Tao Yin

A study of acoustic source generation mechanism of Magnetoacoustic Tomography

Magnetoacoustic Tomography (MAT) is a non-invasive imaging modality for electrical conductivity with good contrast and high spatial resolution. We have analyzed the acoustic source generation mechanism of MAT and presented its physical model, including the simulations and experiments in this paper. In MAT, acoustic sources are generated in a conductive object placed in a static magnetic field. Pulsed current is injected into the object and produces a Lorentz force due to the static magnetic filed. Acoustic vibration was excited by the Lorentz force, and hence, ultrasound waves propagate in all directions and are collected with transducers placed around the object. The conductivity image can then be reconstructed with acoustic waves using some reconstruction algorithms. Because the acoustic source generation mechanism of MAT is the key problem of forward and inverse problems, we analyzed the physical process of acoustic source generation and presented the acoustic dipole source model according to the Lorentz force imposed on the object. In addition, computer simulations and experiments were also conducted. The results of simulations applying an acoustic dipole source model are consistent with experimental results. This study has cardinal significance for the accurate algorithm of MAT and provides a methodology and reference for acoustic source problems.

Image Reconstruction in Magnetoacoustic Tomography With Magnetic Induction With Variable Sound Speeds

Acoustic and electrical characteristics of biological tissue are important factors in magnetoacoustic tomography with magnetic induction (MAT-MI). Acoustic inhomogeneity significantly affects the propagations of sound waves. Differences in sound speed lead to distortions of the sound sources in the reconstruction process. The objective of this study is to develop a novel algorithm to reconstruct the sound source distribution in an acoustically inhomogeneous medium. The proposed algorithm is developed on the basis of the finite-difference time-domain method and time-reversal acoustic theory; it combines the relationship among symmetrical transducers with the back-projection algorithm. An acoustically inhomogeneous model with different regions of variable sound speeds is established to validate the proposed algorithm. From the data collected by a rotated focused transducer, first, the sound speed distribution is reconstructed, and then, the sound sources of the model are reconstructed. The reconstructed sound sources are obviously distorted when the speed differences are not considered. In contrast, the proposed algorithm yields reconstructed sound sources that are consistent with the model in terms of shape and size. Thus, the proposed algorithm is capable of accurately reconstructing the acoustic sources distribution in an acoustically inhomogeneous medium. This method provides a solution reducing the influence of acoustic inhomogeneity in MAT-MI. The distributions of sound speed can be obtained during the process of reconstructing the sound source. Consequently, the imaging of the acoustic speed and the electrical conductivity of biological tissues can be implemented simultaneously in MAT-MI.

Experimental Study on Amplitude Frequency of Acoustic Signal Excited by Coupling Magneto-Acoustic Field

Object To study the relationship between exciting source characteristic and magneto-acoustic signal, also explore the frequency corresponding relationship between exciting signal and acoustic signal applied on the experimental sample. Methods A practical system has been established to detect magneto-acoustic signal. While single period sine pulse current with different amplitude and frequency were loaded in the sample of straight copper wire, acoustic signal was detected synchronously. Amplitude and frequency of exciting current and acoustic signal were analyzed by using time-frequency method, which was adopted by short time Fourier transform STFT and shifted smoothing rectangle window. Then, comparative study was conducted basing on the processed data. Result Giving the current same frequency as well as different amplitude, linear relationship is obtained between the output amplitude of acoustic signal and current amplitude. However, giving the current same amplitude as well as different frequency, the corresponding frequency spectrum of acoustic signal show different variation law, and thus, there is great difference among the system functions. Conclusion Detecting system is highly sensitive to frequency. In order to acquire more information in the magneto-acoustic signal, both of detecting circuit SNR and acoustic transducer bandwidth should be promoted.

Simulation Study to Improve Focalization of a Figure Eight Coil by Using a Conductive Shield Plate and a Ferromagnetic Block

A new method to improve the focalization and efficiency of the Figure of Eight (FOE) coil in rTMS is discussed in this paper. In order to decrease the half width of the distribution curve (HWDC), as well to increase the ratio of positive peak value to negative peak value (RPN) of the induced electric field, a shield plate with a window and a ferromagnetic block are assumed to enhance the positive peak value of the induced electrical field. The shield is made of highly conductive copper, and the block is made of highly permeable soft magnetic ferrite. A computer simulation is conducted on ANSYS® software to conduct the finite element analysis (FEA). Two comparing coefficients were set up to optimize the sizes of the shield window and the block. Simulation results show that a shield with a 60 mm × 30 mm sized window, together with a block 40 mm thick, can decrease the focal area of a FOE coil by 46.7%, while increasing the RPN by 135.9%. The block enhances the peak value of the electrical field induced by a shield-FOE by 8.4%. A real human head model was occupied in this paper to further verify our method.

