Takuso Sato

Ultrasonic imaging of internal vibration of soft tissue under forced vibration

An imaging system that can display both the amplitude and phase maps of internal vibration in soft tissues for forced low-frequency vibration is described. In this method, low-frequency sinusoidal vibration of frequency under several hundred hertz is applied from the surface of the sample and the resulting movement in it is measured from the Doppler frequency shift of the simultaneously transmitted probe ultrasonic waves. Basic experiments are carried out by using 3.0-MHz ultrasonic waves. The two-dimensional maps of the amplitude and phase of internal vibration are shown, and the velocities of vibration are measured for some samples as well as in vivo.<<ETX>>

Sonoelastic determination of human skeletal muscle elasticity

It is not currently practical to directly measure viscoelastic parameters in human muscles in situ. Methods used in vitro cannot readily be applied, and motion analysis provides only a gross estimate. We report on the application of a hybrid approach, sonoelastography, which uses ultrasound to measure the propagation of shear waves induced by externally applied vibrations. Because shear waves predominate in incompressible viscoelastic media at low frequencies, sonoelastic data should be comparable to those obtained using conventional means. We recorded vibration propagation speeds as a function of applied load in the quadriceps muscles of ten volunteers as they underwent a series of static contractions. Data collection during dynamic contractions, not possible with the current equipment, will be the subject of future experimentation. Although statistically significant correlations were not uniformly obtained above 60 Hz nor for propagation perpendicular to the muscle fibers, this is felt to have resulted from deviations from the applied plane wave model. Calculated values of Youngs modulus for 30 Hz propagation parallel to the muscle fibers were 7 +/- 3, 29 +/- 12 and 57 +/- 37 x 10(3) Nm-2 for applied loads of 0, 7.5 and 15 kg, respectively. The corresponding values at 60 Hz were 25 +/- 6, 75 +/- 61 and 127 +/- 65. These values were statistically significant and linearly correlated with the applied load, as expected. Our data represent the first in situ human measurements of their kind. It is anticipated that sonoelastography will provide a useful adjunct to the study of human biomechanics.

Tomographic image reconstruction from limited projections using iterative revisions in image and transform spaces

An iterative technique is proposed for improving the quality of reconstructions from projections when the number of projections is small or the angular range of projections is limited. The technique consists of transforming repeatedly between image and transform spaces and applying a priori object information at each iteration. The approach is a generalization of the Gerchberg-Papoulis algorithm, a technique for extrapolating in the Fourier domain by imposing a space-limiting constraint on the object in the spatial domain. A priori object data that may be applied, in addition to truncating the image beyond the known boundaries of the object, include limiting the maximum range of variation of the physical parameter being imaged. The results of computer simulations show clearly how the process of forcing the image to conform to a priori object data reduces artifacts arising from limited data available in the Fourier domain.

Imaging the nonlinear ultrasonic parameter of a medium

A technique for imaging the nonlinear ultrasonic parameter B/A has been developed. The nonlinear parameter describes the dependence of ultrasonic velocity on pressure and may well provide a new and powerful tool for characterizing both biological and nonbiological media. Our approach is based on observing the interaction of two ultrasonic waves with different frequencies and power levels. A low frequency pump wave, with power level suitable for medical diagnosis (1 mW/cm2), is used to sinusoidally modulate the sound pressure over the region of interest. A much lower intensity high-frequency probe beam is propagated perpendicularly to the pump beam. The phase of the probe wave is modified in proportion to the integral of the product of the nonlinear parameter B/A and the pressure of the pump wave, which varies sinusoidally along the probe beam. This phase change provides a Fourier component of the distribution of the nonlinear parameter B/A for the spatial frequency corresponding to the inverse of the pump wavelength. By changing the frequency of the pump waves, the spatial frequency is changed and a set of spatial Fourier coefficients of the distribution of the nonlinear parameter B/A is obtained. An inverse operation then gives the one-dimensional image along the probe beam. If the probe beam is scanned mechanically, the entire cross-sectional image is obtained. Several images of the nonlinear parameter of biological objects were generated with our system. To our knowledge, these represent the first images of the nonlinear parameter to be reported in the literature. The nonlinear parameter of water was also measured and agreed well with values obtained by other techniques.

