R.S. Yoon

Measurement of thoracic current flow in pigs for the study of defibrillation and cardioversion

Although defibrillation has been in clinical use for more than 50 years, the complete current flow distribution inside the body during a defibrillation procedure has never been directly measured. This is due to the lack of appropriate imaging technology to noninvasively monitor the current flow inside the body. The current density imaging (CDI) technique, using a magnetic resonance (MR) imager, provides a new approach to this problem [Scott et al. (1991)]. CDI measures the local magnetic field generated by the current and calculates the current density by computing its curl. In this study, CDI was used to measure current density at all points within a postmortem pig torso during an electrical current application through defibrillation electrodes. Furthermore, current flow information was visualized along the chest wall and within the chest cavity using streamline analysis. As expected, some of the highest current densities were observed in the chest wall. However, current density distribution varied significantly from one region to another, possibly reflecting underlying heterogeneous tissue conductivity and anisotropy. Moreover, the current flow analysis revealed many complex and unexpected current flow patterns that have never been observed before. This study has, for the first time, noninvasively measured the volume current measurement inside the pig torso.

A new approach to current density impedance imaging

Current density impedance imaging (CDII) is a new impedance imaging technique that utilizes current density vector measurements made using magnetic resonance imager (MRI). CDII provides a simple mathematical expression for the gradient of the logarithm of conductivity, /spl nabla/ln(/spl sigma/), at each point in a region where two current density vector has been measured. From the images of the gradient of the logarithm of conductivity, ln(/spl sigma/) can be reconstructed through integration and of /spl sigma/ by a priori knowledge of the conductivity at a single point in the object. The CDII technique was tested on a conductivity phantom made from tissue mimicking gel. The results showed accurate reconstruction of the gel conductivity from two current density measurements. This study, for the first time, has demonstrated a local reconstruction technique to calculate sample conductivity inside the phantom noninvasively.

Changes in the complex permittivity during spreading depression in rat cortex

With recent developments in current density imaging (CDI), it is feasible to utilize this new technique in brain imaging applications. Since CDIs ability to measure changes in current density depends on a concomitant activity-dependent change in the conductivity of the brain tissue, the authors have examined the changes in complex conductivity during spreading depression (SD) in rodent neocortex using a coaxial probe. SD was chosen because it is often referred to as an animal model of cerebral ischemia and migraine with aura. The conductivity measurements revealed a change with short latency (30-60 s) followed by a change with a longer latency (200-300 s). This change in conductivity with short latency has not been reported before, and the authors conjecture that it may be the priming or triggering mechanism prior to the main SD episode. A 20% change in conductivity during SD is sufficiently large to be measured by CDI. Therefore, the ability to measure changes in the conductivity, as opposed to metabolic changes, makes CDI a viable approach to the study of ischemia and migraine with aura.

Why does MTR change with neuronal depolarization

T1 and T2 relaxation, and magnetization transfer (MT) of the rat brain were measured during experimentally induced spreading depression (SD). All measured MR parameters changed during SD: T1 relaxation increased by approximately 13%, whereas the T2 increase was substantially larger (88%). MT results showed an MT ratio (MTR) decrease of 9%. The lack of change in the MT exchange rate indicated that the MT processes between water and macromolecular protons are not affected by neuronal depolarization. The observed decrease in MTR was only caused by changes in T1 and T2 relaxation. Magn Reson Med 47:472–475, 2002.

A system for in-vivo cardiac defibrillation current density imaging in a pig

Current density imaging (CDI), based on magnetic resonance imaging (MRI), has been used in past studies to measure electrical current pathways associated with external defibrillation electrodes placed on a post-mortem pig. We are preparing to make the same measurements in a living pig. This abstract describes a fast low frequency CDI (FLFCDI) cardiac sequence for this purpose. This sequence is based on a commercial fast gradient recalled echo (FGRE) MRI sequence with cardiac triggering. This sequence can produce phase images of the heart of sufficient quality to be used for CDI. A CDI system appropriate for a living animal is described and solutions are proposed for all of the artifacts and design issues. The CDI system can apply current pulses of 150 mA zero-to-peak amplitude and 5 ms duration. Phantom images, measurements of electrode impedance, and CDI signal-to-noise ratio (SNR) for this sequence are presented.

