吉田 憲司

目的：二つの異なる周波数の超音波ビームを交差させ，交差領域の造影剤の非線形振動により発生する和音もしくは差音の周波数成分を用いて血流の速度を測定する二ビーム交差型コントラストエコー法（CBCE法）を考案した．本手法は，造影エコーと組織エコーの比を改善できるため，MTIフィルタ等のハイパスフィルタを用いることなく，血流速度を測定することができる．本論文では，血流模擬ファントムと造影剤を用いて実験的な検討を行い，基本波，高調波と比較して和音を用いる利点を明らかにする．対象と方法：10 mm/s程度の低流速の定常流を実現できるフローシステムを構築し，CBCE法を用いて流速測定を行った．造影剤としてソナゾイド^®を用いた．エコー信号に設定するレンジゲートの幅を変化させながら造影エコーと組織エコーの比率を変化させ，基本波成分，第二高調波成分および和音成分のドプラ周波数から算出した流速値を比較した．結果と考察：レンジゲート中に組織エコーが含まれる場合において，和音成分の組織エコー対造影エコー比が基本波，高調波に比べて大きいことを確認した．他の周波数成分を用いた結果と比較して，和音のドプラ周波数を用いた計測値の傾向が参照値とよい一致を示すことを確認し，レンジゲート中に組織エコーが含まれる場合でも血流を測定できる可能性を示した．結論：CBCE法における和音成分のドプラ周波数から造影エコー対組織エコー比が改善されることを明らかにした．

In this study, the speed of sound (SOS) of two types of rat livers (eight normal livers, four cirrhotic livers) was measured with a scanning acoustic microscope using two transducers, one of which had an 80-MHz and the other a 250-MHz center frequency. The 250-MHz transducer had a better spatial resolution adapted to studying fiber or hepatic parenchymal cells. In normal livers, averages of the SOS values were from 1598 to 1677 m/s at 80-MHz and from 1568 to 1668 m/s at 250-MHz. In the fiber tissue of cirrhotic livers, averages of the SOS values were from 1645 to 1658 m/s at 80-MHz and from 1610 to 1695 m/s at 250-MHz, while the SOS values in the other tissue of cirrhotic livers ranged from 1644 to 1709 m/s at 80-MHz and from 1641 to 1715 m/s at 250-MHz. In one liver, SOS in fiber tissue was larger than that of tissues without fiber while in others it was lower. The resulting two-dimensional SOS maps provide a unique quantitative insight of liver acoustic microstructures in a healthy liver and in a cirrhotic ones. This study would be helpful to understand the complex relationship between acoustic properties and liver disease including fiber tissue. (C) 2016 Acoustical Society of America.

© 2015 IEEE. The frequency dependence of shear wave velocity provides significant information for evaluating viscoelastic character of tissue relating to liver fibrosis. Although Voigt model has been often used in viscoelastic analysis, several studies showed that the frequency dependence measured by dynamic mechanical analysis (DMA) test was not consistent with the theoretical prediction. To experimentally investigate the relationships of the change of the tissue structure and the viscoelasticity of tissue, the shear wave velocity of fatty and fibrotic livers of rat model was quantitatively measured by using shear wave elastography (SWE) and DMA test. In DMA test, shear wave velocity was calculated from the complex elasticity modulus; storage and loss elastic modulus. The difference in shear wave velocity between fatty and fibrotic livers was evaluated to be 0.27 m/s in SWE and 0.20 m/s in DMA test.

Targeted microbubbles can accumulate in the targeting site via a biochemical reaction. We propose a system for evaluating the accumulation of the bubbles using a quartz crystal microbalance (QCM) as the thickness shear-mode resonator. We derive the formula of the electrical admittance of QCM for the load of bubbles by modeling the reaction force generated by the specifically adsorbed bubbles. The bubble-load model enabled the calculation of the number density of the adsorbed bubbles on the QCM surface. A comparison with experimental results showed that the proposed system yields the possibility of quantitatively estimating the number density. This system will help us to develop new targeted microbubbles. (C) 2015 The Japan Society of Applied Physics