A study on locating the sonic source of sinusoidal magneto-acoustic signals using a vector method

Methods based on the magnetic-acoustic effect are of great significance in studying the electrical imaging properties of biological tissues and currents. The continuous wave method, which is commonly used, can only detect the current amplitude without the sound source position. Although the pulse mode adopted in magneto-acoustic imaging can locate the sonic source, the low measuring accuracy and low SNR has limited its application. In this study, a vector method was used to solve and analyze the magnetic-acoustic signal based on the continuous sine wave mode. This study includes theory modeling of the vector method, simulations to the line model, and experiments with wire samples to analyze magneto-acoustic (MA) signal characteristics. The results showed that the amplitude and phase of the MA signal contained the location information of the sonic source. The amplitude and phase obeyed the vector theory in the complex plane. This study sets a foundation for a new technique to locate sonic sources for biomedical imaging of tissue conductivity. It also aids in studying biological current detecting and reconstruction based on the magneto-acoustic effect.

A 3D reconstruction algorithm for magneto-acoustic tomography with magnetic induction based on ultrasound transducer characteristics

In this study we present a three-dimensional (3D) reconstruction algorithm for magneto-acoustic tomography with magnetic induction (MAT-MI) based on the characteristics of the ultrasound transducer. The algorithm is investigated to solve the blur problem of the MAT-MI acoustic source image, which is caused by the ultrasound transducer and the scanning geometry. First, we established a transducer model matrix using measured data from the real transducer. With reference to the S-L model used in the computed tomography algorithm, a 3D phantom model of electrical conductivity is set up. Both sphere scanning and cylinder scanning geometries are adopted in the computer simulation. Then, using finite element analysis, the distribution of the eddy current and the acoustic source as well as the acoustic pressure can be obtained with the transducer model matrix. Next, using singular value decomposition, the inverse transducer model matrix together with the reconstruction algorithm are worked out. The acoustic source and the conductivity images are reconstructed using the proposed algorithm. Comparisons between an ideal point transducer and the realistic transducer are made to evaluate the algorithms. Finally, an experiment is performed using a graphite phantom. We found that images of the acoustic source reconstructed using the proposed algorithm are a better match than those using the previous one, the correlation coefficient of sphere scanning geometry is 98.49% and that of cylinder scanning geometry is 94.96%. Comparison between the ideal point transducer and the realistic transducer shows that the correlation coefficients are 90.2% in sphere scanning geometry and 86.35% in cylinder scanning geometry. The reconstruction of the graphite phantom experiment also shows a higher resolution using the proposed algorithm. We conclude that the proposed reconstruction algorithm, which considers the characteristics of the transducer, can obviously improve the resolution of the reconstructed image. This study can be applied to analyse the effect of the position of the transducer and the scanning geometry on imaging. It may provide a more precise method to reconstruct the conductivity distribution in MAT-MI.

Translational-circular scanning for magneto-acoustic tomography with current injection

BackgroundMagneto-acoustic tomography with current injection involves using electrical impedance imaging technology. To explore the potential applications in imaging biological tissue and enhance image quality, a new scan mode for the transducer is proposed that is based on translational and circular scanning to record acoustic signals from sources.MethodsAn imaging algorithm to analyze these signals is developed in respect to this alternative scanning scheme. Numerical simulations and physical experiments were conducted to evaluate the effectiveness of this scheme. An experiment using a graphite sheet as a tissue-mimicking phantom medium was conducted to verify simulation results. A pulsed voltage signal was applied across the sample, and acoustic signals were recorded as the transducer performed stepped translational or circular scans. The imaging algorithm was used to obtain an acoustic-source image based on the signals.ResultsIn simulations, the acoustic-source image is correlated with the conductivity at the sample boundaries of the sample, but image results change depending on distance and angular aspect of the transducer. In general, as angle and distance decreases, the image quality improves. Moreover, experimental data confirmed the correlation.ConclusionThe acoustic-source images resulting from the alternative scanning mode has yielded the outline of a phantom medium. This scan mode enables improvements to be made in the sensitivity of the detecting unit and a change to a transducer array that would improve the efficiency and accuracy of acoustic-source images.

Experimental study of the thermoacoustic effect in magnetoacoustic tomography

Magnetoacoustic tomography (MAT) is an emerging noninvasive electrical conductivity imaging that combines the high dielectric contrast of tissues and excellent resolutions of ultrasonography. In this paper, we have found the thermoacoustic (TA) effect in the measurement of MAT. Several materials with different conductivities have been measured by a MAT system with and without a static magnetic field. The acoustic signals have been analyzed and compared in the time domain and the frequency domain, respectively. It is found that the TA effect is related to the material characteristics. For the tissue-like materials with low conductivities, the TA signals caused by the TA effect are observable and cannot be ignored in the time and frequency domains. It means that the TA effect of biological tissues should be considered in MAT system in the future.

Study on electric field in real head model induced by H-coil

To study the distribution of induced electric field in human brain under the H-coil and to explore the deep character of H-coil, this paper builds a real head model with limbic system inside, and an H-coil model close to the real head model. The electric field distribution in scalp and limbic system induced by H-coil was calculated via finite element method and the results were compared with those of figure-of-eight coil. It is found that the deep field performance of H-coil was much better than figure-of-eight coil. Although the induced electric field by H-coil is not focusing on scalp, it can concentrate deeply on anterior cingulate cortex, which is an important part of limbic system. It provides a valid evidence for H-coil to stimulate deep brain structures.