David Hoult: "Multiple RF Coils for MR Reception and Transmission – Problems and Solutions"
von 13:00 bis 13:30
|Wo||FRIAS Seminarraum, Albertstrasse 19, 79104 Freiburg|
|Name||Jan G. Korvink|
Open to University members / universitätsoffen
National Research Council, Winnipeg, Manitoba, Canada
Multiple RF Coils for MR Reception and Transmission – Problems and Solutions
The idea of using more than one coil for pulsed NMR transmission and reception was a natural extension of the employment of crossed coils for continuous wave spectroscopy. Two coils appeared to hold the prospect of a gain of 40% in signal-to-noise ratio and a reduction of a factor of 2 in transmitter power. However, when the idea was first attempted in 1966 in Oxford, it failed badly and was abandoned. It was successfully resurrected in 1983 by the author, who showed that causality demanded a reversal of phase for one coil when switching between transmission and reception. However, numerous subtleties were noted: on the positive side, B1 field homogeneity was improved, but on the negative side, the addition of a second coil could so degrade the performance of the first (and vice versa) that the addition became pointless; mutual inductance and parasitic coupling capacitance could detune high quality-factor coils; the "paddles" needed to annul the mutual inductance could decrease sensitivity, etc.. Notwithstanding, two areas have been found in which multiple coils are beneficial: when the effective sample volume is long and thin and when the sample is "lossy". As a result, the use of multiple coils is now common, but not optimal, in MR imaging. Thus the talk will describe the problems in more detail and discuss solutions that are either in place or have been proposed to facilitate multi-coil use.