Feasibility of Imaging Tissue Electrical Conductivity by Switching Field Gradients with MRI.

Tissue conductivity is a biophysical marker of tissue structure and physiology. Present methods of measuring tissue conductivity are limited. Electrical impedance tomography, and magnetic resonance electrical impedance tomography rely on passing external current through the object being imaged, which prevents its use in most human imaging. Recently, the RF field used for MR excitation has been used to non-invasively measure tissue conductivity. This technique is promising, but conductivity at higher frequencies is less sensitive to tissue structure. Measuring tissue conductivity non-invasively at low frequencies remains elusive. It has been proposed that eddy currents generated during the rise and decay of gradient pulses could act as a current source to map low-frequency conductivity. This work centers on a gradient echo pulse sequence that uses large gradients prior to excitation to create eddy currents. The electric and magnetic fields during a gradient pulse are simulated by a finite-difference time-domain simulation. The sequence is also tested with a phantom and an animal MRI scanner equipped with gradients of high gradient strengths and slew rate. The simulation demonstrates that eddy currents in materials with conductivity similar to biological tissue decay with a half-life on the order of nanoseconds and any eddy currents generated prior to excitation decay completely before influencing the RF signal. Gradient-induced eddy currents can influence phase accumulation after excitation but the effect is too small to image. The animal scanner images show no measurable phase accumulation. Measuring low-frequency conductivity by gradient-induced eddy currents is presently unfeasible

Aircraft electromagnetic compatibility

Illustrated are aircraft architecture, electromagnetic interference environments, electromagnetic compatibility protection techniques, program specifications, tasks, and verification and validation procedures. The environment of 400 Hz power, electrical transients, and radio frequency fields are portrayed and related to thresholds of avionics electronics. Five layers of protection for avionics are defined. Recognition is given to some present day electromagnetic compatibility weaknesses and issues which serve to reemphasize the importance of EMC verification of equipment and parts, and their ultimate EMC validation on the aircraft. Proven standards of grounding, bonding, shielding, wiring, and packaging are laid out to help provide a foundation for a comprehensive approach to successful future aircraft design and an understanding of cost effective EMC in an aircraft setting

Time domain analysis of switching transient fields in high voltage substations

Switching operations of circuit breakers and disconnect switches generate transient currents propagating along the substation busbars. At the moment of switching, the busbars temporarily acts as antennae radiating transient electromagnetic fields within the substations. The radiated fields may interfere and disrupt normal operations of electronic equipment used within the substation for measurement, control and communication purposes. Hence there is the need to fully characterise the substation electromagnetic environment as early as the design stage of substation planning and operation to ensure safe operations of the electronic equipment. This paper deals with the computation of transient electromagnetic fields due to switching within a high voltage air-insulated substation (AIS) using the finite difference time domain (FDTD) metho

Design guidelines for assessing and controlling spacecraft charging effects

The need for uniform criteria, or guidelines, to be used in all phases of spacecraft design is discussed. Guidelines were developed for the control of absolute and differential charging of spacecraft surfaces by the lower energy space charged particle environment. Interior charging due to higher energy particles is not considered. A guide to good design practices for assessing and controlling charging effects is presented. Uniform design practices for all space vehicles are outlined

Determining Enclosure Breach Electromagnetically

A structure breach may be determined. A sensor, provided in the structure, may be driven with a constant frequency signal. The sensor may comprise a first conductive element and a second conductive element. The first conductive element may be substantially parallel with the second conductive element. A standing wave pattern may be induced on the sensor by the constant frequency signal reflecting off a termination point of the sensor. A least one characteristic of the sensor caused by the voltage standing wave pattern may be measured. A breach occurrence in the structure may be determined when the measured at least one characteristic varies from a previously determined value by a predetermined amount. The first conductive element and the second conductive element may be sandwiched between two layers comprising the structure. The structure may comprise a shipping container floor. The detected breach may comprise an opening greater than nine square inches.Georgia Tech Research Corporatio

A Low Temperature Co-fired Ceramic (LTCC) Interposer Based Three-Dimensional Stacked Wire Bondless Power Module

