NMR Imaging of 3-D Snowpack Structures

应该更好地了解积雪的微观结构，因为它会影响积雪的机械和物理性质。目前主要采用薄层切面和平面切面两种方法进行研究。然而，它们只给出了二维信息；因此，重建三维结构需要大量的操作。在本研究中，我们开发了一种核磁共振成像技术来可视化和量化积雪的三维结构，由于来自冰层的核磁共振信号非常弱，雪中的空气中填充了掺杂了乙酰乙酸铁C12H26的十二烷，这种十二烷具有良好的渗透性能和低的冰点(约-12℃)。因此，核磁共振成像显示的是十二烷而不是冰粒所占据的空间。3D图像是用自制的核磁共振成像系统使用4.74-T超导磁体获得的。十二烷的实验成像结果表明，需要0.5~2小时才能获得一个3D图像，因此我们开发了一种样品冷却系统，将温度保持在0℃以下。该系统采用外径40 mm、内径20 mm的亚克力塑料制成的双管圆筒，供冷风通过。制备了两种类型的雪：直径约3毫米的冰球和深度灰白色的晶体。特别是，因为冰球足够大，大小均匀，所以它们是测试成像的理想选择。获得了三维显微图像，图像矩阵为128^3，体素尺寸为0.2 mm^3。从不同角度、不同截面观察，可以清晰地识别填充冰球的三维结构。

Microstructure of the snow pack should be better understood because it affects the mechanical and physical properties of snow. Thin sections and plane-surface sections are currently used to investigate. However, they give only two-dimensional information ; therefore, a large number of operations are necessary to reconstruct the three-dimensional structure. In this study we have developed a NMR imaging technique to visualize and quantify the 3D structure of snow pack.Because the NMR signal from the ice was very weak, the air spaces in the snow were filled with dodecane doped with iron acetylaetonate C12H26, which has a good infiltration property and a low freezing point (about -12℃). Thus, the NMR images the space occupied with dodecane instead of ice particles. The 3D images were obtained with a homebuilt NMR imaging system using a 4.74-T superconducting magnet. The result of test imaging of dodecane showed 0.5 to 2 hours were needed to obtain the one 3D image, and hence we developed a specimen cooling system to keep the temperature below 0℃. This system has a double-pipe cylinder made of acrylic plastic with outer diameter of 40 mm and inner diameter of 20 mm through which cold air was passed. Two types of snow were prepared : ice spheres of about 3-mm diameter and depth-hoar crystals. In particular, because the ice spheres are sufficiently large and uniformly sized, they were ideal for test imaging. 3D microscopic images were obtained successfully ; the image matrix was 128^3 and voxel size was 0.2 mm^3. We could recognize the 3D structure of the packed ice spheres clearly by viewing from various angles and cross sections.