Image brightening in samples of high dielectric constant
Image brightening in samples of high dielectric constant
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DOI：
10.1016/j.jmr.2003.11.003
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发表时间：
2004-03-01
影响因子：
2.2
通讯作者：
Tropp, J
中科院分区：
文献类型：
--
作者：
Tropp, J
给出了在圆极化射频（RF）磁场照射下的无限长有耗介质圆柱体的电磁问题的解析解；通过将RF哈密顿量转换为旋转框架并仅保留与时间无关的项，将圆柱体内场的核磁共振有源分量投影出来；值得注意的是，得到的笛卡儿场分量必须是实的。然后使用核磁共振有源场的平方量级来计算圆柱体的梯度召回图像，对于磁化的小尖端角；结果表明，该结果几乎可以定量地预测3.0和4.0T实验质子图像，在半径为9.25cm的圆柱形幻影中，填充0.05M的水溶液NaCl。特别是，在记录图像中心的人工增亮在模拟中令人信服地再现，其基础模型排除了沿圆柱体轴方向的波传播。工件的形成是根据圆柱体中心射频磁场的聚焦来解释的，如等高线图所示，显示了旋转通量的时间演变。一个扩展的电磁模型——将介电圆柱体封闭在一个长而屏蔽的体积谐振器（例如鸟笼型）中——然后绘制草图。数学细节见附录A；结果表明，模拟图像与简单的原始模型几乎没有区别。介电圆柱体的Q或质量因子的理论——认为它本身是一个谐振物体——是为封闭圆柱体模型开发的，其中屏蔽对通量的限制允许对存储的能量和辐射损失进行明确的处理。将此推广到无屏蔽有耗介质球的Q。进一步给出了在208mhz均匀圆极化场激励下的球体、表面线圈以及在140-160 MHz范围内的封闭圆柱体的磁通等值线图。然后认为，磁共振图像中的中心增亮伪影是由于样品的介质共振欠阻尼，即在Q > 0.5处，而过阻尼条件，Q < 0.5，导致磁通从中心排除，即经典的趋肤效应。术语“介电共振”显示需要仔细解释混合模式激励，如发生与表面线圈。附录b (C) 2003 Elsevier Inc.给出了一个适用于任意电磁格林函数的核磁共振接收的扩展互易公式。版权所有。
An analytic solution is given for the electromagnetic problem of a lossy dielectric cylinder of infinite length, irradiated by a circularly polarized radiofrequency (RF) magnetic field; the NMR-active components of the field inside the cylinder are projected out by transforming the RF Hamiltonian to the rotating frame and retaining only those terms independent of time; it is noted that the resulting cartesian field components are required to be real. The squared magnitude of the NMR-active fields are then used to calculate the gradient-recalled images of the cylinder, for small tip angles of the magnetization; and the result is shown to predict almost quantitatively the intensity patterns of experimental proton images at 3.0 and 4.0T, in a cylindrical phantom of radius 9.25cm, filled with 0.05M aqueous NaCl. In particular, the artifactual brightening at the center of the recorded image is convincingly reproduced in a simulation, whose underlying model excludes wave propagation along the direction of the cylinder axis. Formation of the artifact is explained in terms of the focussing of the RF magnetic field at the center of the cylinder, as illustrated by contour plots showing the time evolution of the rotating flux. An extended electromagnetic model-having the dielectric cylinder enclosed in a long, shielded volume resonator (e.g., of bird cage type)-is then sketched. The mathematical details appear in Appendix A; and the simulated images are shown to be virtually indistinguishable from those of the simpler original model. The theory of the Q, or quality factor, of the dielectric cylinder-considered itself as a resonant object-is developed for the enclosed cylinder model, where flux containment by the shield permits an unambiguous treatment of both the stored energy and the radiative losses. This is extended to treat the Q of a lossy dielectric sphere without shielding. Further plots of flux contours are given for the sphere, excited at 208 MHz with a uniform circularly polarized field, as well as by a surface coil, and for the enclosed cylinder in the range 140-160 MHz. It is then argued that the center brightening artifacts in magnetic resonance images are due to the underdamped dielectric resonance of the sample, i.e., at Q > 0.5, while the overdamped condition, Q < 0.5, leads to exclusion of flux from the center, i.e., to the classic skin effect. The term 'dielectric resonance' is shown to require careful interpretation for mixed-mode excitation, such as occurs with a surface coil. An extended reciprocity formula for NMR reception, valid for an arbitrary electromagnetic Green's function, is also given in Appendix B. (C) 2003 Elsevier Inc. All rights reserved.