Kunio Hashiba

Enhancement of ultrasonic absorption by microbubbles for therapeutic application

It was recently reported that not only the cavitational bioeffect but also the thermal bioeffect of ultrasound can be enhanced with microbubble agents. If the ultrasonic absorption of tissues can be significantly increased in a well-controlled manner with administration of such a microbubble agent, it will be especially useful for the ultrasonic treatment of deep-seated tissues, to where ultrasonic power high enough for the treatment is difficult to deliver. The possibility of this approach was checked in a preliminary in vitro ultrasonic exposure experiment of Levovist suspension. Ultrasonic power absorbed and scattered by a microbubble was calculated through numerically solving Rayleigh-Plesset equation to analyze its nonlinear breathing motion. It is estimated that approximately 3 microbubbles in the resonant size per cubic millimeter of tissue are needed to double the tissue absorption due to ultrasonic exposure at 2 MHz. This microbubble concentration is achievable in a human body. It is also predicted that the microbubble size may need to be controlled small enough to maintain the ultrasonic penetration for efficient ultrasonic heating of tissues.

Measurement of Elastic Properties of Tissue by Shear Wave Propagation Generated by Acoustic Radiation Force

Acoustic radiation force (ARF) imaging has been developed as a novel elastography technology to diagnose hepatic disease and breast cancer. The accuracy of shear wave speed estimation, which is one of the applications of ARF elastography, is studied. The Youngs moduli of pig liver and foie gras samples estimated from the shear wave speed were compared with those measured the static Youngs modulus measurement. The difference in the two methods was 8%. Distance attenuation characteristics of the shear wave were also studied using finite element method (FEM) analysis. We found that the differences in the axial and lateral beam widths in pressure and ARF are 16 and 9% at F-number=0.9. We studied the relationship between two branch points in distance attenuation characteristics and the shape of ARF. We found that the maximum measurable length to estimate shear wave speed for one ARF excitation was 8 mm.

Analysis of the charging problem in capacitive micro-machined ultrasonic transducers

The charging property of dielectrics between the upper and lower electrodes in capacitive micro-machined ultrasonic transducers (CMUTs) was investigated. In particular, the dependence of the capacitance of a CMUT cell on DC voltage (i.e., C-V curve) was measured before and after DC or DC+AC voltage stress was applied between the two electrodes. The charging was evaluated from the C-V curve shift. Silicon dioxide and silicon nitride were investigated as dielectric materials. The investigation found that the charging starts when the stress voltage reaches the collapse voltage of the CMUT cell, and it becomes larger with increasing stress voltage. Silicon dioxide causes less charging than silicon nitride. The charging by DC+AC voltage stress is higher than that by DC voltage stress only. The charge quantity accumulated in the dielectrics saturates at a constant value depending on the stress voltage. Moreover, the CV-curve shift, which degrades the sensitivity of the CMUT, can be estimated from DC bias voltage, AC driving voltage, and collapse voltage of CMUT. To avoid this sensitivity degradation, it is thus necessary that a system using CMUT devices must optimally control the operation voltage in accordance with the estimated shift.

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.

Capacitive micromachined ultrasonic transducer with driving voltage over 100 V and vibration durability over 10 11 cycles

A capacitive micromachined ultrasonic transducer (CMUT) was fabricated using a standard 0.25-µm CMOS backend-of-line process. By adopting the planarization of dielectric material between the upper and lower electrodes by chemical mechanical polishing (CMP), we achieved a time-dependent dielectric breakdown (TDDB) lifetime exceeding 10 years at a DC stress voltage of 192 V. In the case of DC and AC voltage driving, we found degradation of the dielectric at the center of a membrane for the first time. By optimally controlling the driving voltage, membrane vibrations over 1011 cycles were accomplished without dielectric degradation.

Method for estimating uncertainty in individual vector-flow-mapping measurements

An experimental validation system for vector-flow-mapping (VFM) has been developed to verify the VFM accuracy. A VFM measured by ultrasound is a mapping of 2D flow vector fields in the human heart. The validation system consists of a left ventricle (LV) phantom, a stereo particle image velocimetry (stereo-PIV), and an ultrasound scanner. The LV phantom is pulsated by controlling the external pressure. The optically transparent LV phantom enables the PIV to obtain velocity fields. To evaluate the degradation of VFM accuracy in a 3D flow, the VFM results are compared with those obtained from the stereo-PIV, which provides accurate 3D velocity components in a plane. Preliminary results suggest that the VFM and PIV vector fields are in good agreement. However, 3D flow, which resulted from the VFM errors, are also observed in the phantom. A novel method for estimating uncertainty in individual VFM measurements was proposed by considering the velocity discrepancy between VFM and tissue tracking at the LV wall. The VFM error estimated with the proposed method was validated by comparing it with that measured using the PIV. The estimated errors agreed with the measured error from the PIV. The mean difference was about 8% in the experiments.

Acoustic characteristics of CMUT with rectangular membranes caused by higher order modes

The precise relationship between fundamental and higher order modes of capacitive micro-machined ultrasonic transducers (CMUTs) with rectangular membranes were determined. We investigated the effects of higher order modes on acoustical characteristics of CMUTs with rectangular membranes, especially effects on the transmission gain spectrum, through experiments using fabricated rectangular CMUT cells and a modal analysis using 3D simulation. The dips in the transmission gain spectrum corresponded to higher order mode excitation and first-order mode suppression. The frequency and depth of these dips depended on the aspect ratio. The interactions between individual modes in immersion become more complex as the aspect ratio of the rectangular membrane increases.

Tissue harmonic imaging based on amplitude modulation by controlling number of driving elements

A new tissue harmonic imaging (THI) technique, which does not require linearity of a transmission system, based on amplitude modulation by controlling the number of driving elements (AM-CNDE) is proposed. Whether AM-CNDE can achieve THI was investigated by simulation and experiment. It was found out that AM-CNDE can use both harmonics and a fundamental band component although conventional TID (pulse inversion) uses only harmonics to generate images. Due to the fundamental band component, the obtained signal has a broader band than that in the case of pulse inversion. It was confirmed that AM-CNDE achieves TID with almost the same resolution as that by pulse inversion. AM-CNDE is thus considered to be a promising candidate for THI.

Tissue harmonic imaging with CMUT probe by amplitude modulation

Tissue harmonic imaging (THI) based on amplitude modulation by controlling the number of driving elements (AM-CNDE) with a prototype CMUT probe is demonstrated. AM-CNDE requires multiple transmission pulses and the amplitude of each pulse is controlled by changing the number of driving channels in a probe. The harmonic signals are extracted clearly even if the CMUTs have large nonlinearity. The beam patterns of these transmissions and extracted harmonic components are investigated using two approaches: nonlinear propagation simulation and measurement using a manufactured CMUT probe that is able to transmit sufficient pressure (over 1.5 MPa) to generate tissue harmonic components. Results show that the extracted harmonic beam is narrower than that of the fundamental component. Moreover, practical image quality can be obtained by AM-CNDE with the manufactured CMUT probe and a commercially available ultrasound scanner.

Correction to: Accuracy and limitations of vector flow mapping: left ventricular phantom validation using stereo particle image velocimetory

The original version of this article unfortunately contained a mistake. The conflict of interest statement was missing in the article. The CoI statement is given below.