Tjundewo Lawu

Simulation of Ultrasonic Displacement in Random Medium Using Ultrasonic Speckle Tracking

The quantitative evaluation of ultrasonic displacement in tissue under externally applied forces is a necessary step in the field of ultrasonic diagnostics. In this study, the speckle tracking method is used to investigate motion artifact produced by a rotating tissue. An analytic prediction of this motion artifact in relation to system characteristics (i.e., beam width, pulse duration, frequency and phase of the signal) is presented. The tissue is modeled as a random array of numerous point scatterers, and RF signals are computed based on the convolutions between the transmission pulse and the impulse response of each scanning line. The preliminary results show that the artifact resulting from the speckle tracking method depends on the beam width. The method is quite general and can be extended to study the effects of other tissue motion, in particular, the deformation of tissue.

Ultrasonic scattering from a simulated cavity in steel

The scattering of wideband ultrasonic pulses from simulated cavities in a metal block was estimated using a mathematical description of the echo based on the expression for echoes reflected from a strongly scattering object. The theoretically estimated echo waveforms and the corresponding Fourier power spectra of the scattered pulses are compared with those obtained experimentally with good agreement. Interpretations show that both the pulse‐time record and the power spectrum can be utilized to detect roughly the shape of a cavity in a metal block.

Extension of sampling theorem and its applications

The numerical solution of ordinary differential equations has been widely used in many fields including wave propagation analysis. To represent a continuous function in terms of its discrete sampled values in a sequence, it should satisfy the sampling theorem. However, in conventional wave propagation analysis, the experiential finite difference technique has generally been used. In this paper, the sampling extension which converges more rapidly than in the case of classical cardinal series is proposed. The extension and aliasing errors including the truncation error are described specifically. The sampling extention is also generalized to include the sampled values of the derivative and integral of the signal.

Near-field ultrasonic scattering from a cavity in steel considering the velocity amplitude on the transducer face using an error function model

The influence of nonuniform transducer velocity distribution in the near-field echo formation scattered from a cavity in steel is analyzed. In the present study, the velocity amplitude on the transducer face was modeled using an offset error function. Using this distribution, the echo waveforms scattered from a cylinder cavity in steel were estimated and compared with the echo estimated using uniform distribution and echo observed experimentally. The estimation accuracy of the near-field echo waveforms were improved quantitatively.

Analysis of the Near-Field Transient Radiation Considering the Velocity Amplitude on the Transducer Face

The velocity amplitude on the transducer face was estimated using the near-field transient radiation on the axis of the circular transducer obtained experimentally. The velocity amplitude can be obtained by deconvolving the pressure waveform with the electrical characteristics of the transducer and then integrating the result. Using the estimated velocity amplitude, the pressure waveforms were numerically calculated and compared with those obtained experimentally.

FAST CONVERGING BINOMIAL SERIES FOR RADIATION FIELD CALCULATION

An analytical description for the radiation field of a planar source by fast converging binomial series expansion is derived. The terms of the series correspond to the Fresnel solution and its derivatives. Consequently, if the Fresnel solution of the diffraction integral is expressed by differential functions, the radiation field can be obtained by calculating the series. As a result of the expansion, this method can be applied to any region except the points on the surface of transducer. The results are compared with the correct computations and those obtained by conventional Fresnel approximation.

Impulse Representation of Sound Field due to a Rigid Wedge

An impulse representation for calculating a diffraction wave due to a rigid wedge is described. The method is an approximation of the Biot-Tolstoy rigorous closed-form solution for the diffraction of point source radiation by an infinite rigid wedge. The band-limited time-domain function can be reconstructed to the original waveform if it satisfies the sampling theorem, which assumes that sampling takes place at the lowest permissible sampling rate. Therefore, if the energy is concentrated between the first sampling intervals immediately after the rise time of the time-domain function, the rigorous solution can be approximated as a delta function. This paper shows the description methods of the diffraction field near the ridge in three-dimensional space. Using the proposed impulse representation, numerical simulation was performed and the calculation accuracy was examined.

Simulation of tissue displacement using ultrasonic speckle tracking

The quantitative evaluation of ultrasonic displacement in tissue under externally applied forces is a necessary step in the field of ultrasonic diagnostics. In this study, the speckle tracking method is used to investigate the motion artifact produced by a rotating tissue. An analytic prediction of this motion artifact in relation to system characteristics (i.e., beam width, pulse duration, frequency, and phase of the signal) is presented. The tissue is modeled as a random array of numerous point scatterers, and rf signals are computed based on the convolutions between the transmission pulse and the impulse response of each scanning line. These analytical predictions are then compared to the computer simulation of the model. The results show that the artifact resulting from the speckle tracking method depends on the ultrasonic beam parameters. The method is quite general and can be extended to study the effects of other tissue motion, in particular, the deformation of tissue.

Low‐pass filter for the DFT process and its application to impulse response of sound‐field and ultrasonic B‐mode echo simulator

Convolution and correlation are closely related operations that are basic to many areas of signal processing. To obtain the frequency component of the signals using discrete Fourier transform (DFT), the continuous signals must first be sampled. A low‐pass antialiasing filter which is simple to implement is proposed. The low‐pass filter has equally good frequency response characteristics and can be used to eliminate possible aliasing effects by ensuring that the signal to be sampled is limited in the spectral content. The impulse response of the sound field was calculated by using the proposed low‐pass antialiasing filter and compared with the rigorous solution to verify the realizability of the filter. The filter was also applied to construct the ultrasonic B‐mode image of a 3‐D phantom model.

Influence of nonuniform transducer velocity distribution in the near‐field echo formation from a cylindrical rod

The echo formation in the near field of an ultrasonic transducer is strongly influenced by the amplitude distribution of the transducer. In this paper, the velocity amplitude on the face of a plane circular transducer was estimated using on‐axis transient radiation of the actual transducer. The velocity amplitude can be obtained by deconvolving the pressure waveform with the electrical characteristics of the transducer and then integrating the result with respect to time. Using the estimated velocity amplitude, the scattering of wideband ultrasonic pulses from a cylindrical rod was then numerically calculated by applying a mathematical description of the echo [Lawu et al., J. Acoust. Soc. Am. 98, 2809–2818 (1995)]. Results of these estimations agree well with those obtained experimentally. The proposed method improves the accuracy of the echo waveforms scattered by an object in the near field of the transducer.