Although there have been several quantitative ultrasound studies on the methods of estimation of scatterer size and acoustic concentration based on the analysis of RF signals for tissue characterization, some problems, e.g., narrow frequency bandwidths and complex sound fields, have limited the clinical applications of such methods. In this report, two types of ultrasound transducer are investigated for the estimation of the scatterer size and acoustic concentration in two glass bead phantoms of different weight concentrations of 0.25 and 2.50% and those in an excised pig liver. The diameters of the glass beads ranged from 5 to 63 mu m with an average of 50 mu m. The first transducer is a single element and the other is a linear phased array. A comparison of the estimations obtained using both transducers gives an insight into how these methods could be applied clinically. Results obtained using the two transducers were significantly different. One of the possible explanations is that beamforming could significantly affect the backscatter coefficient estimation, which was not taken into account. (C) 2015 The Japan Society of Applied Physics

In this paper, we propose a technique to detect the surface oscillation of an attached bubble with radius ranging from 20 to 200 mm in an acoustic standing wave using a laser Doppler vibrometer (LDV) and a high-speed camera. The threshold condition, where the surface oscillation mode of the bubble was excited, was investigated for three different driving frequencies of 28, 39, and 81 kHz. Frequency spectrum analyses of bubble oscillation measured by the LDV and the images of the bubble simultaneously obtained by the high-speed camera experimentally demonstrated that the surface oscillation was excited when the power-law dependence Delta Rf-th = AR(0)(B) was satisfied. Here, R-0 is the initial bubble radius and Delta Rf-th is the oscillation displacement of the bubble for the fundamental frequency of the incident ultrasound under the threshold condition where the displacement for the subharmonic component abruptly increased. Interestingly, the coefficient B was independent of the driving frequency. This result suggests that the proposed system can be used to check the validity of current models of surface instability on an oscillating bubble.

In this study, we investigated the bubble induced serious damage to tissue mimic exposed to 27-kHz ultrasound. The initial bubble radius ranged from 80 to 100 mu m, which corresponded approximately to the experimentally-evaluated resonant radius of the given ultrasound frequency. The tissue mimic consisted of 10 wt% gelatine gel covered with cultured canine kidney epithelial cells. The collapsing bubble behaviour during the ultrasound exposure with negative peak pressures of several hundred kPa was captured by a high-speed camera system. After ultrasound exposure, a cell viability test was conducted based on microscopic bright-field images and fluorescence images for living and dead cells. In the viability test, cells played a role in indicating the damaged area. The bubble oscillations killed the cells, and on occasion detached layers of cultured cells from the gel. The damaged area was comparable or slightly larger than the initial bubble size, and smaller than the maximum bubble size. We concluded that only a small area in close proximity to the bubble could be damaged even above transient cavitation threshold. (C) 2014 Elsevier B.V. All rights reserved.

Specific adsorption of biotinylated microbubbles to streptavidin was evaluated by measuring the resonant frequency of an AT-cut quartz crystal microbalance (QCM). Streptavidin was fixed via self-assembled monolayers coated onto the QCM electrode. The resonant frequency of the QCM decreased as a result of specific adsorption of the biotinylated microbubbles, compared with the results for microbubbles containing no biotin. Additionally, there was significant evidence indicating that the frequency shift was caused by the internal gas of the microbubble, as well as the mass of the outer-shell material surrounding the gas. These results suggest that the QCM measurement system can be used effectively to evaluate the specific adsorption of targeted microbubbles. (E-mail: buj3076@gmail.com or takashige.nnn@gmail.com) (C) 2014 World Federation for Ultrasound in Medicine & Biology.

Hollow microcapsules have been considered for potential applications as drug or gene carriers. This paper describes an investigation into the mechanical properties of microcapsules with a biocompatible polylactic acid (PLA) shell that can be destroyed using ultrasound irradiation. The microcapsules had a radius of 1 to 25 mu m and a shell thickness of 100 nm to 3 mu m, and their response to ultrasound pulses with a center frequency of 700 kHz to 2 MHz was investigated. It was found that approximately 50% of capsules with a radius of 20 pm were destroyed using pulses with a pressure amplitude of 50 kPa and a frequency of 700 kHz, which is close to the resonance frequency of the capsules. (C) 2013 Elsevier Ltd. All rights reserved.

Ultrasound sonification stimulates the release of pharmaceutical compounds from hydrogels. At the surface of hydrogels, cavitation, cavities formed in liquid, activates to stimulate that release under low-frequency ultrasound. Under high-frequency ultrasound, although cavitation activities are highly suppressed, the compounds are still released. Although it remains elusive how high-frequency ultrasound stimulates this release, one hypothesis is that the internal diffusivity is enhanced. In this study, internal diffusivities in agarose gels were estimated with fluorescent recovery after photobleaching (FRAP) analysis. Under 1-MHz ultrasound sonification, internal diffusivity in agarose gels was enhanced. The enhancement of internal diffusivity was larger than that with temperature elevation alone, although temperature elevation was also observed along with the ultrasound sonification. Thus, we found that high-frequency ultrasound sonification enhances internal diffusivity in agarose gels. This enhancement was, at least in part, independent of temperature elevation. (C) 2013 Elsevier Ltd. All rights reserved.