Image distortion and image mis-registration in low frequency current density imaging

Current density imaging (CDI) is a technique that uses magnetic resonance imaging (MRI) to measure volume current density distributions in tissue. CDI is used to measure current pathways through tissue which adds a much needed tool to electrophysiological research such as in the area of defibrillation research. CDI maps magnetic fields, produced by an externally applied current, onto the phase images of an MRI data set. Current density is computed from the curl of the these magnetic fields. Two CDI artifacts, image distortion and image mis-registration, are studied in this article. Spatial encoding of MR images is achieved by a set of magnetic field gradients. The nonlinearity of these gradient fields causes image distortion. This article reports on the measurement of this distortion using a phantom consisting of a 3D rectangular array of point sources and the subsequent correction of this distortion using feature mapping and interpolation. Image distortion in CDI also causes mis-registration of overlying data sets. Mis-registration leads to incorrect computation of current density due to violation of Maxwells equations. In simulation, mis-registration was also found to cause current density and the curl of current density to exhibit nonzero values in locations where proper registration gives zero current density.

Study of current density distribution in a non-invasive breast cancer detection device

Current density imaging (CDI) using a magnetic resonance (MR) imager has been shown to accurately measure electrical current density in a conductive object. CDI measures the magnetic field generated by the current and converts it to current density (CD) by computing its curl. Therefore, CDI avoids both the inverse problem and invasiveness of other electrical measurement techniques such as electrical impedance tomography and direct electrode measurement. In this study, CDI is used to measure the three dimensional current density distribution inside the breast phantom using the same electrode location as the non-invasive breast cancer detection device. The current flow lines inside the phantom were also examined using the streamline analysis. This study experimentally characterized the sensitivity of the electrode configuration and confirmed its ability to monitor the entire breast.

Defibrillation current density imaging: comparison of in-vivo and post-mortem measurements in a pig

Current density imaging (CDI) is an MRI technique used to measure electrical current density vectors throughout a volume of tissue. Previous work used CDI to measure current pathways through the heart and chest of a post-mortem pig when current is applied using external flexible defibrillation electrodes with typical anterior-anterior positioning. In these post-mortem studies, current pathways were probably influenced by the anisotropic conductivity of the tissues. This work aims to compare post-mortem (/spl sim/15 min. and /spl sim/1 hour after death) results with new in vivo CDI measurements. These measurements indicate that the macroscopic (i.e. across the whole body) current pathways remain similar before and after death, however, at a smaller scale (i.e. distances of a few cm) current pathways are different, particularly in the heart. This comparison demonstrates the influence on current pathways of rapidly changing electric properties of tissue following death.

Current density imaging of electrical current pathways inside the pig torso

Low frequency current density imaging (LFCDI) using a magnetic resonance (MR) imager has been shown to accurately measure electrical current density inside a phantom. CDI measures the magnetic field generated by the current and converts it to current density (CD) by computing its curl. Therefore, CDI avoids both the inverse problem and invasiveness of other electrical measurement techniques such as electrical impedance tomography and direct electrode measurement. This makes CDI an ideal technique for studying the current flow inside the body during electrical therapies such as defibrillation where the current density in tissue is closely associated with the efficacy. Here we report simultaneous measurements of current density at all points within the pig torso during an electrical current application through defibrillation electrodes. Current flow was visualized by computing streamlines from the current density vectors. We observed current flow over the chest walls in agreement with the current literature. However, complex and unexpected current flow patterns were seen inside the heart as well as in the surrounding vasculature. This study represents the first noninvasive volume current measurement inside the pig torso during an electrical current application.