Hiroshi Masuzawa

Relaxor ferroelectric transducers

Novel ultrasonic transducers whose sensitivity can be controlled with a DC bias field have been developed by utilizing relaxor ferroelectrics as a transducer material in place of conventional piezoelectrics. Relaxor ferroelectrics have a large bias-field-induced piezoelectricity as well as a giant electrostrictive effect due to their extremely large dielectric constant ( approximately 10/sup 4/). Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/ (PMN) is a typical relaxor ferroelectric having a diffuse phase transition at around 0 degrees C. The addition of 9 mol.% of PbTiO/sub 3/ to the PMN is found to produce a transducer material whose electromechanical coupling factor for the thickness mode is comparable to that of PZT ceramics for a practical DC bias field. It is suggested that the application of recent piezoceramic/polymer composite technology to this relaxor ferroelectric transducer can play an important role in developing two-dimensional arrays.<<ETX>>

Medical ultrasonic probe using electrostrictive-ceramics/polymer composite

A medical ultrasonic probe the sensitivity of which is controlled by a DC bias field is developed using an electrostrictive-ceramics-polymer composite as a transducer material. (1-x)Pb(Mg/sub 1/3/Nb/sub 2/3/)O/sub 3/-xPbTiO/sub 3/ electrostrictive-ceramics ((1-x)PMN-xPt) are used because they have a large electromechanical coupling factor for the thickness vibrational mode, k/sub t/, under a practical bias field. 0.91PMN-0.09PT/polyurethane 1-3 composites (thickness 0.22 mm, 7.5 MHz) are fabricated using a dicing and filling technique. The 0.91PMN-0.09PT has no residual k/sub t/ after removing the DC bias field around room temperature. Pulse-echo experimentation shows that the echo amplitude increases with the bias field, as expected from the relationship between k/sub t/ and the bias field.<<ETX>>

Electrostrictive Materials for Ultrasonic Probes in the Pb(Mg1/3Nb2/3)O3–PbTiO3 System

Electrostrictive ceramics in the Pb(Mg1/3Nb2/3)O3-PbTiO3 (PMN-PT) solid solution system are investigated as the transducer material for ultrasonic probes whose electro-acoustic conversion efficiency is controlled by an applied DC bias voltage. Considering the electromechanical coupling factors induced under practical DC bias voltage and the normal using temperature, it is concluded that the most promising composition is 0.09 mole fraction of PbTiO3. A basic pulse-echo measurement confirmed that electro-acoustic conversion efficiency of an ultrasonic probe made of electrostrictive ceramics is controlled by an applied DC bias field.

Effect of Micro-Bubbles in Water on Beam Patterns of Parametric Array

The improvement in efficiency of a parametric array by nonlinear oscillation of micro-bubbles in water is studied in this paper. The micro-bubble oscillation can increase the nonlinear coefficient of the acoustic medium. The amplitude of the difference-frequency wave along the longitudinal axis and its beam patterns in the field including the layer with micro-bubbles were analyzed using a Khokhlov-Zabolotskaya-Kuznetsov (KZK) equation. As a result, the largest improvement in efficiency was obtained and a narrow parametric beam was formed by forming a layer with micro-bubbles in front of a parametric sound radiator as thick as about the shock formation distance. If the layer becomes significantly thicker than the distance, the beam of the difference-frequency wave in the far-field will become broader. If the layer is significantly thinner than the distance, the intensity level of the wave in the far-field will be too low.

Precise reconstruction of fast moving cardiac valve in high frame rate synthetic transmit aperture ultrasound imaging

To diagnose heart valve incompetence, i.e., one of the most serious cardiac dysfunctions, it is essential to obtain images of fast-moving valves at high spatial and temporal resolution. Ultrasound synthetic transmit aperture (STA) imaging has the potential to achieve high spatial resolution by synthesizing multiple pre-beamformed images obtained with corresponding multiple transmissions. However, applying STA to fast-moving targets is difficult due to serious target deformation. We propose a high-frame-rate STA (fast STA) imaging method that uses a reduced number of transmission events needed for each image. Fast STA is expected to suppress deformation of moving targets; however, it may result in deteriorated spatial resolution. In this study, we conducted a simulation study to evaluate fast STA. We quantitatively evaluated the reduction in deformation and deterioration of spatial resolution with a model involving a radially moving valve at the maximum speed of 0.5 m/s. The simulated raw channel data of the valve phantom was processed with offline beamforming programs. We compared B-mode images obtained through single received-line in a transmission (SRT) method, STA, and fast STA. The results show that fast STA with four-times-reduced events is superior in reconstructing the original shape of the moving valve to other methods. The accuracy of valve location is 97 and 100% better than those with SRT and STA, respectively. The resolution deterioration was found to be below the annoyance threshold considering the improved performance of the shape reconstruction. The obtained results are promising for providing more precise diagnostic information on cardiovascular diseases.

Suppression of aberration in deeper-region US images using multi-look adaptive beamforming

Multi-look techniques, such as spatial compounding and synthetic aperture, are useful to suppress unwanted aberration with improved coherence by compounding the signals of multi-transmission events. We propose a different approach for multi-look techniques involving just a single transmission event with the help of adaptive beamforming. This approach uses delay and sum output, which is constructed from one transmission event. Adaptive weights are calculated for several virtually oriented mode vector directions. We then obtain compounded output by summing the weighted signals (mode vector compounding; MVC). Because the directions are slightly away from the transmit axis, the output is expected to yield reduced multi-path signals that do not come directly from the axis of dynamic focusing. The results suggest that the effect of spatial correlation of the aberrated wave is suppressed by the compounding of mode directions. For Amplitude and Phase Estimation (APES) with MVC, the spatial resolution does not degrade because of the strong sidelobe reduction by the adaptive weights. With this approach, aberration suppression within single transmission events without the tradeoff of resolution is promising.