Sorgenfrei B L, Edelstein W A
GE Medical Systems, Waukesha, Wisconsin, USA.
Magn Reson Med. 1996 Jul;36(1):104-10. doi: 10.1002/mrm.1910360118.
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 one-preamplifier configuration.
对正交接收线圈的两个输出使用单独的前置放大器（然后在正交混合器中合并前置放大器的输出）比在混合器中直接合并正交输出然后使用单个前置放大器能提供更好的信噪比（SNR）。当体线圈阻抗发生变化且不再匹配50欧姆时，双前置放大器配置的优势会增加。使用噪声系数为0.4 dB的前置放大器，理论预测在体线圈阻抗分别为12.5、25、50、100和200欧姆时，双前置放大器配置相对于单前置放大器配置的信噪比优势分别为1.53、0.42、0、0.42和1.53 dB。实验性的热/冷电阻噪声系数测量表明，在这些阻抗下，双前置放大器配置相对于单前置放大器配置的噪声系数优势分别为2.86、0.65、0.36、0.83和1.40 dB。实际增益大于理论计算值可归因于单前置放大器配置中各种电路元件（如正交混合器）的插入损耗。
