Michael L. G. Joy

Measurement of nonuniform current density by magnetic resonance

A noninvasive tissue current measurement technique and its use in measuring a nonuniform current density are described. This current density image is created by measuring the magnetic field arising from these currents and taking its curl. These magnetic fields are proportional to the phase component of a complex magnetic resonance image. Measurements of all three components of a quasistatic nonuniform current density in a phantom are described. Expected current density calculations from a numerical solution for the magnetic field which was created by the phantom are presented for comparison. The results of a numerical simulation of the experiment, which used this field solution and which included the effects of slice selection and sampling, are also presented. The experimental and simulated results are quantitatively compared. It is concluded that the principle source of systematic error was the finite slice thickness, which causes blurring of boundaries.

Sensitivity of magnetic-resonance current-density imaging

Abstract This paper analyzes the sensitivity of magnetic-resonance current-density imaging (CDI) to random noise and systematic errors with the goal of providing a protocol for achieving a targeted noise performance. All analyses are checked with an electrolytic phantom designed to create a uniform current density or with an equivalent computer model. CDIs sensitivity properties are split between low and high current-time regimes where random noise or systematic error, respectively, dominate. Large current-time products introduce nonlinear signal loss in the magnitude image. Image alignment errors, spatial linearity, pixel size calibration, and current-dependent ringing can exceed random noise. In the low current-time limit, random noise dominates and is proportional to the inverse cube of the planar pixel dimensions. A spin-echo sequence minimizes the current-density noise for current integration times and echo times in the neighborhood of T 2 . The relation between current-density noise and the MR image resolution and signal-to-noise ratio is formulated, allowing one to predict the noise for experimental planning. The lower practical limit of sensitivity is estimated to be 0.1 A/m 2 for a 1.5 × 1.5 × 5 mm voxel.

Computer- and Robot-assisted Resection of Thalamic Astrocytomas in Children

Six children ranging in age from 2 to 10 years who harbored deep benign astrocytomas were operated upon using a computer- and robot-assisted system. A radical excision was achieved in all cases with no significant morbidity nor any mortality. The system consists of an interactive, three-dimensional display of computed tomographic image contours and digitized cerebral angiograms taken using the Brown-Roberts-Wells stereotactic frame. The surgical retractor is held and manipulated using a PUMA 200 robot. The position and orientation of the surgical retractor is displayed on the three-dimensional display. Preoperative planning and simulation are important features of this system. Movement of the brain after removal of the tumor and cerebrospinal fluid is substantial, so the tumor removal is based on visually defined margins. Enhanced computer graphics and robotic devices are important adjuncts to neurosurgical procedures and will find increasing use in the future.

Imaging of current density and current pathways in rabbit brain during transcranial electrostimulation

A magnetic resonance imaging (MRI) method was used for a noninvasive study of current density (CD) and current pathways (CPs) inside the skull during transcranial electrostimulation in rabbits. The transcranial impulse current directions studied were those previously used in transcranial electric treatment either sagittally or bilaterally. MRI data were collected from slices perpendicular to the direction of current application. In these slices, only the perpendicular component of the CD was measured. Computer methods for accurate topographic mapping of the main areas with high CD and for reconstruction of CPs are described. It was revealed that current applied on the head sagittally passed mostly through the cerebrospinal fluid in the basal brain cisternas connected in series, and through the anterior horns of the lateral ventricles, foramina of Monro, ventrocaudal part of the third ventricle, aqueductus, and fourth ventricle. Possible connections between these CPs are suggested. Bilaterally applied current passed through the brain and skull core more diffusely without concentrations in cisternas and ventricles. The results of the present study suggest an explanation for the observation that sagittally applied current more effectively stimulates brain structures with antinociceptive function and elicits more pronounced analgesic effect.

Current Density Impedance Imaging

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 et al., 1991). CDII is based on a simple mathematical expression for nablasigma/sigma = nabla 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 ne 0. From the calculated nabla In sigma and a priori knowledge of the conductivity at the boundary, the logarithm of the conductivity In 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.

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.

MR current density and conductivity imaging: the state of the Aart

Current density imaging (CDI) is an imaging technique that measures electrical current density distributions in a volume of material or tissue, which can be imaged using magnetic resonance imaging (MRI). Measurements of current density are obtained by applying an external current to the material/tissue during an MRI acquisition. The magnetic fields produced by the applied current are mapped onto the phase image of the MRI acquisition. The phase images are processed to compute the current density distribution. Performing CDI requires an MRI system, additional hardware, a modified pulse sequence (PSD) and data processing software. Greig C. Scott, Michael L.G. Joy and R. Mark Henkelman developed CDI in 1988 at the University of Toronto (Canada). The CDI Research Group is presently based at the University of Toronto and is supervised by the author. This paper describes the CDI technique, its applications by this and other groups and recently proposed methods for electrical conductivity imaging based on the technique.

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.

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.

Computer and robotic assisted resection of brain tumours

Six children ages 2 to 10 years harbouring deep brain tumours were operated upon using a computer and robotic assisted system. A radical excision was achieved in all cases with no significant morbidity or any mortality. The system consists of an interactive 3 dimensional (3D) display of computed tomography image contours and digitized cerebral angiograms taken using the BRW stereotactic frame. The surgical retractor is held and manipulated using a PUMA 200 robot. The position and orientation of the surgical retractor is displayed on the 3D display. Preoperative planning and simulation are important features of this system. Movement of the brain following tumour and cerebrospinal fluid removal is substantial so the tumour removal is based on visually defined margins. Enhanced computer graphics and robotic devices are important adjuncts to neurosurgical procedures and will find increasing use in the future.<<ETX>>