Real-Time Nonlinear Parameter Tomography Using Impulsive Pumping Waves

Absrruci-In this paper a novel real-time tomographic system for imaging the nonlinear parameter B/A of biological objects is proposed. This parameter is related closely to the detailed structure and property of tissues, and may well provide a new dimensional and powerful tool for ultrasonic tissue characterization. In this system an impulsive, relatively high-power pumping wave is applied from a direction perpendiculax to the continuous low-intensity high-frequency probing wave so that the phase of the probing wave is modulated instantly by the product of the parameter B/A of the object and pressure of the pumping wave. Then the resulting spatially-modulated probing wave is detected and demodulated to derive the distribution of B/A along the probing beam. An inverse-filtering operation is employed to compensate for the spread of the pumping wave. The processes are repeated by shifting the position of the probing beam and a two-dimensional image is obtained. A prototype of the imaging system is constructed and images related to the nonlinear parameter of phantoms and human tissues are obtained. The usefulness of this method is shown in these experimental results.

Adaptive PVDF piezoelectric deformable mirror system

An adaptive mirror system whose surface deforms smoothly according to the desired curve has been made of polyvinylidene fluoride (PVDF) piezoelectric film and laminar glass plate. One surface of the glass plate was evaporated with silver, and this side was used as the mirror surface. A PVDF film, whose shape was determined by the deformation curve, was pasted tightly on the other surface. The mirror deforms smoothly along this curve with the application of a single voltage to the film. Holographic filter and feedback were lso considered to improve the static and dynamic characteristics. Typically, deformation along ax(2)+bx(3) was obtained.

Imaging through an inhomogeneous layer by least‐mean‐square error fitting

A new active ultrasonic imaging method through an inhomogeneous layer is proposed. It has the special feature that its effectiveness does not depend on the class of the objects to be imaged. In this method, first, a set of data is acquired by repeating transmission and reception for all possible combinations of pairs of transducers on the array, then the spatial frequency components of the object and the structure of the inhomogeneous layer are estimated from these data by means of least‐mean‐square error fitting. Since the data have redundancy for the parameters to be estimated, this process gives an optimum and stable estimation algorithm even when measurement errors and noise are included. The image is reconstructed from the estimated spatial frequency components through inverse Fourier transform. The effectiveness of this method is ensured by several numerical analyses and experiments.

Real‐time bispectral analysis of gear noise and its application to contactless diagnosis

Real‐time bispectral analyses of gear noises are carried out, intending to distinguish abnormal states from the normal one without stopping the machine. Firstly, a real‐time bispectral analyzer which consists of bandpass filters, multipliers, and integrators is constructed. Then, the noises of gears are analyzed. The results show that when scorings have been grown on the gear surfaces the moduli of bispectra of the nosies are reduced markedly comparing to those of normal state, while conventional spectral analysis fails to distinguish the difference. And it is shown that this fact can be used to diagnose the conditions of gear surfaces. Finally, a proper stochastic model of gear noises based on physical considerations as well as the spectral and bispectral characteristics obtained by our study, is proposed.

Imaging system using an intensity triple correlator

A new imaging system for incoherent objects is constructed. In this system, the coherence function of the diffracted field is derived from the signals of three scanning intensity detectors by using computational manipulations. The concrete optical and electronic systems, the details of the signal processings for the derivation of the coherence function, and calculations for image reconstruction are shown. The reconstructed images of asymmetric objects show the usefulness of the system.

Nonlinear parameter tomography system using counter-propagating probe and pump waves

In this paper, a novel tomographic system for imaging the nonlinear parameter (B/A) of biological objects is described. This parameter is closely related to the detailed properties of tissue, and may well provide a new powerful tool for ultrasonic tissue characterization. In our new system, an impulsive, relatively high power (10 mW/cm2), low frequency pump wave is applied from the opposite direction of a cw low intensity probe wave of high frequency (5 MHz) so that the phase of the probe wave is modulated sequentially by the product of the nonlinear parameter (B/A) along the beam (x axis) and the pressure of the impulsive pump wave. This modulated probe wave is detected and demodulated to derive the distribution of (B/A) along the x axis. Many responses are averaged to increase the S/N ratio. Inverse or other filtering operations are applied to widen the frequency bandwidth of the pump wave. The entire system is realized in hardware. The counterbeam orientation makes the imaging system compact, with easy access to many parts of the human body. Its resolution is two times that of the perpendicular system proposed previously by us and the attenuation of the pump wave can also be compensated for easily. A practical system aimed at breast and liver diagnosis is described. The principle of the method and the system construction are described. B/A images of several objects are given.