The objective of this dissertation research is to develop a low temperature co-fired ceramic (LTCC) interposer-based module-level 3-D wire bondless stacked power module. As part of the dissertation work, the 3-D wire bondless stack is designed, simulated, fabricated and characterized. The 3-D wire bondless stack is realized with two stand-alone power modules in a half-bridge configuration. Each stand-alone power module consists of two 1200 V 25 A silicon insulated-gate bipolar transistor (IGBT) devices in parallel and two 1200 V 20 A Schottky barrier diodes (SBD) in an antiparallel configuration. A novel interconnection scheme with conductive clamps and a spring loaded LTCC interposer is introduced to establish electrical connection between the stand-alone power modules to connect them in series to realize a half-bridge stack. Process development to fabricate the LTCC based 3-D stack is performed. In traditional power modules, wire bonds are used as a top side interconnections that introduce additional parasitic inductance in the current conduction path and prone to failure mechanism under high thermomechanical stresses. The loop inductance of the proposed 3-D half-bridge module exhibits 71% lower parasitic inductance compared to a wire bonded module. The 3-D stack exhibits better switching performance compared to the wire bonded counterpart. The measurement results for the 3-D stack shows 30% decrease in current overshoot at turn-on and 43% voltage overshoot at turn-off compared to the wire bonded module. Through measurements, it has been shown that the conducted noise reduces by 20 dB in the frequency range 20-30 MHz for the 3-D stack compared to the wire bonded counterpart. A simulation methodology using co-simulation techniques using ANSYS EM software tools is developed to predict EMI of a power module. Hardware verification of the proposed simulation methodology is performed to validate the co-simulation technique. The correlation coefficient between the measurement and simulation is found to be 0.73. It is shown that 53% of the variability in the simulation can be explained by the simulated result. Moreover, the simulated and measured amplitudes of the EMI spectrum closely match with each other with some variations due to round-off errors due to the FFT conversion

The analysis and modeling of fine pitch laminate interconnect in response to large energy fault transients

In embedded applications, the miniaturization of circuitry and functionality provides many benefits to both the producer and consumer. However, the benefits gained from miniaturization is not without penalty, as the environmental influences may be great enough to introduce system failures in new or different modes and effects;Of particular interest within this research is the effect of fault transients in reduced geometries of printed circuit card interconnect, commonly referred to as fine pitch laminate interconnect. Whereas larger geometries of conductor trace width and spacing may have been immune to circuit failure at a given fault input, the reduction of the trace geometry may introduce failures as the insulating effect of the dielectric is compromised to the point where arcing occurs;To address this concern, a circuit card was designed with fine pitch laminate features in microstrip, embedded microstrip, and stripline constructions. Various trace widths and separations were tested for structural integrity (presence of arcing or fusing) at voltage extremes defined in avionics standard. The specific trace widths in the test were 4 mils, 6 mils, 8 mils, and 12 mils, with the trace separation in each case equal to the trace widths. The results of the tests and methods to artificially improve the integrity of the interconnect are documented, providing a clear region of reliable operation to the designers and the engineering community;Finally, the construction of the interconnect and the results from the test were combined to create an empirical model for circuit analysis. Created for the Saber simulator, but readily adaptable to Spice, this model will describe high-speed operation of a propagating signal before breakdown, and uses data from the experiment to calculate threshold values for the arcing breakdown. The values for the breakdown voltages are correlated to the experimental data using statistical methods of weighted linear regression and hypothesis testing

Nanopulse Generators: Their Design and Application to Cancer Therapy Studies

Effective nanopulse generators have become critical in recent decades concerning the study of subcellular affects in response to nanosecond pulsed electric fields. It has been observed that nanosecond duration electric pulses can target intracellular organelles, ultimately leading to cell apoptosis, suggesting the possibility of a new, minimally invasive, low risk cancer therapy methodology. The standard topology for developing a medical nanopulser is the Blumlein “transmission line” approach. This approach relies on the nearly infinitesimal, yet finite amount of time required for an electromagnetic field to propagate down a short transmission line. Prior to design, requirements and constraints must be defined that are determined by the specific applications and experiments that the nanopulser will be used for. Special effort must be put into nanopulser design to prevent undesirable reflections and oscillations at the load. Critical design objectives common to most nanopulse generators include choosing effective switching elements that facilitate a minimal rise time, configuring the load electrodes to be compatible with experimental setups, and enabling a wide degree of versatility and adjustability concerning pulse parameters