A study of winding interactions in the NMR imagers
In an NMR imaging system, there are five principal sets of windings, namely the main magnet, the radio frequency coil and three sets of field gradient windings. In principle, these windings should all be mutually orthogonal, that is, their mutual inductance should be zero. However, when these windings are assembled together in a limited space inside an NMR imager, they are found to be interacting with each other, which can significantly degrade the imager performance. The experimental investigations on the existing winding sets indicate that there are significant interactions between the rf receiver coil and three distributed gradient windings. In particular, the longitudinal gradient winding, acting along the rf field direction, can interact more severely with the rf coil than the other two gradient windings. Such interactions can seriously damp the desired resonance of the rf receiver coil, and displace its desired resonant frequency, leading to a lower signal-to-noise ratio in the image. To understand the interactions between the rf coil and this particular gradient winding, theoretical models have been developed and experimentally verified. The analysis based on these model calculations shows that there is a mutual coupling between the rf coil and the individual parts of the gradient winding. This can introduce a significant loss into the rf coil and change its resonant frequency. The model study has also been extended to the analysis of the multiple resonance properties of distributed gradient windings. The effects of these resonances on the rf coil are also discussed. The possibility of isolating the rf coil from the gradient windings by means of an rf shield has been considered. The experimental results indicate that, for a 3:2 shield radius to coil radius ratio, the losses of the rf coil due to such a shield are much smaller than those due to the winding interactions. The investigations also indicate a noticeable improvement of the rf field homogeneity when using such a shield.