An ultrasound bubble filter without a mesh structure was developed for extracorporeal circulation. The filter utilizes an ultrasound standing-wave field generated by flexural vibration and removes micro air bubbles from the circulation. The ultrasound filter consists of an aluminum cylinder, two annular ultrasound transducers, and three connectors to the flow path. The configuration of the filter was determined through finite element analysis computations. Flexural vibrations at 46 and 199 kHz were generated in prototypes with lengths of 69 and 130 mm. Two driving modes (both standing-wave and traveling-wave modes) were used to evaluate the filtering characteristics in a water circulation system. The bubble size decreased by increasing the input voltage to the filter; the average diameter changed from 60 to under 10 gm when the input voltage changed from 0 to 200V. The total volume of air in the main flow estimated from the size distribution of the bubbles decreased to 1% for the 130-mm-long filter excited with the traveling-wave mode. (C) 2013 Elsevier B.V. All rights reserved.

Double-strand breaks in giant DNA molecules caused by continuous ultrasound at a frequency of 30 kHz were quantified using single-molecule observations. The effect of the sound pressure was investigated by placing a tube containing DNA solution under an anti-node of the acoustic standing wave. Almost no breaks occurred below the threshold sound pressure. Above this threshold, the probability of strand breaks increased linearly with sound pressure. Acoustic cavitation detected with a hydrophone strongly suggests that the main mechanism of the DNA strand break is via cavitation generated by the ultrasound. (C) 2013 AIP Publishing LLC.

An increase in cytoplasmic calcium (Ca(2+) increase) is a second messenger that is often observed under ultrasound irradiation. We hypothesize that cavitation is a physical mechanism that underlies the increase in Ca(2+) in these experiments. To control the presence of cavitation, the wave type was controlled in a sonication chamber. One wave type largely contained a traveling wave (wave type A) while the other wave type largely contained a standing wave (wave type B). Fast Fourier transform (FFT) analysis of a sound field produced by the wave types ascertained that stable cavitation was present only under wave type A ultrasound irradiation. Under the two controlled wave types, the increase in Ca(2+) in L929 fibroblasts was observed with fluorescence imaging. Under wave type A ultrasound irradiation, an increase in Ca(2+) was observed; however, no increase in Ca(2+) was observed under wave type B ultrasound irradiation. We conclude that stable cavitation is involved in the increase of Ca(2+) in cells subjected to pulsed ultrasound. (C) 2011 Elsevier B.V. All rights reserved.

The direction of the secondary Bjerknes force between a free bubble and an attached bubble was experimentally investigated. The behavior of the two bubbles in an ultrasonic standing wave of 27 kHz was observed using an imaging system with a high-speed video camera. It was demonstrated experimentally that the direction of the force reversed at a specific separation distance between the two bubbles, which was defined as the threshold distance. The threshold distance changed with the radius of the attached bubble. In addition, a theoretical calculation was performed using a previously derived model that coupled the vibrations of two free bubbles [Ida, Phys. Lett. A 297, 210-217 (2002)]. The experiment data for the threshold distance qualitatively agreed with the theoretical predictions, except when the separation distance was very small. Then, it was discovered that the free bubble became trapped near the attached bubble when the separation distance between the two bubbles was very small. This indicated that a stable equilibrium point for the separation distance exists that cannot be predicted by the theoretical model. (C) 2011 Acoustical Society of America. [DOI: 10.1121/1.3592205]

We constructed the experimental system with a laser Doppler vibrometer (LDV) for measuring the vibration of a single microbubble. It was demonstrated that the system enabled the capture of the vibration with an amplitude of nanometer order. We attempted to experimentally measure the resonant characteristics of a bubble attached to a wall by using the system. As a result, we succeeded in measuring the characteristics and evaluating the Q factor and the resonant radius at a driving frequency of 27.8 kHz, although these values are different from those predicted on the basis of the theory for a single free bubble. The LDV measurement system is expected to an effective tool for evaluating bubble vibrations with very small displacement amplitudes, such as the vibration of a microcapsule. (c) 2011 The Japan Society of Applied Physics

