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Optimizing MRI signal‐to‐noise ratio for quadrature unmatched RF coils: Two preamplifiers are better than one
Author(s) -
Sorgenfrei Birgit L.,
Edelstein William A.
Publication year - 1996
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.1910360118
Subject(s) - preamplifier , electrical impedance , physics , quadrature (astronomy) , electromagnetic coil , noise (video) , acoustics , electrical engineering , nuclear magnetic resonance , amplifier , computer science , engineering , optics , optoelectronics , cmos , quantum mechanics , artificial intelligence , image (mathematics)
Using separate preamplifiers for the two outputs of a quadrature receive coil (and then combining the preamplifier outputs in a quadrature hybrid) provides a better signal‐to‐noise ratio (SNR) than is obtained by directly combining the quadrature outputs in a hybrid followed by a single preamplifier. The advantage of the two‐preamplifier configuration increases when the body coil impedance changes and is no longer matched to 50 ohms. Using 0.4 dB noise figure preamplifiers, theory predicts 1.53, 0.42, 0, 0.42, and 1.53 dB SNR advantage of the two‐preamplifier configuration over the one‐preamplifier arrangement at body coil impedances of 12.5, 25, 50, 100, and 200 ohms, respectively. Experimental hot/cold resistor noise figure measurements indicate 2.86, 0.65, 0.36, 0.83, and 1.40 dB noise figure advantage for the two preamplifier configuration relative to the one‐preamplifier configuration at those impedances. Empirical gains larger than theoretically calculated are attributable to insertion losses of various circuit elements, such as the quadrature hybrid, for the onepreamplifier configuration.