Kumar Ananda, Edelstein William A, Bottomley Paul A
Department of Radiology, Division of MR Research, Johns Hopkins University, Baltimore, Maryland 21205, USA.
Magn Reson Med. 2009 May;61(5):1201-9. doi: 10.1002/mrm.21948.
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.
圆形线圈是最常见的磁共振探测器。线圈阵列可提高信噪比（SNR）和空间分辨率，并实现并行成像。随着线圈尺寸减小，线圈噪声相对于样本噪声增加，最终主导SNR。在此，样本和线圈的相对噪声贡献通过线圈噪声系数（NF），即NF（线圈）来量化，它与传统系统NF相加。NF（线圈）由空载与加载线圈品质因数Q的比值确定。使用解析和全波数值电磁分析，在40至400MHz范围内测量和/或计算导体、电容器、焊点、重叠阵列线圈中的涡流以及样本的损耗。对50至400MHz调谐的圆线和带状线圈测量Q值。NF（线圈）被确定为半径、频率和调谐电容器数量的函数。计算得到的Q值和NF（线圈）值与实验值在约10%的范围内相符。直径3cm的线绕线圈在1T、1.5T、3T、7T和9.4T时的NF（线圈）值分别为3dB、1.9dB、0.8dB、0.2dB和0.1dB。圆线和带状线圈表现相似，但阵列中的带状线圈存在大量涡流损耗。表征并可靠预测与组件和几何形状相关的线圈损耗的能力是设计SNR优化的线圈和相控阵探测器的关键。
