Experimental performance evaluation of multi‐echo ICNE pulse sequence in magnetic resonance electrical impedance tomography

Minhas, Atul S.,Jeong, Woo Chul,Kim, Young Tae,Han, Yeqing,Kim, Hyung Joong,Woo, Eung Je Wiley Subscription Services, Inc., A Wiley Company 2011 Magnetic resonance in medicine Vol.66 No.4

<P><B>Abstract</B></P><P>Latest experimental results in magnetic resonance electrical impedance tomography (MREIT) demonstrated high‐resolution in vivo conductivity imaging of animal and human subjects using imaging currents of 5 to 9 mA. Externally injected imaging currents induce magnetic flux density distributions, which are affected by a conductivity distribution. Since we extract the induced magnetic flux density images from MR phase images, it is essential to reduce noise in the phase images. In vivo human and disease model animal experiments require reduction of imaging current amplitudes and scan times. In this article, we investigate a multi‐echo based MREIT pulse sequence where we utilize a remaining time after the first echo within one TR to obtain more echo signals. It also allows us to prolong the total current injection time. From phantom and animal imaging experiments, we found that this method significantly reduces the noise level in measured magnetic flux density images. We describe experimental validation of the multi‐echo sequence by comparing its performance with a single‐echo method using 3 mA imaging currents. The proposed method will be advantageous for an imaging region with long T2 values such as the brain and knee. Depending on T2 values, we suggest using two or three echoes in future experimental studies. Magn Reson Med, 2011. © 2011 Wiley‐Liss, Inc.</P>

MREIT of Postmortem Swine Legs using Carbon-hydrogel Electrodes

Minhas, Atul S.,Jeong, Woo-Chul,Kim, Young-Tae,Kim, Hyung-Joong,Lee, Tae-Hwi,Woo, Eung-Je The Korean Society of Medical and Biological Engin 2008 의공학회지 Vol.29 No.6

Magnetic resonance electrical impedance tomography(MREIT) has been suggested to produce cross-sectional conductivity images of an electrically conducting object such as the human body. In most previous studies, recessed electrodes have been used to inject imaging currents into the object. An MRI scanner was used to capture induced magnetic flux density data inside the object and a conductivity image reconstruction algorithm was applied to the data. This paper reports the performance of a thin and flexible carbon-hydrogel electrode that replaces the bulky and rigid recessed electrode in previous studies. The new carbon-hydrogel electrode produces a negligible amount of artifacts in MR and conductivity images and significantly simplifies the experimental procedure. We can fabricate the electrode in different shapes and sizes. Adding a layer of conductive adhesive, we can easily attach the electrode on an irregular surface with an excellent contact. Using a pair of carbon-hydrogel electrodes with a large contact area, we may inject an imaging current with increased amplitude primarily due to a reduced average current density underneath the electrodes. Before we apply the new electrode to a human subject, we evaluated its performance by conducting MREIT imaging experiments of five swine legs. Reconstructed conductivity images of the swine legs show a good contrast among different muscles and bones. We suggest a future study of human experiments using the carbon-hydrogel electrode following the guideline proposed in this paper.

Chemical Shift Artifact Correction in MREIT

Minhas, Atul S.,Kim, Young-Tae,Jeong, Woo-Chul,Kim, Hyung-Joong,Lee, Soo-Yeol,Woo, Eung-Je The Korean Society of Medical and Biological Engin 2009 의공학회지 Vol.30 No.6

Magnetic resonance electrical impedance tomography (MREIT) enables us to perform high-resolution conductivity imaging of an electrically conducting object. Injecting low-frequency current through a pair of surface electrodes, we measure an induced magnetic flux density using an MRI scanner and this requires a sophisticated MR phase imaging method. Applying a conductivity image reconstruction algorithm to measured magnetic flux density data subject to multiple injection currents, we can produce multi-slice cross-sectional conductivity images. When there exists a local region of fat, the well-known chemical shift phenomenon produces misalignments of pixels in MR images. This may result in artifacts in magnetic flux density image and consequently in conductivity image. In this paper, we investigate chemical shift artifact correction in MREIT based on the well-known three-point Dixon technique. The major difference is in the fact that we must focus on the phase image in MREIT. Using three Dixon data sets, we explain how to calculate a magnetic flux density image without chemical shift artifact. We test the correction method through imaging experiments of a cheese phantom and postmortem canine head. Experimental results clearly show that the method effectively eliminates artifacts related with the chemical shift phenomenon in a reconstructed conductivity image.

