Trudeau J D, Dixon W T, Hawkins J
Frederik Philips Magnetic Resonance Research Center, Emory University School of Medicine, Atlanta, Georgia 30322, USA.
J Magn Reson B. 1995 Jul;108(1):22-30. doi: 10.1006/jmrb.1995.1098.
Water diffusion measurements in white matter of freshly excised pig spinal cord and in parenchyma of fresh celery (excluding the fibers along the edge of the stalk) were performed using NMR at 200 MHz. In white matter of pig spinal cord, the measured diffusion coefficient is anisotropic and independent of sample orientation with respect to the magnetic field. In celery parenchyma, diffusion is isotropic and independent of orientation in the magnetic field when using a diffusion sequence that gives results independent of self-induced magnetic-field gradients. However, when the standard diffusion pulse sequence that gives results dependent upon self-induced magnetic-field gradients is used, diffusion in celery appears isotropic when the stalk is oriented parallel to the magnetic field but anisotropic when oriented perpendicular. Susceptibility variations leading to anisotropic self-induced magnetic-field gradients approximately 3 kHz/cm in magnitude when the celery is oriented perpendicular to the magnetic field can explain this apparent anisotropic diffusion. A study of the apparent diffusion coefficient (ADC) in celery as a function of diffusion times ranging from 8 to 22 ms indicates that the motion is at most only slightly restricted. Therefore, although the effect is not seen in all types of samples, one must be aware that self-induced gradients may affect the ADC and may cause isotropic diffusion to appear anisotropic. In addition, NMR experiments that change diffusion-sensitizing gradient timings to study restricted diffusion change the effects of the self-induced gradients as well as the effect of barriers on the ADC, complicating interpretation.
使用200兆赫兹的核磁共振仪对刚切除的猪脊髓白质和新鲜芹菜实质（不包括茎边缘的纤维）进行了水扩散测量。在猪脊髓白质中，测得的扩散系数是各向异性的，且与样品相对于磁场的取向无关。在芹菜实质中，当使用能给出与自感应磁场梯度无关的结果的扩散序列时，扩散是各向同性的，且与磁场中的取向无关。然而，当使用能给出依赖于自感应磁场梯度的结果的标准扩散脉冲序列时，芹菜茎平行于磁场取向时扩散呈各向同性，而垂直于磁场取向时扩散呈各向异性。当芹菜垂直于磁场取向时，导致大小约为3千赫兹/厘米的各向异性自感应磁场梯度的磁化率变化可以解释这种明显的各向异性扩散。一项关于芹菜中表观扩散系数（ADC）作为扩散时间（范围为8至22毫秒）的函数的研究表明，运动最多只是略有受限。因此，尽管并非在所有类型的样品中都能看到这种效应，但必须意识到自感应梯度可能会影响ADC，并可能导致各向同性扩散看起来呈各向异性。此外，通过改变扩散敏感梯度定时来研究受限扩散的核磁共振实验，既改变了自感应梯度的影响，也改变了屏障对ADC的影响，使解释变得复杂。
