Gibbs Eric, Liu Chunlei
Brain Imaging and Analysis Center, Duke University School of Medicine, Durham, NC 27710.
Brain Imaging and Analysis Center, Duke University School of Medicine, Durham, NC 27710; Department of Radiology, Duke University School of Medicine, Durham, NC 27710.
Tomography. 2015 Dec;1(2):125-135. doi: 10.18383/j.tom.2015.00142.
Tissue conductivity is a biophysical marker of tissue structure and physiology. Present methods of measuring tissue conductivity are limited. Electrical impedance tomography, and magnetic resonance electrical impedance tomography rely on passing external current through the object being imaged, which prevents its use in most human imaging. Recently, the RF field used for MR excitation has been used to non-invasively measure tissue conductivity. This technique is promising, but conductivity at higher frequencies is less sensitive to tissue structure. Measuring tissue conductivity non-invasively at low frequencies remains elusive. It has been proposed that eddy currents generated during the rise and decay of gradient pulses could act as a current source to map low-frequency conductivity. This work centers on a gradient echo pulse sequence that uses large gradients prior to excitation to create eddy currents. The electric and magnetic fields during a gradient pulse are simulated by a finite-difference time-domain simulation. The sequence is also tested with a phantom and an animal MRI scanner equipped with gradients of high gradient strengths and slew rate. The simulation demonstrates that eddy currents in materials with conductivity similar to biological tissue decay with a half-life on the order of nanoseconds and any eddy currents generated prior to excitation decay completely before influencing the RF signal. Gradient-induced eddy currents can influence phase accumulation after excitation but the effect is too small to image. The animal scanner images show no measurable phase accumulation. Measuring low-frequency conductivity by gradient-induced eddy currents is presently unfeasible.
组织电导率是组织结构和生理功能的生物物理标志物。目前测量组织电导率的方法存在局限性。电阻抗断层成像和磁共振电阻抗断层成像依赖于让外部电流通过被成像物体，这使得其无法用于大多数人体成像。最近，用于磁共振激发的射频场已被用于非侵入性测量组织电导率。这项技术很有前景，但高频下的电导率对组织结构的敏感度较低。在低频下非侵入性测量组织电导率仍然难以实现。有人提出，梯度脉冲上升和衰减期间产生的涡流可作为映射低频电导率的电流源。这项工作围绕一种梯度回波脉冲序列展开，该序列在激发前使用大梯度来产生涡流。通过时域有限差分模拟来模拟梯度脉冲期间的电场和磁场。该序列还在一个体模以及一台配备有高梯度强度和 slew 率梯度的动物磁共振成像扫描仪上进行了测试。模拟表明，在与生物组织电导率相似的材料中，涡流以纳秒量级的半衰期衰减，并且在激发前产生的任何涡流在影响射频信号之前就完全衰减了。梯度感应涡流会影响激发后的相位积累，但这种影响太小以至于无法成像。动物扫描仪图像显示没有可测量的相位积累。目前，通过梯度感应涡流测量低频电导率是不可行的。
