Motoyoshi Okujima

Underground Imaging Using Shear Waves: Stacking Method of the Reflected Scattered Waves

Electrical and electromagnetic methods are widely used to detect buried relics. However, in marshes or swamps where the moisture constant is high, these methods are difficult to use, because the electromagnetic wave is strongly attenuated by the presence of water. Although seismic prospecting utilizing elastic waves and the common depth point stacking method (CDP method), which assumes reflection at the stratum plane, are suitable for the investigation of stratigraphic structure ( e.g., at depths of 100 to 1000 m ), commercial equipment and the CDP method are not suitable for detecting relics which exist at shallow depths ( e.g., 0–10 m ). Therefore, we proposed an underground imaging method using shear waves and a method based on stacking of reflected scattered waves in which point reflection is assumed. The experimental results confirm the feasibility of our proposed method. However, the resolution of the underground image is insufficient to detect relics. Therefore, an improvement in the resolution is necessary.

Effect of Calcium Chloride on Sodium Alginate Microbubbles as Ultrasound Contrast Agent

In order to stabilize microbubbles without coating layers to be used as ultrasound contrast agent, the viscosity of the liquid for storage has to be increased. Sodium alginate is a material which forms viscous liquid when its powder is dissolved in water. The viscosity may vary from 1 cP to about 100 cP when the amount of material used is increased from 0.1% to 1%. However, the viscosity is still not enough to make stable microbubbles which can be stored for over several weeks. In this paper, by adding calcium chloride to the sodium alginate solution, a gel-type solution is produced which has a high viscosity capable of holding microbubbles for a long time. By measuring the viscosity of the sodium alginate solution and the solution with calcium chloride solution added, it is known that the solution is changed from a Newtonian liquid to a non-Newtonian liquid when calcium chloride solution is added. Since the attenuation coefficient of microbubble dispersion is proportional to the number of microbubbles, the disappearance of microbubbles lowers the attenuation coefficient. Therefore, in this study, the attenuation coefficient is used as an estimating parameter for the lifetime of microbubbles.

Simulations of Contrast Effects from Free Microbubbles in Relation to Their Size, Concentration and Acoustic Properties.

To gain a deeper insight into the diagnostic images produced by microbubbles as ultrasound contrast agents, the basic acoustical properties such as the scattering cross section, extinction cross section and the expected backscattered power from different sized microbubbles are calculated. The results proved that the ratio of scattering to extinction cross section, size and concentration of microbubbles are the factors determining the maximum backscattered power received from microbubble dispersions. When microbubbles decrease in size, the energy loss by microbubbles upon ultrasound exposure gradually changes from loss by scattering to loss by absorption. As a result, the concentration of microbubbles required to produce a B mode image of the comparable brightness is increased. However, an increased concentration of microbubbles does not necessarily produce a clearer image. In this paper, it is further established that an optimum concentration exists at which the microbubbles give maximum backscattered power. The optimum microbubble concentration and the maximum backscattered power also depend on the depth of the microbubbles in the microbubble dispersion. In addition, the concentration of microbubbles necessary to produce the maximum backscattered power during resonance is less than that at other frequencies.

Change in size and number of sodium laurate microbubbles with time in saline at different air concentrations

The study of the annihilation mechanisms of microbubbles is a very important, topic in the development of ultrasound contrast agents. To gain better insight into the annihilation mechanisms, knowledge of the change in the number and size of microbubbles with time in saline is necessary. In this study, the use of a Coulter Multisizer is proposed, and the change in number and size of microbubbles is accurately measured. As a result, mechanisms for longevity and annihilation are suggested. Sodium laurate is used for tlle coating layer of tlle microbubbles. The measured longevity of sodium laurate microbubbles is compared with the results of a simulation of free air microbubbles. Although the coating layers lower the diffusion rate of air from microbubbles, diffusion still occurs. Tlle diffusion of air contributes to the size of the microbubbles and the dissolution of the surfactant coating layer causes a decrease in the number of microbubbles. The reasons for these assertions are discussed in this paper. When the concentration of dissolved air in saline is close to saturation, the annhilation mechanism of sodium laurate microbubbles changes to the dissolution of tlle coating layer into the solution. In addition, it is proposed from the experimental results that small particles less than 10 μm in size which dissolve slowly in saline are formed after the shrinking of microbubbles by diffusion.

