Vashaee S, Goora F, Britton M M, Newling B, Balcom B J
UNB MRI Centre, Department of Physics, University of New Brunswick, Fredericton, New Brunswick E3B 5A3, Canada.
School of Chemistry, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK.
J Magn Reson. 2015 Jan;250:17-24. doi: 10.1016/j.jmr.2014.10.016. Epub 2014 Nov 8.
Magnetic resonance imaging (MRI) in the presence of metallic structures is very common in medical and non-medical fields. Metallic structures cause MRI image distortions by three mechanisms: (1) static field distortion through magnetic susceptibility mismatch, (2) eddy currents induced by switched magnetic field gradients and (3) radio frequency (RF) induced eddy currents. Single point ramped imaging with T1 enhancement (SPRITE) MRI measurements are largely immune to susceptibility and gradient induced eddy current artifacts. As a result, one can isolate the effects of metal objects on the RF field. The RF field affects both the excitation and detection of the magnetic resonance (MR) signal. This is challenging with conventional MRI methods, which cannot readily separate the three effects. RF induced MRI artifacts were investigated experimentally at 2.4 T by analyzing image distortions surrounding two geometrically identical metallic strips of aluminum and lead. The strips were immersed in agar gel doped with contrast agent and imaged employing the conical SPRITE sequence. B1 mapping with pure phase encode SPRITE was employed to measure the B1 field around the strips of metal. The strip geometry was chosen to mimic metal electrodes employed in electrochemistry studies. Simulations are employed to investigate the RF field induced eddy currents in the two metallic strips. The RF simulation results are in good agreement with experimental results. Experimental and simulation results show that the metal has a pronounced effect on the B1 distribution and B1 amplitude in the surrounding space. The electrical conductivity of the metal has a minimal effect.
在医学和非医学领域，金属结构存在时的磁共振成像（MRI）非常常见。金属结构通过三种机制导致MRI图像失真：（1）通过磁敏感性失配引起的静磁场失真；（2）由切换磁场梯度感应产生的涡流；（3）射频（RF）感应产生的涡流。具有T1增强的单点斜坡成像（SPRITE）MRI测量在很大程度上不受敏感性和梯度感应涡流伪影的影响。因此，人们可以分离金属物体对RF场的影响。RF场会影响磁共振（MR）信号的激发和检测。这对于传统的MRI方法来说具有挑战性，因为传统方法无法轻易分离这三种影响。通过分析围绕两条几何形状相同的铝和铅金属条的图像失真，在2.4T下对RF感应的MRI伪影进行了实验研究。将金属条浸入掺有造影剂的琼脂凝胶中，并采用锥形SPRITE序列进行成像。使用纯相位编码SPRITE的B1映射来测量金属条周围的B1场。选择金属条的几何形状来模拟电化学研究中使用的金属电极。采用模拟来研究两条金属条中RF场感应产生的涡流。RF模拟结果与实验结果吻合良好。实验和模拟结果表明，金属对周围空间中的B1分布和B1幅度有显著影响。金属的电导率影响最小。
