Disentangling contributions from iron and myelin architecture to brain tissue magnetic susceptibility by using Quantitative Susceptibility Mapping ( QSM )-猫眼课题宝

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2011

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F. Schweser;A. Deistung;K. Sommer;J. Reichenbach

导言-磁化率是一种固有的物理组织属性，最近通过一种名为定量磁化率映射(QSM)的新成像技术在体内获得了这种属性[1，2]。人脑的敏感度图谱显示了惊人的解剖对比[1，2]，目前认为这主要是由于铁(顺磁性)和髓磷脂(抗磁性)[3]。然而，这两种因素的混合使易感性变化的解释变得复杂，特别是在神经退行性疾病中，炎性髓鞘丢失和局灶性铁聚集可能同时发生。此外，最近发现，脑组织敏感性存在相当大的方位依赖性，这进一步使解释变得复杂。我们提出了一种新的技术，通过利用额外的R2*信息来显著提高QSM的特异性。这项技术产生了两种新颖的对比，一种与取向效应无关，另一种与组织铁浓度无关。理论-假设一个三室组织模型，在一个均匀的组织基质中含有点状颗粒包涵体(以下称为铁)和有髓轴突。在这个模型中，体素的磁化率可以用方程来表示。1(忽略铁的体积分数)[1]。有效横向弛豫速率R2~*的对应关系由方程给出。2[4，5]。在这两个方程中，与髓鞘相关的术语取决于轴突相对于主磁场的方向(θ角)，如公式所述。3[5]和公式。4[6，7]。通过方程的线性组合，可以从方程中消除对铁浓度的依赖。1和等式。2根据公式2。5，得到了一种新的铁不依赖的对比剂ξ。系数1ˆˆ−Fe Ferχ可根据文献值估算(本研究；见表2)。2)或来自R2*和髓鞘贡献相似的区域的敏感值。对比度ξnoFe与髓磷脂体积分数呈线性关系，并包括与髓鞘相关的取向依赖性。旋转不变的对比度可以通过等式的线性组合来产生。1和2，根据公式1和2消除θ项。6.这种对比度与铁浓度和髓鞘体积分数都是线性的。材料和方法：使用ToF-SWI序列[9](TE1/TE2=3.38ms/22ms，tr=30ms，FA=20°，600μm各向同性体素；采集时间：15min)从一名志愿者(男性，26岁)的脑部采集高分辨率双回波磁共振成像数据。在3Tesla全身核磁共振扫描仪(德国Erlangen的西门子医疗解决方案公司的Tim Trio)上，使用12通道接收磁头矩阵线圈。志愿者的头对着脖子的姿势被重复扫描，以研究方位效应。使用FSL-Folrt(FMRIB，牛津大学)将得到的复数值图像配准到正常的头部位置。R2*图由幅度回波计算，并补偿了里氏噪声[10]，磁化率图由相位图用Heidi算法重建[提交给ISMRM]。最后，根据EQ将这些地图组合在一起。5和6.未知常数1||ˆ−⊥⋅my rχin Eq.6是通过最小化两个头部方向(A，B)的方向无关对比ξnoOrient之间的差异来确定的：2

INTRODUCTION – Magnetic susceptibility is an intrinsic physical tissue property which recently became accessible in vivo by a novel imaging technique called quantitative susceptibility mapping (QSM) [1,2]. Susceptibility maps of the human brain demonstrate astounding anatomical contrast [1,2], which is currently believed to be predominantly due to iron (paramagnetic) and myelin-lipids (diamagnetic) [3]. The intermixing of both contributions, however, complicates interpretation of susceptibility changes in particular in neurodegenerative diseases where inflammatory myelin-loss and focal iron accumulation may occur simultaneously. It has, furthermore, recently been discovered that a considerable orientation dependence of brain tissue susceptibility exists, which further complicates interpretation. We present a novel technique for substantially increasing the specificity of QSM by utilizing additional R2* information. The technique yields two novel contrasts, one that is independent of orientation effects, whereas the other is independent of tissue iron concentration. THEORY – A three compartment tissue model was assumed with punctuate particle inclusions (called iron in the following) and myelinated axons in a homogenous tissue matrix. In this model, the bulk voxel susceptibility can be expressed by Eq. 1 (volume fraction of iron neglected) [1]. The corresponding relation for the effective transverse relaxation rate, R2*, is given by Eq. 2 [4,5]. In both equations the terms associated with myelin depend on the orientation of the axons relative to the main magnetic field (angle θ ) as described by Eq. 3 [5] and Eq. 4 [6,7]. The dependence on iron concentration may be eliminated from the equations by linear combination of Eq. 1 and Eq. 2 according to Eq. 5, yielding a novel iron-independent contrast ξnoFe. The coefficient 1 ˆ ˆ − Fe Fer χ may be estimated from literature values (this study; see Tab. 2) or from R2* and susceptibility values in regions with a similar contribution of myelin. The contrast ξnoFe depends linearly on the myelin-lipid volume fraction and includes the myelin-related orientation dependencies. A rotation invariant contrast may be generated by a linear combination of Eqs. 1 and 2 that eliminates the θ -terms according to Eq. 6. This contrast is linear with respect to both the iron concentration and the myelin volume fraction. MATERIALS AND METHODS – To demonstrate the technique, high-resolution double-echo GRE data was acquired from the brain of a volunteer (male, 26y) using the ToF-SWI-sequence [9] (TE1/TE2=3.38ms/22ms, TR=30ms, FA=20°, 600μm isotropic voxels; acquisition time: 15min.) on a 3 Tesla whole-body MRI scanner (Tim Trio, Siemens Medical Solutions, Erlangen, Germany) using a 12-channel receive head-matrix coil. The scan was repeated with the volunteer’s head in head-to-neck position to investigate orientation effects. The resulting complex-valued images were registered to the normal head position using FSL-FLIRT (FMRIB, Oxford University). R2* maps were computed from the magnitude echoes with compensation of Rician noise [10] and susceptibility maps were reconstructed from the phase images using the HEIDI algorithm [submitted to ISMRM]. The maps were, finally, combined according to Eqs. 5 and 6. The unknown constant 1 || ˆ − ⊥ ⋅ My r χ in Eq. 6 was determined by minimizing the difference between the orientation independent contrasts, ξnoOrient, of the two head orientations (A,B) in the corpus callosum: 2

Disentangling contributions from iron and myelin architecture to brain tissue magnetic susceptibility by using Quantitative Susceptibility Mapping ( QSM )

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