Magnetoacoustic Tomography With Magnetic Induction Research Articles

Imaging biological tissues with electrical conductivity contrast below 1 S m−1 by means of magnetoacoustic tomography with magnetic induction

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently introduced imaging modality for noninvasive electrical impedance imaging, with ultrasound imaging resolution and a contrast reflecting the electrical conductivity properties of tissues. However, previous MAT-MI systems can only image samples that are much more conductive than real human or animal tissues. To image real biological tissue samples, a large-current-carrying coil that can give stronger magnetic stimulations and stronger MAT-MI acoustic signals is employed in this study. The conductivity values of all the tissue samples employed in this study are also directly measured using a well calibrated four-electrode system. The experimental results demonstrated the feasibility to image biological tissues with electrical conductivity contrast below 1.0 S∕m using the MAT-MI technique with safe level of electromagnetic energy applied to tissue samples.

Multi-excitation Magnetoacoustic Tomography With Magnetic Induction for Bioimpedance Imaging

Magnetoacoustic tomography with magnetic induction (MAT-MI) is an imaging approach proposed to conduct noninvasive electrical conductivity imaging of biological tissue with high spatial resolution. In the present study, based on the analysis of the relationship between the conductivity distribution and the generated MAT-MI acoustic source, we propose a new multi-excitation MAT-MI approach and the corresponding reconstruction algorithms. In the proposed method, multiple magnetic excitations using different coil configurations are employed and ultrasound measurements corresponding to each excitation are collected to derive the conductivity distribution inside the sample. A modified reconstruction algorithm is also proposed for the multi-excitation MAT-MI imaging approach when only limited bandwidth acoustic measurements are available. Computer simulation and phantom experiment studies have been done to demonstrate the merits of the proposed method. It is shown that if unlimited bandwidth acoustic data is available, we can accurately reconstruct the internal conductivity contrast of an object using the proposed method. With limited bandwidth data and the use of the modified algorithm we can reconstruct the relative conductivity contrast of an object instead of only boundaries at the conductivity heterogeneity. Benefits that come with this new method include better differentiation of tissue types with conductivity contrast using the MAT-MI approach, specifically for potential breast cancer screening application in the future.

MAT-MI acoustic source reconstruction using ultrasound B-Scan imaging

We present in this study an ultrasound B-scan based imaging approach for magnetoacoustic tomography with magnetic induction (MAT-MI) to reconstruct electrical conductivity distribution. In MAT-MI acoustic waves are generated in the sample by placing it in a static and a time-varying magnetic field. The acoustic waves from these sources propagate in all directions. In the present approach these acoustic signals are collected with a focused ultrasound transducer which confines the collected signal to that from sources along a line. The focused transducer also gives signal gain in the focus region improving the MAT-MI signal quality. The time-resolved acoustic signals are back projected to form a one-dimensional (1D) image of the source distribution along the line. The complete cross-section of the object is obtained by acquiring 1D images along multiple directions in the cross-sectional plane. A simulation model of the image reconstruction method is developed with ultrasound simulations using the Field II program. The present reconstruction results suggest that acoustic source imaging in MAT-MI can be achieved using the much practical ultrasound B scan imaging technique. The developed method is applied to MAT-MI in experiments. This method should allow combining MAT-MI with clinical ultrasound imaging methods and broadening the potential applicability of the technique.

Comparison Study of Three Different Image Reconstruction Algorithms for MAT-MI

In this paper, we report a theoretical study on magnetoacoustic tomography with magnetic induction (MAT-MI). According to the description of signal generation mechanism using Green's function, the acoustic dipole model was proposed to describe acoustic source excited by the Lorentz force. Using Green's function, three kinds of reconstruction algorithms based on different models of acoustic source (potential energy, vectored acoustic pressure, and divergence of Lorenz force) are deduced and compared, and corresponding numerical simulations were conducted to compare these three kinds of reconstruction algorithms. The computer simulation results indicate that the potential energy method and vectored pressure method can directly reconstruct the Lorentz force distribution and give a more accurate reconstruction of electrical conductivity.

Development of magnetoacoustic tomography

Magnetoaconstic tomography(MAT)is the combination of electrical impedance tomography(EIT)and ultrasonic imaging.In detection using magnetoacoustic tomography with magnetic induction(MAT-MI),sample is located in the area where both static magnetic field and a time-varying magnetic field exist.The time-racing magnetic field induces an eddy current in the sample and the sample will emit ultrasonic waves by Lorentz force.The ultrasonic signals are collected to reconstruct images related to the electrical impedance distribution.This method prorides images with higher resolution and higher sensitivity which are valuable in clinical diagnoses.Based on the MAT-MI,the application of MAT-MI with no rotation of the static magnetic field is proposed. Key words: Electrical impedance tomography; Magnetoacoustic tomography; Magnetoacoustic tomography with nagnetic induction; Ultrasonic signals

Solving the forward problem of magnetoacoustic tomography with magnetic induction by means of the finite element method

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently proposed imaging modality to image the electrical impedance of biological tissue. It combines the good contrast of electrical impedance tomography with the high spatial resolution of sonography. In this paper, a three-dimensional MAT-MI forward problem was investigated using the finite element method (FEM). The corresponding FEM formulae describing the forward problem are introduced. In the finite element analysis, magnetic induction in an object with conductivity values close to biological tissues was first carried out. The stimulating magnetic field was simulated as that generated from a three-dimensional coil. The corresponding acoustic source and field were then simulated. Computer simulation studies were conducted using both concentric and eccentric spherical conductivity models with different geometric specifications. In addition, the grid size for finite element analysis was evaluated for the model calibration and evaluation of the corresponding acoustic field.