Observation techniques for measuring the small vibration of a single microcapsule of tens of nanometers in an acoustic standing wave field are discussed. First, simultaneous optical observation of a microbubble vibration by two methods is investigated, using a high-speed video camera, which permits two-dimensional observation of the bubble vibration, and a laser Doppler vibrometer (LDV), which can observe small bubble vibration amplitudes at high frequency. Bubbles of tens of micrometers size were trapped at the antinode of an acoustic standing wave generated in an observational cell. Bubble vibration at 27 kHz could be observed and the experimental results for the two methods showed good agreement. The radial vibration of microcapsules with a hard plastic shell was observed using the LDV and the measurement of the capsule vibration with radial oscillation amplitude of tens of nanometers was successful. The acoustic radiation force acting on microcapsules in the acoustic standing wave was measured from the trapped position of the standing wave and the radial oscillation amplitude of the capsules was estimated from the theoretical equation of the acoustic radiation force, giving results in good agreement with the LDV measurements. The radial oscillation amplitude of a capsule was found to be proportional to the amplitude of the driving sound pressure. A larger expansion ratio was observed for capsules closer to the resonance condition under the same driving sound pressure and frequency.

Observation techniques for measuring the vibration of a single microcapsulein an acoustic standing wave field were discussed. First, optical simultaneousobservation of a microcapsule vibration by two methods was investigated, using ahigh-speed video camera, which permits two-dimensional observation of thecapsule vibration, and a laser Doppler vibrometer (LDV), which can observe smallcapsule vibration amplitudes at high-frequency. Capsule of tens of micrometersize was attached to a tape and oscillation behavior was observed at theantinode of an acoustic standing wave generated in an observational cell. Also,the biodegradable microcapsules are trapped at the antinode of the standing waveand the possibility of measuring vibration amplitude of microcapsule in thefree field was suggested. © 2010 IEEE.

We propose a new contrast-echo method using the counter-crossed beams of two ultrasonic frequencies for ultrasound diagnosis of cancer and call it the counter-cross beam contrast echo (C-CBCE) method. In this C-CBCE method, the sum-or difference-frequency components generated by the nonlinear vibration of microbubbles driven by dual-frequency ultrasound are used for measuring flow velocity. The C-CBCE method has the potential to measure low blood flow velocities, since the echo from the blood flow is efficiently separated from the echo from the biological soft tissues using the sum-or difference-frequency component. Sonazoid is used as microbubbles, and the sum frequency is used in the experiments. We prototyped the measurement system, which consists of commercially available ultrasound diagnostic equipment with a convex array probe and another ultrasonic probe, in order to determine the feasibility of the C-CBCE method in clinical diagnosis. In this study, we employ the C-CBCE method to measure the flow velocity of microbubbles in a channel using the sum-frequency component generated by the nonlinear vibration of microbubbles. We demonstrate theoretically the relationship between the Doppler-shift frequency of the sum-frequency component and the flow velocity in a channel. In order to confirm the theory on which this method is based, we then determine flow velocities on the basis of the sum-frequency component, fundamental component, and second-harmonic component. (C) 2010 The Japan Society of Applied Physics

In this paper, we demonstrate the cleaning ability of oscillating bubbles, on the basis of optical observations of bubble behaviors and cleaning process using a high-speed video camera. It was shown that a bubble cluster was formed after the collapse of the initial bubble. The bubble cluster consisted of a main bubble and many small bubbles. In addition, optical observations clearly revealed that the oscillations of these microbubbles removed a thin ink film bonded to the surface of a slide glass. From the analysis of the relationship between bubble oscillation and cleaning effect, it was confirmed that the cleaned area increased with an increasing amplitude of bubble oscillation, although there was a poor correlation between cleaned area and bubble oscillation. Moreover, the observed results suggested that a pressure ranging from several tens kPa to MPa was generated on the slide glass by hydrodynamic phenomena; micro jet and secondary wave were emitted by the oscillating bubble. Our experimental results lead to the understanding of the cleaning mechanism in ultrasonic cleaning techniques. (C) 2010 The Japan Society of Applied Physics

In order to clarify the mechanical effects caused by micro-bubbles in liquids, the detailed behavior of a single bubble adhered to a solid surface was observed was observed optically using an ultra high-speed video camera (maximum recording rate: 1,000,000 frames/s). Making use of a thin piezoelectric transducer on the solid surface, the pressure signal caused by the bubble behavior was simultaneously observed. In addition, the influence of the initial bubble radius R(0) on mechanical effects was investigated. It was confirmed that a micro-jet was generated from the center of the bubble, which was followed by a counter-jet, in the range of R(0)/R(res) approximate to 0.65-1.10 (R(res): the resonance radius of the free bubble). An impulsive positive pressure to the surface was also observed synchronously with the momentary increase of bubble volume acceleration. Moreover, it was found that the strength maximum value when R(0)/R(res) was approximately 0.9.