Underground Imaging Using Shear Waves –Study on Three Dimentional Underground Imaging–

We proposed an underground imaging technique that uses shear waves and a method based on the stacking of reflected scattered waves. In order to study a three-dimensional underground image, geophones were arranged in a plane and three targets were buried at different depths. We tried to detect these targets using cross-sectional images. From the experimental results, we were able to find the buried targets; however, improvements are necessary to make a three-dimensional image.

Study on Detection of Buried Steel Bar in Concrete with Electromagnetic Impact Driving Method

The authors devised a new method of detecting the location of a steel bar within concrete from the concrete surface by an electromagnetic impact driving method. Various experiments were performed to test the possibility of detection by this method. The amplitude characteristics of radiated sound from such a bar were also examined. Consequently, it is clear that the electromagnetic impact driving method is useful for detection of the location of a steel bar buried in concrete.

Acoustic properties of surfactant microbubbles in relation to their lifetime in vitro as determined by diffusion

In this study, to make microbubbles less than 10 µm and to extend their lifetime, sodium laurate is chosen as the material for the coating of microbubbles. In studies of ultrasound contrast agents, the authors are the first to report on the use of sodium laurate as the material for manufacturing the microbubble contrast agent. In addition, the method of producing microbubbles using a syringe is proposed. Using sodium laurate and the syringe method, microbubbles with diameter less than 10 µm are produced. From the result of simulation, it is found that the dissolved gas concentration in the solution surrounding microbubbles plays an important role in determining microbubble lifetime. Microbubbles without coating are calculated to have a lifetime of less than 1 min when the dissolved concentration of gas is about 90%. On the other hand, the lifetime of microbubbles made from sodium laurate is about 5 min longer than the calculated lifetime. Therefore, it is hypothesized that when the dissolved gas in the solution is above a certain level, the mechanism governing annihilation of microbubbles is the dissolution of the surfactant coating layer into the solution, instead of the diffusion of gas from the microbubbles. Since the dissolution time of surfactants is longer than the dissolution time of gas, the lifetime becomes longer.

Relationship between the Harmonic Components and Illuminating Frequency for the Microbubbles : Experimental and Numerical Consideration

The nonlinear response of microbubbles has been considered as a basis for ultrasound blood vascular image enhancement system using nonlinear behavior of microbubbles. In this paper, the relationship between the condition of microbubble samples and illuminating conditions of ultrasound for the microbubbles coated with a surfactant membrane are studied experimentally and by numerical simulation. The effect of the number of ultrasound pulse shots, illuminating frequency and size distribution of bubbles are considered, experimentally. To simulate the relationship between the illuminating frequency of ultrasound exposed to the bubbles and the behavior of the harmonic component, a mathematical model, taking into consideration the compressibility of the surrounding liquid around the bubbles and the effect of the coated membrane is proposed.

Fundamental examination of cattle red blood cells damage with ultrasound exposure microscopic system (UEMS)

In order to define the safety limit of the medical ultrasound equipment and to study the ultrasonic bioeffects, ultrasound exposure microscopic system (UEMS) was developed in our laboratory. We intended to observe the tissue damage caused by ultrasound exposure in real time. The change of size and shape of the cattle red blood cells was investigated experimentally with UEMS. It was observed that, at a driving frequency of 2.2 MHz, when the ultrasound intensity ISPTP was 28.4 W/cm2, the change of shape of the cattle red blood cells, due to the ultrasound exposure, occurred after 30 min of exposure. The change of size of the cattle red blood cells was investigated with a Coulter counter. The results, indicated that the size of the red blood cells after exposure to ultrasound was smaller than before exposure. Moreover, the results also indicated that, there was a tendency of the red blood cells to decrease in size after the ultrasound exposure when the ultrasound intensity was increased. At the two driving frequencies of 2.2 MHz and 0.55 MHz, in this study, the size of the cattle red blood cells decreased when the ultrasound intensities were increased.

Observation of Underwater Ultrasound Propagation in the Sea by Two-Period M-Sequence Signal Method

The velocity of a current going in and out through a strait can be measured by the differences in underwater sound propagation times with and against the current. However, ambient pulsive noises often interfere with the measurements of sound propagation times when a pulsed sound is used. A two-period M-sequence signal method is therefore proposed which is free from ambient pulsive noises. A sound propagation test was carried out. Short term propagation stability and tidal effects on multipath propagation were observed.