Design, development and characterization of wideband polymer ultrasonic probes for medical ultrasound applications

This dissertation deals with the design, development and characterization of non-resonantpolymer ultrasonic probes for medical ultrasound applications. Both single element andmultielement imaging transducer design having single layer and multilayer configurationwere developed with the primary goal of minimizing the trade off between resolution andpenetration depth. The simultaneous improvement in the transducers' pulse-echo sensitivity and bandwidthwas achieved by employing a multilayer structure made of thin piezopolymer films andutilizing the concept of Barker code. The results of the experiments indicated that themultilayer Barker code transducers provided the widest bandwidth in comparison withthe imaging transducers made of conventional piezoelectric ceramic material. Also, theyexhibited enhanced sensitivity compared to a single layer piezopolymer transducer. Specifically, the results indicated that the '6 dB fractional bandwidth extending over 2decades (20 MHz) could be achieved in the case of non-resonant transducers, whereas theconventional resonant design imaging transducer could provide only about one half of thebandwidth. The polymer array transducers also showed uniform acoustic response fromelement to element, which is desirable in order to obtain high quality ultrasound images. The double layer hydrophone probes used for characterization of the imaging transducerswere fabricated using dissimilar thickness of PVDF polymer film. This techniqueensured simultaneous enhancement of sensitivity and bandwidth. Also, a measurementprocedure employing planar scanning technique was developed to calibrate thehydrophone probes below 1 MHz for adequate characterization of acoustic field producedby the imaging transducers. The results of this work indicate that the new class of transducers developed featuressignificantly enhanced bandwidth. Such transducers hold promise to be capable ofoperating at clinically relevant frequencies and suitable for use at fundamental,subharmonic and higher harmonics imaging. It is expected that this non-resonantlyoperating imaging transducer would become a useful clinical tool in medical imaging andcould improve diagnostic efficacy.