Search

The speed of sound (SoS), which is the propagation speed of longitudinal waves, is expected to be effective for the early detection and monitoring of diseases. Several methods that use handheld probes have been proposed to estimate the SoS. In this study, a modified average SoS estimation method, referred to as the improved focusing method, is proposed to increase the accuracy of average SoS estimation. In the proposed method, a plane wave was transmitted, and the center-element position-dependent variations in the SoS estimation were reduced. Ex vivo experiments were conducted using chicken liver to evaluate the applicability and sensitivity of the proposed method to actual tissues. The livers were subjected to continuous temperature changes to induce changes in the SoS. The results suggest that the improved focusing method measures the average SoS accurately as well as small SoS changes of approximately 1.5 m·s−1·°C−1.

The elastic modulus of tissue as a useful biomarker of disease detection can be quantitatively evaluated based on shear wave speed (SWS) measurements in shear wave elastography. Although the longitudinal wave speed (LWS) is also expected to be a promising biomarker for disease detection, the elasticity is not always dominant because the LWS is affected by the bulk modulus. In other words, LWS and SWS may reflect different tissue properties. Therefore, in this study, based on the improvement in LWS measurement, the relationship between the composition of a phantom mixed with agar and glycerol and ultrasonically measured LWS and SWS was investigated. The LWS had a good sensitivity in detecting glycerol, while the SWS had a good sensitivity in detecting agar. The calculated Poisson’s ratio had a better sensitivity in detecting agar. In conclusion, a simultaneous measurement of LWS and SWS may help identify the tissue composition.

The speed of sound (SOS), which is the propagation speed of longitudinal waves, is a promising index for early detection and monitoring of diseases, and many measurement methods have been proposed. It has already been found that the average SOS can be easily and reasonably estimated for a homogeneous medium based on time-delay processing by utilizing the method using backscattered waves, which enables the use of handheld probes. However, the error caused by the application of the method to an inhomogeneous medium has not yet been clarified. In this study, we investigate the relationship between statistical properties of the SOS distribution and the measurement error through a simulation and a phantom experiment. As a result, it is revealed that if the spatial variation of the SOS distribution is below a certain level, the average SOS can be estimated under the assumption of a homogeneous medium.

The relationships between the intravascular wall shear stress, controlled by flow dynamics, and the progress of arteriosclerosis plaque have been clarified by various studies. Since the shear stress is determined by the viscosity coefficient and shear rate, both factors must be estimated accurately. In this paper, an adaptive method for improving the accuracy of quantitative shear rate estimation was investigated. First, the parameter dependence of the estimated shear rate was investigated in terms of the differential window width and the number of averaged velocity profiles based on simulation and experimental data, and then the shear rate calculation was optimized. The optimized result revealed that the proposed adaptive method of shear rate estimation was effective for improving the accuracy of shear rate calculation.

The mechanical properties, or elasticity and viscosity, of tissues reflect the pathological change of tissues caused by lifestyle-related diseases such as cancer and cardiac disease. Therefore, tissue elasticity imaging has been investigated for several years and a practical system applicable to clinical diagnosis has recently been developed. To diagnose the stage of disease progression and differentiate malignant from benign tumors precisely using parameters that represent tissue elasticity and viscosity, it is required to elucidate the quantitative relationship between these parameters and disease. However, no database for such a relationship has been yet established. With the aim of constructing such a database, we undertook to develop a three-dimensional (3D) tissue viscoelasticity microscope, with which we can quantitatively measure the 3D distribution of parameters of tissue elasticity and viscosity. Results of experiments using a phantom model made of konnyaku and acrylamide confirmed that the system could reveal differences in the elasticity and viscosity of tissues.

Mechanical load to the blood vessel wall, such as shear stress and pressure, which occurs in blood flow dynamics, contribute greatly to plaque rupture in arteriosclerosis and to biochemical activation of endothelial cells. Therefore, noninvasive estimations of these mechanical loads are able to provide useful information for the prevention of vascular diseases. Although the pressure is the dominant component of mechanical load, for practical purposes, the pressure gradient is also often important. So far, we have investigated the estimation of the kinematic viscosity coefficient using a combination of the Navier-Stokes equations and ultrasonic velocity measurement. In this paper, a method for pressure gradient estimation using the estimated kinematic viscosity coefficient is proposed. The validity of the proposed method was investigated on the basis of the analysis with the data obtained by computer simulation and a flow phantom experiment. These results revealed that the proposed method can provide a valid estimation of the pressure gradient.