Three-dimensional MREIT Simulator of Static Bioelectromagnetism and MRI

우응제,Atul S. Minhas,Zijun Meng,Young Tae Kim,김형중,김혜현 대한의용생체공학회 2011 Biomedical Engineering Letters (BMEL) Vol.1 No.2

Purpose Magnetic resonance electrical impedance tomography (MREIT) aims to produce high-resolution cross-sectional images of a conductivity distribution inside the human body. We perform conductivity image reconstructions based on a relation between the conductivity distribution and induced magnetic flux density distributions subject to externally injected currents. This induced magnetic flux density is measured in MREIT using an MRI scanner. To facilitate MREIT research, we need a numerical simulator including static bioelectromagnetism and MRI data collection process. In this paper, we describe the development of a threedimensional MREIT simulator (MREITSim). Methods We describe various features of MREITSim including geometry modeling, meshing, finite element modeling and numerical computations of magnetic flux density and k-space MR data. We demonstrate the underlying bioelectromagnetic phenomena and MR data collection process using phantom models of without and with anomaly. We illustrate effects of noise in MR data and echo time on magnetic flux density computations. Results We demonstrate numerical computations of current density and magnetic flux density distributions for current injections orthogonal to z-direction, the direction of the main magnetic field of an MRI scanner. The k-space MREIT data generation procedure is illustrated using a phantom model with an insulating anomaly. Conclusions The simulator functions as a virtual MREIT scanner and provides quantitative numerical results of intended experimental studies. We suggest the simulator as a basic research tool for future MREIT studies of its theory, algorithm,experimental techniques and pulse sequence design.

MREIT conductivity imaging of the postmortem canine abdomen using CoReHA

Jeon, Kiwan,Minhas, Atul S,Kim, Young Tae,Jeong, Woo Chul,Kim, Hyung Joong,Kang, Byeong Teck,Park, Hee Myung,Lee, Chang-Ock,Seo, Jin Keun,Woo, Eung Je IOP PUBLISHING 2009 PHYSIOLOGICAL MEASUREMENT Vol.30 No.9

<P>Magnetic resonance electrical impedance tomography (MREIT) is a new bio-imaging modality providing cross-sectional conductivity images from measurements of internal magnetic flux densities produced by externally injected currents. Recent experimental results of postmortem and <I>in vivo</I> imaging of the canine brain demonstrated its feasibility by showing conductivity images with meaningful contrast among different brain tissues. MREIT image reconstructions involve a series of data processing steps such as <I>k</I>-space data handling, phase unwrapping, image segmentation, meshing, modelling, finite element computation, denoising and so on. To facilitate experimental studies, we need a software tool that automates these data processing steps. In this paper, we summarize such an MREIT software package called CoReHA (conductivity reconstructor using harmonic algorithms). Performing imaging experiments of the postmortem canine abdomen, we demonstrate how CoReHA can be utilized in MREIT. The abdomen with a relatively large field of view and various organs imposes new technical challenges when it is chosen as an imaging domain. Summarizing a few improvements in the experimental MREIT technique, we report our first conductivity images of the postmortem canine abdomen. Illustrating reconstructed conductivity images, we discuss how they discern different organs including the kidney, spleen, stomach and small intestine. We elaborate, as an example, that conductivity images of the kidney show clear contrast among cortex, internal medulla, renal pelvis and urethra. We end this paper with a brief discussion on future work using different animal models.</P>

An Iterative Method for Problems with Multiscale Conductivity

Kim, Hyea Hyun,Minhas, Atul S.,Woo, Eung Je Hindawi Publishing Corporation 2012 Computational and mathematical methods in medicine Vol.2012 No.-

<P>A model with its conductivity varying highly across a very thin layer will be considered. It is related to a stable phantom model, which is invented to generate a certain apparent conductivity inside a region surrounded by a thin cylinder with holes. The thin cylinder is an insulator and both inside and outside the thin cylinderare filled with the same saline. The injected current can enter only through the holes adopted to the thin cylinder. The model has a high contrast of conductivity discontinuity across the thin cylinder and the thickness of the layer and the size of holes are very small compared to the domain of the model problem. Numerical methods for such a model require a very fine mesh near the thin layer to resolve the conductivity discontinuity. In this work, an efficient numerical method for such a model problem is proposed by employing a uniform mesh, which need not resolve the conductivity discontinuity. The discrete problem is then solved by an iterative method, where the solution is improved by solving a simple discrete problem with a uniform conductivity. At each iteration, the right-hand side is updated by integrating the previous iterate over the thin cylinder. This process results in a certain smoothing effect on microscopic structures and our discrete model can provide a more practical tool for simulating the apparent conductivity. The convergence of the iterative method is analyzed regarding the contrast in the conductivity and the relative thickness of the layer. In numerical experiments, solutions of our method are compared to reference solutions obtained from COMSOL, where very fine meshes are used to resolve the conductivity discontinuity in the model. Errors of the voltage in <I>L<SUP>2</SUP></I> norm follow <I>O(h)</I> asymptotically and the current density matches quitewell those from the reference solution for a sufficiently small mesh size <I>h</I>. The experimental results present a promising feature of our approach for simulating the apparent conductivity related to changes in microscopic cellular structures.</P>

