Schäfer Andreas, Wharton Sam, Gowland Penny, Bowtell Richard
Sir Peter Mansfield Magnetic Resonance Centre, School of Physics and Astronomy, University of Nottingham, University Park, Nottingham, NG7 2RD, UK.
Neuroimage. 2009 Oct 15;48(1):126-37. doi: 10.1016/j.neuroimage.2009.05.093. Epub 2009 Jun 8.
Using a Fourier-based method for rapidly calculating the spatially varying magnetic field perturbation generated by a general susceptibility distribution, chi(r)<<1, maps of the NMR frequency variation in a structured phantom, the human cortex and mid-brain structures have been simulated and compared with experimental data acquired at 7 T. The results from the phantom indicate that the Fourier method can generate an accurate estimate of the NMR frequency variation in a complex structure where the magnitude of susceptibility variation is similar to that in the human brain. Simulations based on a 1-mm-resolution model of the human head show that susceptibility variation can give rise to sharp boundaries between grey and white matter regions. The frequency difference in the simulated data was found to be significantly less than the volume susceptibility difference scaled by three. Simulations based on a simple model of the substantia nigra, red nuclei and surrounding white matter, made up of three homogeneous compartments, showed patterns of field perturbation that were very similar to those seen in experimental data. These results indicate that care must be exercised in interpreting phase or susceptibility weighted images, since the simulations show that features such as rings and graded phase variation can easily be generated by simple homogeneous structures. The results also indicate that even after spatial filtering, frequency variation can be projected outside of localised structures. k-space analysis provides some insight into the conditions needed to ensure that the measured frequency perturbation directly represents the anatomical features.
使用一种基于傅里叶的方法来快速计算由一般磁化率分布（χ(r)<<1）产生的空间变化磁场扰动，已对结构化体模、人类皮层和中脑结构中的核磁共振频率变化图谱进行了模拟，并与在7T下获取的实验数据进行了比较。体模实验结果表明，傅里叶方法能够准确估计复杂结构中核磁共振频率的变化，该结构中磁化率变化的幅度与人类大脑中的相似。基于1毫米分辨率的人类头部模型进行的模拟显示，磁化率变化会导致灰质和白质区域之间出现清晰的边界。发现模拟数据中的频率差明显小于按三倍缩放的体积磁化率差。基于由三个均匀隔室组成的黑质、红核及周围白质的简单模型进行的模拟显示，磁场扰动模式与实验数据中观察到的非常相似。这些结果表明，在解释相位或磁化率加权图像时必须谨慎，因为模拟表明，简单的均匀结构很容易产生诸如环和渐变相位变化等特征。结果还表明，即使经过空间滤波，频率变化仍可能投影到局部结构之外。k空间分析为确保测量的频率扰动直接代表解剖特征所需的条件提供了一些见解。
