Karl Hollaus

Bio: Karl Hollaus is an academic researcher from Vienna University of Technology. The author has contributed to research in topics: Finite element method & Eddy current. The author has an hindex of 14, co-authored 75 publications receiving 794 citations. Previous affiliations of Karl Hollaus include Graz University of Technology & University of Graz.

Detection of brain oedema using magnetic induction tomography: a feasibility study of the likely sensitivity and detectability.

Abstract: The detection and continuous monitoring of brain oedema is of particular interest in clinical applications because existing methods (invasive measurement of the intracranial pressure) may cause considerable distress for the patients A new non-invasive method for continuous monitoring of an oedema promises the use of multi-frequency magnetic induction tomography (MIT) MIT is an imaging method for reconstructing the changes of the conductivity Δκ in a target object The sensitivity of a single MIT-channel to a spherical oedematous region was analysed with a realistic model of the human brain The model considers the cerebrospinal fluid around the brain, the grey matter, the white matter, the ventricle system and an oedema (spherical perturbation) Sensitivity maps were generated for different sizes and positions of the oedema when using a coaxial coil system The maps show minimum sensitivity along the coil axis, and increasing values when moving the perturbation towards the brain surface Parallel to the coil axis, however, the sensitivity does not vary significantly When assuming a standard deviation of 10−7 for the relative voltage change due to the system's noise, a centrally placed oedema with a conductivity contrast of 2 with respect to the background and a radius of 20 mm can be detected at 100 kHz At higher frequencies the sensitivity increases considerably, thus suggesting the capability of multi-frequency MIT to detect cerebral oedema

Solution of the inverse problem of magnetic induction tomography (MIT).

Abstract: Magnetic induction tomography (MIT) of biological tissue is used to reconstruct the changes in the complex conductivity distribution inside an object under investigation. The measurement principle is based on determining the perturbation ?B of a primary alternating magnetic field B0, which is coupled from an array of excitation coils to the object under investigation. The corresponding voltages ?V and V0 induced in a receiver coil carry the information about the passive electrical properties (i.e. conductivity, permittivity and permeability). The reconstruction of the conductivity distribution requires the solution of a 3D inverse eddy current problem. As in EIT the inverse problem is ill-posed and on this account some regularization scheme has to be applied. We developed an inverse solver based on the Gauss?Newton-one-step method for differential imaging, and we implemented and tested four different regularization schemes: the first and second approaches employ a classical smoothness criterion using the unit matrix and a differential matrix of first order as the regularization matrix. The third method is based on variance uniformization, and the fourth method is based on the truncated singular value decomposition. Reconstructions were carried out with synthetic measurement data generated with a spherical perturbation at different locations within a conducting cylinder. Data were generated on a different mesh and 1% random noise was added. The model contained 16 excitation coils and 32 receiver coils which could be combined pairwise to give 16 planar gradiometers. With 32 receiver coils all regularization methods yield fairly good 3D-images of the modelled changes of the conductivity distribution, and prove the feasibility of difference imaging with MIT. The reconstructed perturbations appear at the right location, and their size is in the expected range. With 16 planar gradiometers an additional spurious feature appears mirrored with respect to the median plane with negative sign. This demonstrates that a symmetrical arrangement with one ring of planar gradiometers cannot distinguish between a positive conductivity change at the true location and a negative conductivity change at the mirrored location.

Numerical solution of the general 3D eddy current problem for magnetic induction tomography (spectroscopy)

Abstract: Magnetic induction tomography (MIT) is used for reconstructing the changes of the conductivity in a target object using alternating magnetic fields. Applications include, for example, the non-invasive monitoring of oedema in the human brain. A powerful software package has been developed which makes it possible to generate a finite element (FE) model of complex structures and to calculate the eddy currents in the object under investigation. To validate our software a model of a previously published experimental arrangement was generated. The model consists of a coaxial coil system and a conducting sphere which is moved perpendicular to the coil axis (a) in an empty space and (b) in a saline-filled cylindrical tank. The agreement of the measured and simulated data is very good when taking into consideration the systematic measurement errors in case (b). Thus the applicability of the simulation algorithm for two-compartment systems has been demonstrated even in the case of low conductivities and weak contrast. This can be considered an important step towards the solution of the inverse problem of MIT.