In vivo magnetic resonance electrical impedance tomography of canine brain: Disease model study of ischemia and abscess

Kim, Young Tae,Jeong, Woo Chul,Minhas, Atul S.,Lim, Chae Young,Park, Hee Myung,Kim, Hyung Joong,Woo, Eung Je Springer-Verlag 2011 Biomedical Engineering Letters (BMEL) Vol.1 No.1

In Vivo Magnetic Resonance Electrical Impedance Tomography of Canine Brain: Disease Model Study of Ischemia and Abscess

김형중,Young Tae Kim,Woo Chul Jeong,Atul S. Minhas,우응제,Chae Young Lim,Hee Myung Park 대한의용생체공학회 2011 Biomedical Engineering Letters (BMEL) Vol.1 No.1

Purpose In this study, we performed in vivo disease model animal experiments to validate the MREIT technique in terms of its capability to produce a conductivity contrast corresponding to brain ischemia and abscess. Methods Injecting 5 mA imaging currents into the head of an anesthetized dog, we collected induced magnetic flux density data using a 3T MRI scanner. Applying the harmonic Bz algorithm to the data, we reconstructed scaled conductivity images providing conductivity contrast information. To investigate any change of electrical conductivity due to brain diseases of ischemia and abscess, we scanned an animal with such a regional brain disease along with a separate prior scan of the same animal having no disease model. Results In the brain ischemic region, conductivity images show a significantly decreased contrast. The conductivity images of brain abscess show a significantly increased contrast, which is not apparent in the normal brain. Conclusions The results indicate that MREIT conductivity images provide meaningful diagnostic information that is not available from other imaging modalities. We suggest further animal imaging experiments with numerous disease models to support clinical significance of the MREIT conductivity imaging method.

MREIT Conductivity Imaging of Pneumonic Canine Lungs: Preliminary Post-mortem Study

Kim, Hyung-Joong,Kim, Young-Tae,Jeong, Woo-Chul,Minhas, Atul S.,Lee, Tae-Hwi,Lim, Chae-Young,Park, Hee-Myung,Kwon, O-Jung,Woo, Eung-Je The Korean Society of Medical and Biological Engin 2010 의공학회지 Vol.31 No.2

In magnetic resonance electrical impedance tomography (MREIT), a current-injection MR imaging method is adopted to produce a cross-sectional image of an electrical conductivity distribution in addition to MR images. The purpose of this study was to test the feasibility of MREIT for differentiating the canine lung parenchyma without and with pneumonia. Three normal healthy beagles and two mixed breed dogs with pneumonia were used. After attaching electrodes around the chest, we placed the dog inside our MR scanner. We injected as much as 30 mA current in a form of short pulses into the chest region. Reconstructed conductivity images of normal canine lungs exhibit a peculiar pattern of a relatively coarse salt and pepper noise. On the contrary, conductivity images of pneumonic canine lungs show significantly enhanced contrast of the lesions while the corresponding MR images show a little bit of contrast in the middle and caudal lung parenchyma due to the accumulation of pleural fluid. This preliminary study indicates that MREIT imaging of the chest may deliver unique new diagnostic information.

3T MREIT 시스템을 이용한 실험견 사체의 두부 도전율 영상

정우철,김영태,김형중,이태휘,강병택,박희명,우응제,Jeong, Woo-Chul,Kim, Young-Tae,Minhas, Atul S.,Kim, Hyung-Joong,Lee, Tae-Hwi,Kang, Byeong-Teck,Park, Hee-Myung,Woo, Eung-Je 대한의용생체공학회 2009 의공학회지 Vol.30 No.2

Magnetic Resonance Electrical Impedance Tomography (MREIT) is a new bio-imaging modality providing cross-sectional conductivity images from measurements of internal magnetic flux densities produced by externally injected currents. Recent MREIT studies demonstrated successful conductivity image reconstructions of postmortem and in vivo canine brain. However, the whole head imaging was not achieved due to technical issues related with electrodes and noise in measured magnetic flux density data. In this study, we used a new carbon-hydrogel electrode with a large contact area and injected 30 mA imaging current through a canine head. Using a 3T MREIT system, we performed a postmortem canine experiment and produced high-resolution conductivity images of the entire canine head. Collecting magnetic flux density data inside the head subject to multiple injection currents, we reconstructed cross-sectional conductivity images using the harmonic $B_z$ algorithm. The conductivity images of the canine head show a good contrast not only inside the brain region including white and gray matter but also outside the brain region including the skull, temporalis muscle, mandible, lingualis proprius muscle, and masseter muscle.