Tissue elasticity imaging technology is expected to be a new technique for breast disease diagnosis. In clinical measurement, high-speed and freehand manipulation of the probe is required for a practical system. Thus, we developed a tissue elasticity imaging system which performs a stable strain measurement with freehand tissue compression based on the extended combined autocorrelation method. The method enables us to obtain tissue strain distribution at high-speed and suppressing errors due to lateral slip of the probe caused by freehand compression. The developed method can estimate the strain images at about 5 frames/s and was applied to breast disease measurement in vivo. Consequently, it is shown that the system is effective not only for the diagnosis of tissue hardness but also to determine the disease expansion area. It is also confirmed that the method can stably estimate the strain during the breast measurement in vivo.

In this paper, a new parameter that quantifies the intensity of tissue nonlinear elasticity is introduced as the nonlinear elasticity parameter. This parameter is defined based on the empirical information that the nonlinear elastic behavior of soft tissues exhibits an exponential character. To visualize the quantitative nonlinear elasticity parameter, an ultrasonic imaging procedure involving the three-dimensional finite element method (3-D FEM) is presented. Experimental investigations that visualize the nonlinear elasticity parameter distribution of a chicken gizzard and a pig kidney embedded in a gelatin-based phantom were performed. The values extracted by ultrasound and 3-D FEM were compared with those measured by the direct mechanical compression test. Experimental results revealed that the nonlinear elasticity parameter values extracted by ultrasound and 3-D FEM exhibited good agreement with those measured by the mechanical compression test, and that the intensity of tissue nonlinear elasticity could be visualized quantitatively by the defined nonlinear elasticity parameter.

Since the speed of sound (SOS) reflects tissue characteristics and is expected as an evaluation index of elasticity and water content, the noninvasive measurement of SOS is eagerly anticipated. However, it is difficult to measure the SOS by using an ultrasound device alone. Therefore, we have presented a noninvasive measurement method of SOS using ultrasound (US) and magnetic resonance (MR) images. By this method, we determine the longitudinal SOS based on the thickness measurement using the MR image and the time of flight (TOF) measurement using the US image. The accuracy of SOS measurement is affected by the accuracy of image registration and the accuracy of thickness measurements in the MR and US images. In this study, we address the accuracy improvement in the latter thickness measurement, and present an image-processing-based method for improving the accuracy of thickness measurement. The method was investigated by using in vivo data obtained from a tissue-engineered cartilage implanted in the back of a rat, with an unclear boundary.

It is important in regenerative medicine to evaluate the maturity of regenerating tissue. In the maturity evaluation of regenerating cartilage, it is useful to measure the temporal change in elasticity because the maturity of regenerating tissue is closely related to its elasticity. In this study, a quantitative elasticity evaluation of extracted regenerating cartilage samples, which is based on the laser Doppler measurement of ultrasonic particle velocity and calibration, was experimentally investigated using agar-based phantoms with different Young's moduli and regenerating cartilage samples extracted from beagles in animal experiments. The experimental results verified the feasibility of the proposed method for the elasticity evaluation of regenerating cartilage samples.

It is important for regenerative medicine to evaluate the maturity of regenerating tissue. In the maturity evaluation of regenerating cartilage, it is useful to measure the temporal change of elasticity because the maturity of regenerating tissue is closely related to its elasticity. In this study, an elasticity evaluation method for the extracted regenerating cartilage sample, which is based on the laser Doppler measurement of ultrasonic particle velocity, was experimentally investigated using agar-based phantoms with different elastic moduli and the regenerating cartilage samples extracted from beagles in animal experiments. In addition, the experimentally-obtained elasticity was compared with the result of a static compression test. These results verified the feasibility of the proposed method in the elasticity evaluation of regenerating cartilage samples.

