PVdF array characterisation for high frequency ultrasonic imaging

Polyvinylidenefluoride (PVdF) has been utilized for a number of years within ultrasonic hydrophones. However, polymeric materials have rarely been incorporated into medical imaging phased arrays due to lower emitted power levels (relative to PZT transducers), and higher transducer element impedance. PVdF´s advantages as a transducer material lie in its inherent wide bandwidth and the potential to create high-resolution images whilst maintaining low transducer manufacturing costs. Here we report the fabrication and test of PVdF linear arrays with 28 μm PVdF film with elements on a 250 μm pitch. These arrays are connected to equipment that has been developed to perform transmit beamforming to a variable focal point, and receive echoes from single transducer elements that are close-coupled to a 48-channel array of amplifiers. A-line data can then be post processed to perform dynamic receive beamforming. Utilizing this equipment, measurements of pressure field distributions are presented, and compared with simulations, to determine the optimum number of pulsed elements. Using the arrays in pulse-echo mode, imaging quality is assessed with biological tissue samples and ultrasound phantoms. A prototype transducer, operated to produce ultrasound with >20MHz centre frequency, realized spatial resolution of <0.4mm laterally and 0.1mm axially, at a distance of 15mm from the transducer.