Reconstruction of Vectorial Acoustic Sources in Time-Domain Tomography

A new theory is proposed for the reconstruction of curl-free vector field, whose divergence serves as acoustic source. The theory is applied to reconstruct vector acoustic sources from the scalar acoustic signals measured on a surface enclosing the source area. It is shown that, under certain conditions, the scalar acoustic measurements can be vectorized according to the known measurement geometry and subsequently be used to reconstruct the original vector field. Theoretically, this method extends the application domain of the existing acoustic reciprocity principle from a scalar field to a vector field, indicating that the stimulating vectorial source and the transmitted acoustic pressure vector (acoustic pressure vectorized according to certain measurement geometry) are interchangeable. Computer simulation studies were conducted to evaluate the proposed theory, and the numerical results suggest that reconstruction of a vector field using the proposed theory is not sensitive to variation in the detecting distance. The present theory may be applied to magnetoacoustic tomography with magnetic induction (MAT-MI) for reconstructing current distribution from acoustic measurements. A simulation on MAT-MI shows that, compared to existing methods, the present method can give an accurate estimation on the source current distribution and a better conductivity reconstruction.

The Effect of Electrical Anisotropy During Magnetoacoustic Tomography With Magnetic Induction

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a technique for imaging electrical conductivity in tissue. A time-varying magnetic field induces currents that interact with a static magnetic field to produce a Lorentz force, initiating ultrasonic waves. The goal of this communication is to examine the effect of anisotropy during MAT-MI.

Magnetoacoustic tomographic imaging of electrical impedance with magnetic induction

Magnetoacoustic tomography with magnetic induction (MAT-MI) is a recently introduced method for imaging tissue electrical impedance properties by integrating magnetic induction and ultrasound measurements. In the present study, we have developed a focused cylindrical scanning mode MAT-MI system and the corresponding reconstruction algorithms. Using this system, we demonstrated 3-dimensional MAT-MI imaging in a physical phantom, with cylindrical scanning combined with ultrasound focusing, and the ability of MAT-MI in imaging electrical conductivity properties of biological tissue.

Imaging Electrical Impedance From Acoustic Measurements by Means of Magnetoacoustic Tomography With Magnetic Induction (MAT-MI)

We have conducted computer simulation and experimental studies on magnetoacoustic-tomography with magnetic induction (MAT-MI) for electrical impedance imaging. In MAT-MI, the object to be imaged is placed in a static magnetic field, while pulsed magnetic stimulation is applied in order to induce eddy current in the object. In the static magnetic field, the Lorentz force acts upon the eddy current and causes acoustic vibrations in the object. The propagated acoustic wave is then measured around the object to reconstruct the electrical impedance distribution. In the present simulation study, a two-layer spherical model is used. Parameters of the model such as sample size, conductivity values, strength of the static and pulsed magnetic field, are set to simulate features of biological tissue samples and feasible experimental constraints. In the forward simulation, the electrical potential and current density are solved using Poisson's equation, and the acoustic pressure is calculated as the forward solution. The electrical impedance distribution is then reconstructed from the simulated pressure distribution surrounding the sample. The present computer simulation results suggest that MAT-MI can reconstruct conductivity images of biological tissue with high spatial resolution and high contrast. The feasibility of MAT-MI in providing high spatial resolution images containing impedance-related information has also been demonstrated in a phantom experiment.

Magnetoacoustic tomography with magnetic induction for imaging electrical impedance of biological tissue

An experimental feasibility study was conducted on Magnetoacoustic-Tomography with Magnetic Induction (MAT-MI). It is demonstrated that the 2-dimensional (2D) MAT-MI system can detect and image the boundaries between regions of different electrical conductivity with high spatial resolution. Utilizing a magnetic stimulation coil, MAT-MI evokes magnetically induced eddy current in an object which is placed in a static magnetic field. Because of the existence of Lorenz forces, the eddy current in turn causes acoustic vibrations, which are measured around the object in order to reconstruct the electrical impedance distribution of the object. The present experimental results from the saline and gel phantoms are promising and suggest the merits of MAT-MI in imaging electrical impedance of biological tissue with high spatial resolution.

Magnetoacoustic tomography with magnetic induction (MAT-MI)

We report our theoretical and experimental investigations on a new imaging modality, magnetoacoustic tomography with magnetic induction (MAT-MI). In MAT-MI, the sample is located in a static magnetic field and a time-varying (µs) magnetic field. The time-varying magnetic field induces an eddy current in the sample. Consequently, the sample will emit ultrasonic waves by the Lorentz force. The ultrasonic signals are collected around the object to reconstruct images related to the electrical impedance distribution in the sample. MAT-MI combines the good contrast of electrical impedance tomography with the good spatial resolution of sonography. MAT-MI has two unique features due to the solenoid nature of the induced electrical field. Firstly, MAT-MI could provide an explicit or simple quantitative reconstruction algorithm for the electrical impedance distribution. Secondly, it promises to eliminate the shielding effects of other imaging modalities in which the current is applied directly with electrodes. In the theoretical part, we provide formulae for both the forward and inverse problems of MAT-MI and estimate the signal amplitude in biological tissues. In the experimental part, the experimental setup and methods are introduced and the signals and the image of a metal object by means of MAT-MI are presented. The promising pilot experimental results suggest the feasibility of the proposed MAT-MI approach.