Danish Research Centre for Magnetic Resonance, Centre for Functional and Diagnostic Imaging and Research, Copenhagen University Hospital Hvidovre, Denmark.
High-Field Magnetic Resonance Center, Max-Planck-Institute for Biological Cybernetics, Tübingen, Germany; Department of Biomedical Magnetic Resonance, University of Tübingen, Tübingen, Germany.
Phys Med. 2019 Mar;59:142-150. doi: 10.1016/j.ejmp.2019.02.022. Epub 2019 Mar 8.
MR Current Density Imaging (MRCDI) involves weak current-injection into the head. The resulting magnetic field changes are measured by MRI. Stray fields pose major challenges since these can dominate the fields caused by tissue currents. We analyze the sources and influences of stray fields.
First, we supply validation data for a recently introduced MRCDI method with an unprecedented noise floor of ∼0.1 nT in vivo. Second, we assess the accuracy limit of the method and our corresponding cable current correction in phantoms ensuring high signal-to-noise ratio (SNR). Third, we simulate the influence of stray fields on current flow reconstructions for various realistic experimental set-ups. Fourth, we experimentally determine the physiological field variations. Finally, we explore the consequences of head positioning in an exemplary head coil, since off-center positioning provides space for limiting cable-induced fields.
The cable correction method performs well except near the cables. Unless correcting for cable currents, the reconstructed current flow is easily misestimated by up to 45% for a realistic experimental set-up. Stray fields dominating the fields caused by tissue currents can occur, e.g. due to a wire segment 20 cm away from the imaged region, or due to a slight cable misalignment of 3°. The noise is increased by 40% due to physiological factors. Minor patient movements can cause field changes of ∼40 nT. Off-centered head positioning can locally reduce SNR by e.g. 30%.
Quantification of stray fields showed that MRCDI requires careful field correction. After cable correction, physiological noise is a limiting factor.
磁共振电流密度成像（MRCDI）涉及向头部弱电流注入。通过 MRI 测量由此产生的磁场变化。杂散场构成了主要挑战，因为这些场可能会主导组织电流产生的场。我们分析了杂散场的来源和影响。
首先，我们提供了一种最近引入的 MRCDI 方法的验证数据，该方法在体内具有前所未有的噪声基底约为 0.1nT。其次，我们在体模中评估了该方法的精度极限和我们相应的电缆电流校正，以确保高信噪比（SNR）。第三，我们模拟了杂散场对各种现实实验设置下电流流动重建的影响。第四，我们实验确定了生理场变化。最后，我们探索了头部在典型头部线圈中的定位的后果，因为偏心定位为限制电缆引起的场提供了空间。
除了靠近电缆的位置外，电缆校正方法表现良好。除非校正电缆电流，否则对于现实的实验设置，重建的电流流动很容易被高估多达 45%。杂散场可能会主导组织电流产生的场，例如由于距离成像区域 20cm 的一段电线，或者由于电缆轻微的不对准 3°。由于生理因素，噪声增加了 40%。患者的轻微移动会导致约 40nT 的场变化。偏心头部定位会导致局部 SNR 降低，例如降低 30%。
杂散场的量化表明，MRCDI 需要仔细的场校正。在进行电缆校正后，生理噪声是一个限制因素。
