Tim P. DeMonte

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.

Current density imaging and electrically induced skin burns under surface electrodes

The origin of electrical burns under gel-type surface electrodes is a controversial topic that is not well understood. To investigate the phenomenon, we have developed an excised porcine skin-gel model, and used low-frequency current density imaging (LFCDI) to determine the current density (CD) distribution through the skin before and after burns were induced by application of electrical current (200 Hz, 70% duty cycle, 20-35mA monophasic square waveform applied to the electrodes for 30-135min). The regions of increased CD correlate well with the gross morphological changes (burns) observed. The measurement is sensitive enough to show regions of high current densities in the pre-burn skin, that correlate with areas were burn welts were produced, thus predicting areas where burns are likely to occur. Statistics performed on 28 skin patches revealed a charge dependency of the burn areas and a relatively uniform distribution. The results do not support a thermal origin of the burns but rather electro-chemical mechanisms. We found a statistically significant difference between burn area coverage during anodic and cathodic experiments.

Investigation of current densities produced by surface electrodes using finite element modeling and current density imaging

Designers of gel-type surface electrodes, used in medical applications such as pain relief and neuromuscular stimulation, require a more thorough understanding of current pathways in tissue in order to design more effective electrical stimulation systems. To investigate these pathways, a finite element model (FEM) was used to compute current density distributions produced by an electrode placed on the surface of a homogeneous, tissue-mimicking gel slab. The gel slab phantom was constructed and the current densities were measured using a recently developed technique called current density imaging (CDI). CDI uses the phase data produced by magnetic resonance imaging (MRI) as a measure of the magnetic fields produced by the externally applied current. The results of the FEM simulation and CDI measurements compare well. CDI has several potential advantages over conventional FEM techniques including: no requirement for knowledge of local tissue conductivities, low and constant computational overhead regardless of tissue complexity, and the potential to perform in-vivo measurements.

Multislice Radio-Frequency Current Density Imaging

Radio-frequency current density imaging (RF-CDI) is an imaging technique that noninvasively measures current density distribution at the Larmor frequency utilizing magnetic resonance imaging (MRI). Previously implemented RF-CDI techniques were only able to image a single slice transverse to the static magnetic field B0 . This paper describes the first realization of a multislice RF-CDI sequence on a 1.5 T clinical imager. Multislice RF current density images have been reconstructed for two phantoms. The influence of MRI random noise on the sensitivity of the multislice RF-CDI measurement has also been studied by theoretical analysis, simulation and phantom experiments.

In-vivo measurement of relationship between applied current amplitude and current density magnitude from 10 mA to 110 mA

Current density imaging (CDI) is a magnetic resonance imaging (MRI) technique used to quantitatively measure current density vectors throughout the volume of an object/subject placed in the MRI system. Electrical current pulses are applied externally to the object/subject and are synchronized with the MRI sequence. In this work, CDI is used to measure average current density magnitude in the torso region of an in-vivo piglet for applied current pulse amplitudes ranging from 10 mA to 110 mA. The relationship between applied current amplitude and current density magnitude is linear in simple electronic elements such as wires and resistors; however, this relationship may not be linear in living tissue. An understanding of this relationship is useful for research in defibrillation, human electro-muscular incapacitation (e.g. TASER®) and other bioelectric stimulation devices. This work will show that the current amplitude to current density magnitude relationship is slightly nonlinear in living tissue in the range of 10 mA to 110 mA.

Experimental implementation of a new method of imaging anisotropic electric conductivities

This paper presents the first experiment of imaging anisotropic impedance using a novel technique called Diffusion Tensor Current Density Impedance Imaging (DTCD-II). A biological anisotropic tissue phantom was constructed and an experimental implementation of the new method was performed. The results show that DT-CD-II is an effective way of non-invasively measuring anisotropic conductivity in biological media. The cross-property factor between the diffusion tensor and the conductivity tensor has been carefully determined from the experimental data, and shown to be spatially inhomogeneous. The results show that this novel imaging approach has the potential to provide valuable new information on tissue properties.

Measurement of current density vectors in a live pig for the study of human electro-muscular incapacitation devices

Current density imaging (CDI) is an MRI technique used to quantitatively measure current density vectors in biological tissue. A CDI sequence and corresponding experimental method were developed for the study of human electro-muscular incapacitation (HEMI) devices using an animal model. Measurements of current density vectors were performed in piglets weighing 4 to 5 kg. Pathways of current density vectors in the region of the chest and heart were investigated using vector plotting and streamline integration methods. Measurement of current density vectors were also used to analyze the relationship between applied current amplitude and measured current density magnitude in the range of 10 mA to 45 mA of applied current.

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.

Detecting skin burns induced by surface electrodes

The origin of electrical burns under gel-type surface electrodes is a controversial topic that is not well understood. To investigate the phenomenon, we have developed an excised porcine skin+gel model. In the present paper, we describe methods to detect these burns in the skin+gel model in an effort to understand the genesis of these burns. Burns were induced by severe electrical stimulation and changes in the impedance spectra and current density measured. We found that the changes in impedance spectrum were characterized by a significant drop in the low frequency (<1 kHz) impedance magnitude and the formation of welts in the skin. Low frequency current density imaging (LFCDI) revealed regions of high current density beneath the electrode before burns were induced suggesting the possibility of predicting the locations where welts from burns will form and the importance of current density and local tissue impedance in the formation of these burns.

Noise Analysis for Multi-slice Radio Frequency Current Density Imaging

Radio frequency current density imaging (RF-CDI) is an imaging technique that measures current density distribution at the Larmor frequency utilizing magnetic resonance imaging (MRI). The multi-slice RF-CDI sequence has extended the ability of RF-CDI to image multiple slices and thus has enhanced its capacity for biomedical applications. In this paper, the influence of MRI random noise on the sensitivity of multi-slice RF-CDI measurement is studied. The formula of current noise is derived, which is verified by both simulation and phantom experiments. A 3-D finite-difference time-domain (FDTD) model is employed to compute the electromagnetic fields in the simulation