Multilayer transducer for nonlinear imaging with application to targeting and monitoring of therapeutic ultrasound

Nonlinear acoustic wave propagation can improve the resolution of ultrasound imaging, and could be used to dynamically estimate the physical properties of tissue. However, transducers capable of launching a wave that becomes nonlinear through propagation can typically detect only the fundamental and second harmonic. Here we present the design and characterization of a multilayer transducer with a high power transmit layer to generate nonlinear waves and a broadband receive layer to detect nonlinear scatter. The transmit array was made from a narrow-band PZT, with nominal frequency of 2.0 MHz, that was diced into an array of 32 elements. Elements had 0.300 mm width and 6.3 mm elevation, and the pitch was 0.400 mm. The receive array, placed directly above the transmit array, was made from PVDF elements that were patterned by flex circuit pads that replicated the PZT element dimensions. The PZT and PVDF elements had identical apertures. Simulations were performed to guide the selection of the transducer materials and thicknesses. Characterization of electrical parameters and acoustic output were performed per standard methods, in which transmit and receive events were driven by a software-controlled ultrasound system. Nonlinear waveforms with peak positive pressure up to 2.14 MPa were measured by a calibrated hydrophone. Echo data, collected from ex vivo tissue and digitized at 45 MS/s, exhibited frequency content up to the 4th harmonic of the 2 MHz transmit frequency.