Redpath T W
Department of Clinical Physics, Walsgrave Hospital, Coventry, England.
Magn Reson Med. 1992 Mar;24(1):85-9. doi: 10.1002/mrm.1910240109.
When two or more rf coils are used to receive NMR signals from the same part of the patient, the overall sensitivity of an image formed from some combination of these signals may be degraded if noise voltages in the coils are correlated. Whereas it is straightforward to calculate correlation caused by noise sources in the coils themselves, there is disagreement about noise correlation when that noise arises from thermally driven currents within the body itself. A general analysis based on the principles of reciprocity, superposition, and energy conservation yields an equation for sample-induced noise correlation in terms of circuit impedances. Furthermore, the analysis yields a very simple expression for the total noise correlation (i.e., noise from patient and coil elements) between the coils. This latter expression is applicable to any two-port receiver coil assembly, including single coils with two modes, such as a birdcage coil, and in general, to any two ports of an n-port receiver system.
当使用两个或更多射频线圈从患者的同一部位接收核磁共振信号时，如果线圈中的噪声电压相关，那么由这些信号的某种组合形成的图像的整体灵敏度可能会降低。虽然计算由线圈自身噪声源引起的相关性很简单，但当噪声源于身体自身的热驱动电流时，对于噪声相关性存在不同看法。基于互易性、叠加原理和能量守恒原理的一般分析得出了一个关于样本引起的噪声相关性与电路阻抗关系的方程。此外，该分析还得出了一个关于线圈之间总噪声相关性（即来自患者和线圈元件的噪声）的非常简单的表达式。后一个表达式适用于任何双端口接收线圈组件，包括具有两种模式的单线圈，如鸟笼线圈，并且一般适用于n端口接收系统的任何两个端口。
