer Films

Presently used ultrasound imaging transducers employ almost exclusively PZT or PZT composite materials, and are designed to operate at fundamental half wavelength resonance frequency. These transducers use front matching layers which limit the transducer's bandwidth and overall resolution of the imaging system. This thesis describes the development of a new type of wideband imaging transducers. The approach proposed here, namely, a switchable Barker code transducer (SBCT) design, uses multiple piezopolymer films and allows transducer to be operated at off-resonance mode. More specifically, the transducer described here can be driven by an arbitrary excitation signal, which will allow image resolution to be tailored to diagnostic needs at clinically relevant frequency, between 2-10 MHz. Also, the design proposed improves electrical matching between the polymer transducer and the excitation source, and allows the pulse-echo sensitivity of the polymer transducers to be comparable with that of the PZT or PZT composite transducers. The principle of the multilayer transducer design is described together with a new model developed to predict the transducer performance. The model accounts for all losses in piezoelectric material and allows the influence of tissue, backing material and excitation signals to be determined. Comprehensive computer simulations of both multilayer SBCT and conventional PZT transducer behavior were carried out. The computer predictions indicate that the design proposed here will outdo resonant PZT transducers with respect to the axial resolution and sensitivity. To verify the computer modeling, several prototypes of Barker code transducers were constructed and tested. The experimental data were found to be in good agreement with the theoretical predictions. Fundamental limitations of the non-resonant Barker code design are discussed and the advantages of using the multilayer transducers in the 2-10 MHz frequency range, typical of that presently used in medical diagnostic ultrasound are pointed out. Finally, the potential use of the multilayer transducers in the 25-100 MHz frequency range is examined and the suggestions for future improvements of the design are also included.