Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities
Gaussian approximation in the theory of MR signal formation in the presence of structure-specific magnetic field inhomogeneities
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DOI：
10.1016/s1090-7807(03)00131-9
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发表时间：
2003-08-01
影响因子：
2.2
通讯作者：
Yablonskiy, DA
中科院分区：
文献类型：
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作者：
Sukstanskii, AL;Yablonskiy, DA
在高斯相位分布近似的框架下，对存在介观结构特定磁场不均匀性的情况下自由感应衰减（FID）和自旋回波（SE）MR信号形成进行了详细的理论分析。该理论考虑了具有可渗透边界的任意形状的磁化物体产生的不均匀磁场中核自旋的扩散。在短时限制内，FID 信号随时间呈二次方衰减，并且仅通过体积分数取决于物体的几何形状，而 SE 信号以时间的 5/2 次方衰减，其系数取决于磁化物体的体积分数及其表面与体积之比。在运动窄化状态下，有限尺寸物体的 FID 和 SE 信号单指数衰减；得到了弛豫速率常数DeltaR(2)的简单通用表达式。在运动变窄状态下无限长圆柱体的情况下，理论预测 S 中的非指数信号衰减类似于根据先前的结果 -tlnt。对于物体的特定几何形状（球体和无限长圆柱体），给出了 FID 和 SE 信号的精确解析表达式。例如，该理论可应用于由脱氧红细胞、毛细管网络、造影剂等引起介观磁场不均匀性的生物系统。(C) 2003 Elsevier Science（美国）。版权所有。
A detailed theoretical analysis of the free induction decay (FID) and spin echo (SE) MR signal formation in the presence of mesoscopic structure-specific magnetic field inhomogeneities is developed in the framework of the Gaussian phase distribution approximation. The theory takes into account diffusion of nuclear spins in inhomogeneous magnetic fields created by arbitrarily shaped magnetized objects with permeable boundaries. In the short-time limit the FID signal decays quadratically with time and depends on the objects' geometry only through the volume fraction, whereas the SE signal decays as 5/2 power of time with the coefficient depending on both the volume fraction of the magnetized objects and their surface-to-volume ratio. In the motional narrowing regime, the FID and SE signals for objects of finite size decay mono-exponentially; a simple general expression is obtained for the relaxation rate constant DeltaR(2). In the case of infinitely long cylinders in the motional narrowing regime the theory predicts non-exponential signal decay In S similar to -tlnt in accordance with previous results. For specific geometries of the objects (spheres and infinitely long cylinders) exact analytical expressions for the FID and SE signals are given. The theory can be applied, for instance, to biological systems where mesoscopic magnetic field inhomogeneities are induced by deoxygenated red blood cells, capillary network, contrast agents, etc. (C) 2003 Elsevier Science (USA). All rights reserved.