Noise figure limits for circular loop MR coils.

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 Qs 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 Qs and NF(coil)s agree within approximately 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.