https://ieeexplore.ieee.org/iel4/5388/14578/00664417.pdf

Ultrasound Imaging

A method is presented for determining the piezoelectric constants and the frequency-dependent dielectric properties of the polymers from a five-step algorithm based on analysis of air-loaded broadband impedance measurements. It is shown how to account for the frequency-dependent lossy properties of these films in an equivalent impedance circuit model and a modified Mason's model. Comparisons between the models and actual film transducers show excellent broadband simulation of both electrical input impedance and ultrasonic pulse-echo performance. >

Ultrasound transducer models for piezoelectric polymer films

A dual frequency probe using a multilayer ceramic is proposed for simultaneously obtaining a high resolution B mode and a high sensitivity Doppler mode image. This ceramic consists of two layers in which the poling directions are opposite and the individual thicknesses are different. It is possible to control the values of relative electromechanical coupling factors in the fundamental and the second harmonic by changing the thickness ratio. A thickness ratio of 1:0.7 was decided from computer simulation based on the Mason's model. A sufficient resolution has been shown from the fact that the intima of the carotid artery could be distinguished by an actually fabricated probe with dual frequencies of 3.75 and 7.5 MHz. Also, the sensitivity of this probe in the Doppler mode at 5 cm depth from the surface has been improved as much as 5 dB over that of a conventional one. >

A dual frequency ultrasonic probe for medical applications

Current commercial applications of ferroelectric polymers for both nondestructive testing (NDT) and medical ultrasound are reviewed. Cases are made for their future need in such applications as embedded sensors for structural NDT and health monitoring, extremely high-frequency broadband materials characterization, and 'disposable' invasive medical imaging. A brief overview of some of these fabrication techniques includes multilayer film-forming, robust electrical interconnects, direct polymer deposition, and combining the ferroelectric polymer and semiconductor technologies to produce integrated ultrasonic devices. >

Ferroelectric polymers: current and future ultrasound applications

Electromechanical properties of thin films of vinylidene fluoride‐trifluoroethylene copolymer [P(VDF‐TrFE)] and their application to ultrasonic transducers were studied in the frequency range of 50–250 MHz at room temperature. A 5‐μm‐thick P(VDE‐TrFE) film resonating at 250 MHz in thickness extensional mode exhibited a large electromechanical coupling factor of 0.26. The dielectric and elastic constants were also studied in this frequency region. A transducer using a 3‐μm‐thick film exhibited a large conversion efficiency (6.5 dB at 150 MHz), and good impulse response (30 ns ring‐down time width) in water. A concave transducer transmitted a well‐focused ultrasonic beam (35‐μm beamwidth).

Generation of very high‐frequency ultrasonic waves using thin films of vinylidene fluoride‐trifluoroethylene copolymer

A 5-MHz 64-element linear array transducer composed of vinylidene fluoride and trifluoroethylene copolymer (P(VDF-TrFE)) has been developed and its performance studied. The transducer has high resolution in both lateral and depth directions and has energy conver- sion efficiency high enough to be used for practical applicalions. The electroacoustic conversion loss (CL) is 6 dB at 5 MHz, and the pulse- echo ring-down period at -20 dB is 1.0 PS. These experimental values are in good agreement with theoretical ones calculated with Mason's equivalent circuit in which the internal losses of a piezoelectric material are taken into account. The ultrasonic field distribution from the array is also in good agreement with the simulations obtained using a numer- ical Huygens' wavelet approach for pulse waves. The maximum cross coupling voltage between the nearest neighbor elements is len percent of the driving voltage. This degree of cross coupling does not appreci- ably degrade the ultrasonic field distribution. A medical echogram taken with this transducer is shown. Its lateral and depth resolutions nn the focal plane are 1.5 mm and 0.75 mm, respectively.

http://ieeexplore.ieee.org/iel5/10326/32874/01539708.pdf

N. Hashimoto

Papers

Performance of a Linear Array Transducer of Vinylidene Fluoride Trifluoroethylene Copolymer