Magnetic Resonance Imaging: Applied Physics and Electromagnetics – William A. Edelstein

Date: Thu. April 11th, 2002, 4:15 pm-5:15 pm
Location: Rockefeller 301

Magnetic Resonance Imaging (MRI) is the most novel and important medical imaging modality since the advent of the X-ray. MRI grew out of the long development by physicists of atomic spectroscopy, atomic and molecular beam resonance and, finally, nuclear magnetic resonance (NMR) in condensed matter. The operation and economics of MRI systems depend critically on the performance of magnets, pulsed magnetic field gradient windings and rf coils, and necessity has spurred development of much science and innovative technology in these areas. Superconducting magnets have come to be the magnet of choice because of their ability to produce strong, stable, homogeneous magnetic fields (0.5 T to 8 T) in large enough volumes to accommodate human subjects. Gradient windings present both theoretical electromagnetic and MRI challenges. The need for rf (radiofrequency) coils which resonate at high frequencies while surrounding large spatial regions has inspired coils that produce uniform rf magnetic fields over a substantial volume and minimize electric interactions with the imaging subject. These coils can be configured to create (or receive) a rotating rf magnetic field that reduces applied rf power and improves MRI signal-to-noise ratio. Additionally, it is possible to use arrays of small rf coils (the “NMR phased array”) to obtain MRI images with the high signal-to-noise ratio of a small surface coil and the field of view of a large volume coil. Recently we investigated, and devised ways to mitigate, the intense acoustic noise (up to 110 dB) produced in MRI scanners. Surprisingly, some of the loudest acoustic noise is generated by eddy-current-induced forces in the magnet and imager structure.