Giovannetti Giulio, Landini Luigi, Santarelli Maria Filomena, Positano Vincenzo
Institute of Clinical Physiology, National Council of Researches, Via Moruzzi 1, 56124 S Cataldo, Pisa, Italy.
MAGMA. 2002 Nov;15(1-3):36-44. doi: 10.1007/BF02693842.
The birdcage coils are extensively used in MRI systems since they introduce a high signal to noise ratio and a high radiofrequency magnetic field homogeneity that guarantee a large field of view. The present article describes the implementation of a birdcage coil simulator, operating in high-pass and low-pass modes, using magnetostatic analysis of the coil. Respect to other simulators described in literature, our simulator allows to obtain in short time not only the dominant frequency mode, but also the complete resonant frequency spectrum and the relevant magnetic field pattern with high accuracy. Our simulator accounts for all the inductances including the mutual inductances between conductors. Moreover, the inductance calculation includes an accurately birdcage geometry description and the effect of a radiofrequency shield. The knowledge of all the resonance modes introduced by a birdcage coil is twofold useful during birdcage coil design: --higher order modes should be pushed far from the fundamental one, --for particular applications, it is necessary to localize other resonant modes (as the Helmholtz mode) jointly to the dominant mode. The knowledge of the magnetic field pattern allows to a priori verify the field homogeneity created inside the coil, when varying the coil dimension and mainly the number of the coil legs. The coil is analyzed using equivalent circuit method. Finally, the simulator is validated by implementing a low-pass birdcage coil and comparing our data with the literature.
鸟笼式线圈在磁共振成像（MRI）系统中被广泛使用，因为它们能带来高信噪比和高射频磁场均匀性，从而保证大视野。本文描述了一种鸟笼式线圈模拟器的实现，该模拟器通过对线圈进行静磁分析，工作在高通和低通模式。与文献中描述的其他模拟器相比，我们的模拟器不仅能在短时间内获得主导频率模式，还能高精度地获得完整的谐振频率谱和相关磁场模式。我们的模拟器考虑了所有电感，包括导体之间的互感。此外，电感计算包括精确的鸟笼几何形状描述和射频屏蔽的影响。了解鸟笼式线圈引入的所有谐振模式在鸟笼式线圈设计过程中有两方面的用处：——高阶模式应远离基模；——对于特定应用，有必要将其他谐振模式（如亥姆霍兹模式）与主导模式一起定位。磁场模式的知识允许在改变线圈尺寸，主要是线圈臂数量时，预先验证线圈内部产生的场均匀性。使用等效电路方法对线圈进行分析。最后，通过实现一个低通鸟笼式线圈并将我们的数据与文献进行比较，对模拟器进行了验证。
