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Noise figure limits for circular loop MR coils
Author(s) -
Kumar Ananda,
Edelstein William A.,
Bottomley Paul A.
Publication year - 2009
Publication title -
magnetic resonance in medicine
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.696
H-Index - 225
eISSN - 1522-2594
pISSN - 0740-3194
DOI - 10.1002/mrm.21948
Subject(s) - electromagnetic coil , physics , noise (video) , capacitor , electrical conductor , radiofrequency coil , eddy current , acoustics , radius , detector , nuclear magnetic resonance , optics , materials science , voltage , computer science , computer security , quantum mechanics , artificial intelligence , image (mathematics)
Circular loops are the most common MR detectors. Loop arrays offer improved signal‐to‐noise ratios (SNRs) and spatial resolution, and enable parallel imaging. As loop size decreases, loop noise increases relative to sample noise, ultimately dominating the SNR. Here, relative noise contributions from the sample and the coil are quantified by a coil noise figure (NF), NF coil , which adds to the conventional system NF. NF coil is determined from the ratio of unloaded‐to‐loaded coil quality factors Q . Losses from conductors, capacitors, solder joints, eddy currents in overlapped array coils, and the sample are measured and/or computed from 40 to 400 MHz using analytical and full‐wave numerical electromagnetic analysis. The Q s are measured for round wire and tape loops tuned from 50 to 400 MHz. NF coil is determined as a function of the radius, frequency, and number of tuning capacitors. The computed and experimental Q s and NF coil s agree within ∼10%. The NF coil values for 3 cm‐diameter wire coils are 3 dB, 1.9 dB, 0.8 dB, 0.2 dB, and 0.1 dB, at 1T, 1.5T, 3T, 7T, and 9.4T, respectively. Wire and tape perform similarly, but tape coils in arrays have substantial eddy current losses. The ability to characterize and reliably predict component‐ and geometry‐associated coil losses is key to designing SNR‐optimized loop and phased‐array detectors. Magn Reson Med, 2009. © 2009 Wiley‐Liss, Inc.