Matthias Hamsch

A measurement system and image reconstruction in magnetic induction tomography

Magnetic induction tomography (MIT) is a technique for imaging the internal conductivity distribution of an object. In MIT current-carrying coils are used to induce eddy currents in the object and the induced voltages are sensed with other coils. From these measurements, the internal conductivity distribution of the object can be reconstructed. In this paper, we introduce a 16-channel MIT measurement system that is capable of parallel readout of 16 receiver channels. The parallel measurements are carried out using high-quality audio sampling devices. Furthermore, approaches for reconstructing MIT images developed for the 16-channel MIT system are introduced. We consider low conductivity applications, conductivity less than 5 S m(-1), and we use a frequency of 10 MHz. In the image reconstruction, we use time-harmonic Maxwells equation for the electric field. This equation is solved with the finite element method using edge elements and the images are reconstructed using a generalized Tikhonov regularization approach. Both difference and static image reconstruction approaches are considered. Results from simulations and real measurements collected with the Philips 16-channel MIT system are shown.

Imaging hemorrhagic stroke with magnetic induction tomography: realistic simulation and evaluation.

Magnetic induction tomography (MIT) is a noncontact method for detecting the internal conductivity distribution of an object. This technology has the potential to be used in the biomedical area to check bio-impedance change inside the human body, for example to detect hemorrhage in the human brain. In this study the hemorrhagic stroke detectability with a 16-channel MIT system operating at 10 MHz was evaluated. Since the conductivity distribution is changed by the hemorrhagic stroke as well as the squeezed brain tissue around the stroke, deformation of the brain tissue is also considered and simulated with the help of a FEM-based linear bio-mechanical model in this paper. To simulate the raw measurement data as realistically as possible, the noise estimated from the experimental MIT system with hypothesis testing methods at 95% confidence level is added to the simulated measurements. Stroke images of 600 noisy samples for each detection assignment are reconstructed by the one-step Tikhonov-regularized inverse eddy current solution. Under the statistical framework, the detection failure is in control of a high false negative rate which represents a large artifact visualized in the reconstruction domain. The qualitative detectability of 18 detecting assignments, with three hemorrhagic positions (shallow, medial and center of the cerebrum) and two volume values (10 ml and 20 ml), overlaid by noise with three levels (standard deviation of phase change at 5 x 10(-3) degrees , 2.5 x 10(-3) degrees , 10 x 10(-3) degrees ), are investigated. These detecting assignments are compared with each other to find out which volumes of deformed spherical hemorrhagic stroke can be detected by the modeled MIT system.

16 Channel Magnetic Induction Tomography System Featuring Parallel Readout

Magnetic induction tomography (MIT) allows the reconstruction of conductivity distributions for a wide variety of industrial and medical applications. Philips is interested in using the MIT technique for acquiring information of conductivity distribution and conductivity changes in human tissue. The advantage of this technique is the contactless and non-invasive way of collecting information on the tissue. An MIT system consists of excitation coils that produce a primary magnetic field that causes eddy currents in a conductive object. The eddy current produces a secondary magnetic field that can be detected by an array of receiving coils. This paper presents the setup of a 16 channel MIT system featuring parallel readout of the 16 receiver coil array. To achieve the parallel readout the high-frequency signals used for the measurements are converted to a lower frequency by heterodyne downconversion and then sampled by high quality audio sampling equipment. The sampled low frequency signals are processed by digital signal processing algorithms in a standard computer. This allows the replacement of the commonly used lock-in amplifier and enables the processing of all 16 receiving channels in parallel.

Contact-less human vital sign monitoring with a 12 channel synchronous parallel processing magnetic impedance measurement system

Magnetic impedance measurements allow monitoring conductivity changes in the tissue. The idea is to generate a primary electromagnetic field which permeates the human body and to evaluate the received electromagnetic field. The received electromagnetic field is a superposition of the primary field and a secondary field which is caused by eddy currents in the human body. Changes of the conductivity over time e.g. due to breathing or heartbeats can be measured.

Sensitivity Comparisons of Cylindrical and Hemi-Spherical Coil Setups for Magnetic Induction Tomography

In this simulation study, we evaluate and compare the cylindrical and the hemi-spherical coil setups of two Magnetic Induction Tomography (MIT) systems using sensitivity analysis. Furthermore, different parameters for the size and the number of measurement and excitation coils are tested. For evaluating the sensitivity to conductivity values, the edge finite element method with uniform tetrahedral elements is utilized. The volume of interest is defined by a sphere, which represents a generic measurement object similar to the human head. A figure of merit that describes the general sensitivity to conductivity changes within the upper half of this volume, and two plots representing the distribution of sensitivity values are computed. Our findings indicate that the hemi-spherical MIT system with a smaller distance between the layer of coils and the measurement object shows a clearly higher sensitivity compared to the cylindrical MIT system. In addition, the two simulated setups with larger coil areas provide higher sensitivities in relation to the standard setups, while the difference between the hemi-spherical setups using a different number of coils with identical areas is relatively small.

A comparison of two phase measurement techniques for Magnetic Impedance Tomography

Magnetic induction tomography (MIT) allows the reconstruction of conductivity distributions inside a given volume for a wide variety of industrial and medical applications. Philips is interested in using the MIT technique for acquiring information of conductivity distribution and conductivity changes in human tissue. The advantage of this technique is the contactless and non-invasive way of collecting information on the tissue. An MIT system consists of excitation coils that produce a primary magnetic field that causes eddy currents in a conductive object. The eddy current produces a secondary magnetic field that can be detected by an array of receiving coils. Improved Philips MIT setups based on IF down-conversion and high-speed sampling are compared. MIT demands an accurate measure of phase and the new setups are found to offer improvements in noise, linearity and drift over our previous system.

Imagereconstruction approaches for Philips magnetic induction tomograph

Magnetic induction tomography (MIT) is a technique for imaging internal conductivity distribution of an object. In MIT current-carrying coils are used to induce eddy currents in the object. The secondary magnetic field due to the eddy currents is sensed with other coils. From these measurements an image can be reconstructed. The image reconstruction is anonlinear, three-dimensional, illposed inverse problem which necessitates a solution of the time harmonic Maxwells equations as well as some regularization due to the ill-posedness. In this paper we introduce approaches for reconstructing MIT images developed for a Philips 16 channel MIT system. We consider low conductivity applications, conductivity less than 10 Sm-1, and we use frequency of 10 MHz. The Maxwells equation in frequency domain for the electric field is solved with finite element method usingedge elements and regularized inversion techniques are used in solving the inverse problem. Both difference and static image reconstruction approaches are considered. Results from simulations and real measurements collected with the Philips 16 channel MIT system are shown.