It has been reported that the hydroxyl (OH) radical is produced by irradiating ultrasound to titanium dioxide (TiO2). On the other hand, infection at the exit site of catheter retained in the body has become a serious problem. Therefore, we are developing a novel anti-infective catheter system in which the exit site of a catheter is covered with an aminated titanium dioxide (AmTiO2) composite, which consists of AmTiO2 nanoparticles immobilized on a substrate and exhibits cell adhesiveness, and the anti-infective effect (OH radical production) is induced by irradiating low-intensity focused ultrasound (LIFU) to the composite. In this study, the ability to activate TiO2 nanoparticles by ultrasound irradiation was verified in a preliminary experiment, and then the feasibilities that the AmTiO2 composite is activated and that the OH radical is produced by LIFU irradiation were verified by dye decomposition experiment and dihydroxybenzoic acid (DHBA) measurement. In addition, as a trigger for activating the AmTiO2 composite by LIFU irradiation and for producing the OH radical, the decrease in cavitation threshold was suggested.

The abnormalities of myocardial wall motion caused by changes in wall stiffness often appear in the early stage of ischemic heart disease. Since the myocardium exhibits complex and large motion, a two-dimensional (2D) or three-dimensional (3D) assessment of stiffness distribution is required for accurate diagnosis. Although a 3D assessment is ultimately required, as a stepped approach for practical use, we propose novel methods for tracking the 2D motion using a one-dimensional (1D) phased array and for assessing myocardial malfunction by visualizing the invariant of a strain tensor. The feasibilities of the proposed methods were evaluated by numerically simulating the short-axis imaging of a 3D myocardial model. This model includes a hard infarction located between 1 and 3 o'clock, which is difficult to detect by conventional tissue Doppler and strain rate imaging, and the motions of the model were assigned by referring to actual myocardial motion. These results revealed that the proposed imaging methods clearly depicted the hard infarction area which conventional imaging could not detect.

For assessing the tissue elastic properties based on 3D information of displacement and strain induced by static deformation, we propose an estimator of 3D displacement vectors. This method is composed of the combination of the weighted phase gradient method (WPG), which utilizes the gradient of the plane obtained from the phase shift at each receiving element on the 2D array, and combined autocorrelation method (CA), which is phase domain processing without aliasing. Large displacements required for highly accurate measurements can be measured by this method. In addition, we develop a tissue elasticity reconstruction method using the measured 3D displacement vectors. Computer simulations reveal that our methods are valid for evaluating the elastic structures of tissues based on 3D information.

Although the technique for measuring blood flow based on the ultrasonic Doppler method is an important means for clinical diagnosis, it is inappropriate for quantitative assessment of blood flow since the true three-dimensional (3-D) velocity component cannot be obtained in many cases. In this paper, to overcome the restriction, a new method of 3-D velocity vector measurement is proposed. By detecting the gradient of the weighted phase shift distribution on the receiving aperture, the proposed method enables us to employ a smaller single aperture compared with those used in conventional techniques, and its sensitivity is independent of the direction of transverse flow. Furthermore, real-time measurement can be realized since the autocorrelation technique is utilized for the phase shift extraction as well as the present Doppler method. The basic performance of the method is evaluated by computer simulation. Moreover, sector scan imaging is constructed by the proposed method and compared with the conventional method. The obtained results validated the feasibility and ability of the proposed method for measuring 3-D velocity.

Ultrasonic pulse Doppler tomography is generally applied to noninvasive vascular measurement. The present method, however, measures only one component along the beam direction of a three-dimensional (3-D) velocity vector and provides useful information only if the beam direction in relation to the flow velocity vector is known. Images obtained by such systems are not only unsuitable for quantitative evaluation and but also prone to misinterpretation. On the other hand, in terms of real-time data acquisition achieved by scanning 3-D space, a two-dimensional (2-D) array probe is expected to become indispensable for future clinical applications. From this point of view, we have proposed a method of measuring a 3-D velocity vector using the 2-D array probe. In this paper, the principles of the proposed approach are introduced and experimental data obtained using a prototype system and the flow phantom are processed. Experimental results validate the usefulness of the method and its potential for practical use.