Fast calculation of the sensitivity matrix in magnetic induction tomography by tetrahedral edge finite elements and the reciprocity theorem

Abstract: Magnetic induction tomography of biological tissue is used to reconstruct the changes in the complex conductivity distribution by measuring the perturbation of an alternating primary magnetic field. To facilitate the sensitivity analysis and the solution of the inverse problem a fast calculation of the sensitivity matrix, i.e. the Jacobian matrix, which maps the changes of the conductivity distribution onto the changes of the voltage induced in a receiver coil, is needed. The use of finite differences to determine the entries of the sensitivity matrix does not represent a feasible solution because of the high computational costs of the basic eddy current problem. Therefore, the reciprocity theorem was exploited. The basic eddy current problem was simulated by the finite element method using symmetric tetrahedral edge elements of second order. To test the method various simulations were carried out and discussed.

Single-step 3-d image reconstruction in magnetic induction tomography: theoretical limits of spatial resolution and contrast to noise ratio.

Abstract: Magnetic induction tomography (MIT) is a low-resolution imaging modality for reconstructing the changes of the complex conductivity in an object. MIT is based on determining the perturbation of an alternating magnetic field, which is coupled from several excitation coils to the object. The conductivity distribution is reconstructed from the corresponding voltage changes induced in several receiver coils. Potential medical applications comprise the continuous, non-invasive monitoring of tissue alterations which are reflected in the change of the conductivity, e.g. edema, ventilation disorders, wound healing and ischemic processes. MIT requires the solution of an ill-posed inverse eddy current problem. A linearized version of this problem was solved for 16 excitation coils and 32 receiver coils with a model of two spherical perturbations within a cylindrical phantom. The method was tested with simulated measurement data. Images were reconstructed with a regularized single-step Gauss–Newton approach. Theoretical limits for spatial resolution and contrast/noise ratio were calculated and compared with the empirical results from a Monte-Carlo study. The conductivity perturbations inside a homogeneous cylinder were localized for a SNR between 44 and 64 dB. The results prove the feasibility of difference imaging with MIT and give some quantitative data on the limitations of the method.

Magnetic induction tomography

Abstract: A magnetic induction tomography (MIT) apparatus comprises an excitation signal generator (70) for generating an excitation signal; a primary excitation coil (50) arranged to receive the excitation signal from the excitation signal generator (70) and to convert the excitation signal into electromagnetic radiation and to emit said radiation to excite a sample having at least one of an electrical conductivity distribution, an electrical permittivity distribution or a magnetic permeability distribution; a primary receiver coil (60) arranged to receive electromagnetic radiation from the excited sample and to convert the received radiation into a detection signal; and a signal distribution network (115) arranged to receive the detection signal from the primary receiver coil (60). The apparatus further comprises a passive reference detector arranged to detect the excitation signal and to convert the detected signal into a passive reference signal. The apparatus further comprises an active reference signal generator (230) for generating an active reference signal; and an active reference source (175) arranged to receive the active reference signal from the active reference signal generator (230) and to supply the active reference signal to the signal distribution network (115).

Magnetoacoustic tomography with magnetic induction (MAT-MI)

Abstract: 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 (micros) 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

A Broadband High-Frequency Electrical Impedance Tomography System for Breast Imaging

Abstract: Bio-electric impedance signatures arise primarily from differences in cellular morphologies within an organ and can be used to differentiate benign and malignant pathologies, specifically in the breast. Electrical impedance tomography (EIT) is an imaging modality that determines the impedance distribution within tissue and has been used in prior work to map the electrical properties of breast at signal frequencies ranging from a few kHz to 1 MHz. It has been suggested that by extending the frequency range, additional information of clinical significance may be obtained. We have, therefore, developed a new EIT system for breast imaging which covers the frequency range from 10 kHz to 10 MHz. The instrument developed here is a distributed processor tomograph with 64 channels, capable of generating and measuring voltages and currents. Electrical benchmarking has shown the system to have a SNR greater than 94 dB up to 2 MHz, 90 dB up to 7 MHz, and 65 dB at 10 MHz. In addition, the system measures impedances to an accuracy of 99.7% and has channel-to-channel variations of less than 0.05%. Phantom imaging has demonstrated the ability to image across the entire frequency range in both single-and multiplane configurations. Further, 96 women have participated safely in breast exams with the system and the associated conductivity spectra obtained from 3-D image reconstructions range from 0.0237 S/m at 10 kHz to 0.2174 S/m at 10 MHz. These findings are consistent with impedance values reported in the literature.