预测体外猪视神经中等尺度拉莫尔频移
Predicting Mesoscopic Larmor Frequency Shifts in Ex Vivo Porcine Optic Nerve
AI总结：
研究通过μQSM预测视神经白质中纤维取向依赖的亚体素频移，揭示其微结构起源，提升定量磁 susceptibility 映射的精度。
摘要：
白质中的拉莫尔频移随纤维取向变化，由于各向异性微结构。由于临床体素显著大于这些微观频移，测得信号代表局部频移的总体平均。因此，准确估计磁化率需考虑低于成像分辨率的底层频移分布。我们评估了Microstructure-informed Quantitative Susceptibility Mapping (μQSM) 是否能从取向分散的空心圆柱和球形包含物中预测取向依赖的亚体素频移。使用3T Siemens Connectom扫描仪，从体外猪视神经在多个相对于主磁场方向获取扩散加权和多梯度回波图像。我们还分析去铁化视神经以尝试分离髓鞘和铁对磁化率的影响。估计的亚体素频移与μQSM预测高度一致，与由均匀磁化的轴突生成的中等尺度磁场扰动一致。去铁对频移影响极小，表明铁贡献可忽略。μQSM准确再现了视神经白质中拉莫尔频移的取向依赖性，为理解其微结构起源提供了新见解，并支持改进定量磁化率映射中组织磁化率的估计。
英文摘要：
Larmor frequency shifts in white matter (WM) vary with fiber orientation due to anisotropic microstructure. Since clinical voxels are significantly larger than these microscopic frequency variations, the measured signal represents a bulk average of local shifts. Accurate estimation of magnetic susceptibility therefore requires accounting for these underlying frequency distributions that exist below the imaging resolution. We evaluated whether Microstructure-informed Quantitative Susceptibility Mapping (μQSM) can predict orientation-dependent sub-voxel frequency shifts from orientationally dispersed hollow cylinders and spherical inclusions. Diffusion-weighted and multi-gradient-echo images were acquired from ex vivo pig optic nerves at multiple orientations relative to the main magnetic field using a 3T Siemens Connectom scanner. We also analyzed de-ironed optic nerves to try and separate the effects of myelin and iron on susceptibility. The estimated sub-voxel frequency shifts closely matched μQSM predictions, consistent with mesoscopic field perturbations generated by uniformly magnetized axons. De-ironing had minimal effect on the frequency shifts, indicating negligible iron contribution. μQSM accurately reproduces the orientation dependence of Larmor frequency shifts in optic nerve WM, providing new insight into their microstructural origin and supporting improved estimation of tissue magnetic susceptibility in Quantitative Susceptibility Mapping.
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