Department of Physics and Astronomy, University of Western Ontario, London, ON, Canada N6A 3K7.
Proc Natl Acad Sci U S A. 2014 Jan 7;111(1):E159-67. doi: 10.1073/pnas.1306516111. Epub 2013 Dec 27.
Estimates of the apparent transverse relaxation rate (R2*) can be used to quantify important properties of biological tissue. Surprisingly, the mechanism of R2* dependence on tissue orientation is not well understood. The primary goal of this paper was to characterize orientation dependence of R2* in gray and white matter and relate it to independent measurements of two other susceptibility based parameters: the local Larmor frequency shift (fL) and quantitative volume magnetic susceptibility (Δχ). Through this comparative analysis we calculated scaling relations quantifying R2' (reversible contribution to the transverse relaxation rate from local field inhomogeneities) in a voxel given measurements of the local Larmor frequency shift. R2' is a measure of both perturber geometry and density and is related to tissue microstructure. Additionally, two methods (the Generalized Lorentzian model and iterative dipole inversion) for calculating Δχ were compared in gray and white matter. The value of Δχ derived from fitting the Generalized Lorentzian model was then connected to the observed R2* orientation dependence using image-registered optical density measurements from histochemical staining. Our results demonstrate that the R2* and fL of white and cortical gray matter are well described by a sinusoidal dependence on the orientation of the tissue and a linear dependence on the volume fraction of myelin in the tissue. In deep brain gray matter structures, where there is no obvious symmetry axis, R2* and fL have no orientation dependence but retain a linear dependence on tissue iron concentration and hence Δχ.
组织表观横向弛豫率（R2*）的估计可用于量化生物组织的重要特性。令人惊讶的是，组织取向对 R2的影响机制尚未得到很好的理解。本文的主要目标是描述灰质和白质中 R2的取向依赖性，并将其与另外两个基于磁化率的独立测量参数（局部拉莫尔频率偏移（fL）和定量体积磁化率（Δχ））相关联。通过这种比较分析，我们计算了在给定局部拉莫尔频率偏移测量值的情况下，体素中 R2'（来自局部磁场不均匀性的横向弛豫率的可逆贡献）的标度关系。R2'是扰动物体几何形状和密度的度量，与组织微观结构有关。此外，在灰质和白质中比较了两种计算Δχ的方法（广义洛伦兹模型和迭代偶极子反演）。然后，使用来自组织化学染色的图像配准光密度测量值，将拟合广义洛伦兹模型得出的Δχ值与观察到的 R2取向依赖性联系起来。我们的结果表明，白质和皮质灰质的 R2和 fL 可以很好地用组织取向的正弦依赖性和组织中髓鞘体积分数的线性依赖性来描述。在深部脑灰质结构中，由于没有明显的对称轴，因此 R2*和 fL 没有取向依赖性，但与组织铁浓度保持线性依赖性，因此与Δχ保持线性依赖性。
