Sapkota Nabraj, Yoon Sook, Thapa Bijaya, Lee YouJung, Bisson Erica F, Bowman Beth M, Miller Scott C, Shah Lubdha M, Rose John W, Jeong Eun-Kee
Utah Center for Advanced Imaging Research, University of Utah, Salt Lake City, UT, USA; Department of Physics and Astronomy, University of Utah, Salt Lake City, UT, USA.
Utah Center for Advanced Imaging Research, University of Utah, Salt Lake City, UT, USA.
J Magn Reson. 2016 Nov;272:53-59. doi: 10.1016/j.jmr.2016.09.001. Epub 2016 Sep 4.
Signal measured from white matter in diffusion-weighted imaging is difficult to interpret because of the heterogeneous structure of white matter. Characterization of the white matter will be straightforward if the signal contributed from the hindered space is suppressed and purely restricted signal is analyzed. In this study, a Monte Carlo simulation (MCS) of water diffusion in white matter was performed to understand the behavior of the diffusion-weighted signal in white matter. The signal originating from the hindered space of an excised pig cervical spinal cord white matter was suppressed using the ultrahigh-b radial diffusion-weighted imaging. A light microscopy image of a section of white matter was obtained from the excised pig cervical spinal cord for the MCS. The radial diffusion-weighted signals originating from each of the intra-axonal, extra-axonal, and total spaces were studied using the MCS. The MCS predicted that the radial diffusion-weighted signal remains almost constant in the intra-axonal space and decreases gradually to about 2% of its initial value in the extra-axonal space when the b-value is increased to 30,000s/mm. The MCS also revealed that the diffusion-weighted signal for a b-value greater than 20,000s/mm is mostly from the intra-axonal space. The decaying behavior of the signal-b curve obtained from ultrahigh-b diffusion-weighted imaging (b∼30,000s/mm) of the excised pig cord was very similar to the decaying behavior of the total signal-b curve synthesized in the MCS. A mono-exponential plus constant fitting of the signal-b curve obtained from a white matter pixel estimated the values of constant fraction and apparent diffusion coefficient of decaying fraction as 0.32±0.05 and (0.16±0.01)×10mm/s, respectively, which agreed well with the results of the MCS. The signal measured in the ultrahigh-b region (b>20,000s/mm) is mostly from the restricted (intra-axonal) space. Integrity and intactness of the axons can be evaluated by assessing the remaining signal in the ultrahigh-b region.
由于白质结构的异质性，在扩散加权成像中从白质测量的信号难以解释。如果抑制受阻空间贡献的信号并分析纯受限信号，白质的特征描述将变得简单直接。在本研究中，进行了白质中水扩散的蒙特卡罗模拟（MCS），以了解白质中扩散加权信号的行为。使用超高b值径向扩散加权成像抑制了取自猪颈脊髓白质的受阻空间产生的信号。从切除的猪颈脊髓获取白质切片的光学显微镜图像用于MCS。使用MCS研究了源自轴突内、轴突外和总体空间的径向扩散加权信号。MCS预测，当b值增加到30000s/mm²时，轴突内空间的径向扩散加权信号几乎保持恒定，而在轴突外空间中逐渐降低到其初始值的约2%。MCS还表明，b值大于20000s/mm²时的扩散加权信号主要来自轴突内空间。从切除的猪脊髓的超高b值扩散加权成像（b∼30000s/mm²）获得的信号-b曲线的衰减行为与MCS中合成的总信号-b曲线的衰减行为非常相似。对从白质像素获得的信号-b曲线进行单指数加常数拟合，估计常数分数和衰减分数的表观扩散系数值分别为0.32±0.05和(0.16±0.01)×10⁻³mm²/s，这与MCS的结果非常吻合。在超高b值区域（b>20000s/mm²）测量的信号主要来自受限（轴突内）空间。通过评估超高b值区域中的剩余信号，可以评估轴突的完整性和完好性。