To investigate the physical effects of a collapsing bubble oil a soft material, the bubble collapse phenomenon oil agar gel under ultrasonic irradiation was experimentally examined. Using a high-speed video camera, we observed two effects of the bubble on the agar gel. One effect was the formation of a hole on the afar surface by "microjet generation". It was revealed that the hole was formed under the initial condition of resonant bubble size. The other effect was the local surface fluctuation of the agar gel by "bubble vibration". This surface fluctuation is the observed phenomenon in our observations and indicates that a localized pressure stimulus was generated on the agar surface by bubble vibration. These revealed effects show the significant contributions of the bubble techniques in ultrasonic therapy techniques, e.g., ultrasonic drug delivery. [DOI: 10.1143/JJAP.47.4200]

We propose a novel contrast echo method using crossed beams of two different ultrasonic frequencies as an ultrasound method of diagnostic cancer, which we call the crossed beam contrast echo (CBCE) method. The sum or difference frequency components that are derived from the nonlinear oscillation of the microbubbles driven by dual-frequency ultrasound are used in the CBCE method. The blood flow information call only be obtained in the area of the crossed beams, because no sum or difference, frequency components are generated by the propagation of biological soft tissues except in the crossed beam area. We discussed the driving conditions under which the sum frequency component is most efficiently generated and experimentally confirmed the area in which the sum frequency component is generated. As a result, the sum frequency component was larger than the other harmonic components when its frequency was equal to the resonance frequency of the microbubbles. It was confirmed that the area in which the large sum frequency component is generated was equal to the crossing area of the two beams. [DOI: 10.1143/JJAP.47.4188]

The contrast echo imaging is based on detecting second order harmonic components in echo signals, because nonlinear vibration of microbubbles in the blood vessels gives second order harmonic components in scattered waves. However, the separation of the echoes from the microbubbles in the blood flow to the echoes from biological soft tissues is an important issue, because the harmonic components are also generated during the propagation through biological soft tissues as well as the nonlinear oscillation of the microbubbles. In order to solve this problem we propose a novel contrast echo imaging method using the sum or difference frequency component in the echoes reflected from the contrast agents in the crossing area of the two ultrasonic beams with different frequencies, which we call Crossed Beams Contrast Echo (CBCE) method. In CBCE method, the sum or difference frequency components are generated only in the crossing area of beams. Therefore it is capable of separating the microbubble echoes from the soft tissue echoes, so that the microvascular blood flow information can be obtained in detail by using these specific frequency components. In this study, the sum frequency components which were generated by the commercially available contrast agents "Sonazoid" in the flow tube were measured by transmitting two ultrasonic waves of 2.8MHz and 2.0MHz. As a result, it is indicated that CBCE method is applicable for the blood flow measurement, especially for the separation of the microbubble echoes from the tissue echoes.

マイクロカプセルはシェルを持ちその内部に気体もしくは薬液などの液体を保持することができる。シェルの役割は、周囲物質と内部物質を隔離すること、内部物質の拡散を防ぐことである。このためマイクロカプセルは様々な分野で利用されている。特に、医療分野において、遺伝子治療やドラッグデリバリーシステム(DDS)における搬送体としての役割が期待されている。搬送体として応用する場合、内容物を放出させるためにカプセルを崩壊させる必要があり、マイクロカプセルの崩壊を制御することが重要な技術課題となる。またこれらの応用技術にあたって、マイクロカプセルの崩壊情報は、崩壊するマイクロカプセルが発する音響信号を解析することにより得ることが望ましい。しかしながら、マイクロカプセル崩壊時の音響信号特性について詳述した報告は少ない。そこで、マイクロカプセル崩壊時の音響特性をより明確に得ることを目的とし、高速度ビデオカメラを用いてマイクロカプセルの崩壊挙動を明確に確認するとともに、カプセルによる二次的な音響放射の観測を試みた。