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 impedance imaging (CDII) is a new impedance imaging technique that can noninvasively measure the conductivity distribution inside a medium. It utilizes current density vector measurements which can be made using a magnetic resonance imager (MRI) (Scott , 1991). CDII is based on a simple mathematical expression for inverted Delta sigma / sigma = inverted Delta ln sigma, the gradient of the logarithm of the conductivity sigma, at each point in a region where two current density vectors J1 and J2 have been measured and J1 x J2 not equal 0. From the calculated inverted Delta ln sigma and a priori knowledge of the conductivity at the boundary, the logarithm of the conductivity ln sigma is integrated by two different methods to produce an image of the conductivity sigma in the region of interest. The CDII technique was tested on three different conductivity phantoms. Much emphasis has been placed on the experimental validation of CDII results against direct bench measurements by commercial LCR meters before and after CDII was performed.

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, nablaln(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(sigma) can be reconstructed through integration and of 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.

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