Otward M. Mueller

Comparison of linear and circular polarization for magnetic resonance imaging

Abstract A comparison of experimental imaging results obtained with linearly polarized and circularly polarized radiofrequency excitation and reception is presented. Simulation images in good agreement with the experimental scans are described. The simulations are calculated with a model in which a homogeneous, isotropic cylinder of lossy dielectric material and infinite axial extent is immersed in a uniform rf magnetic field perpendicular to the axis. It is found that with the usual linear polarization, reconstructions of uniform objects have regions of decreased intensity. These artifacts are shown to arise from dielectric standing wave effects and eddy currents. The effects become more severe as the frequency or object size is increased, and depend upon the complex conductivity of the object. Results indicate that a significant reduction in the artifact intensity is achieved when circular polarization is employed for both transmission and reception. The expected benefits of circular polarization over linear polarization in reduction of excitation power (up to 50% reduction) and signal-to-noise advantage (√2) have been realized in practice with cylindrical objects and human subjects.

Technical alternatives in nuclear magnetic resonance (NMR) imaging

In this paper we consider the choice of the magnetic field for an imaging system based on the nuclear magnetic resonance of hydrogen. We show by analysis that the quality, or contrast-to-noise ratio, of images based on T1 discrimination increases with field or frequency up to 2 T or 85 MHz. After a brief discussion of potential engineering limitations we present results showing that images of the human head with excellent anatomic detail can be produced at 1.5 T or 64 MHz.

SiGe and Ge Cryogenic Power Transistors

We have undertaken development of power semiconductor devices based on SiGe and Ge, intended for operation over a wide temperature range, from room temperature down to deep cryogenic temperatures, ~30 K (~ -240{degree sign}C). We summarize results for Ge diodes (~10 A/400 V), Ge junction field-effect transistors (Ge JFETs, ~0.5 A/30 V), Ge metal-insulator field-effect transistors (Ge MIS-FETs ~1 A/20 V) and SiGe heterojunction bipolar transistors (SiGe HBTs, ~10 A/50 V). As a practical demonstration, we have used SiGe HBTs and SiGe diodes as the active devices in 100-W power-conversion circuits, operating from room temperature down to ~30 K (~ -240{degree sign}C) with increased efficiency as temperature de-creases. Our developments demonstrate the potential of alternative semiconductor materials to provide excellent performance for power applications down to deep cryogenic temperatures.