Toshihiro Ozeki

Three-dimensional MR microscopy of snowpack structures

Abstract MR microscopy was designed to visualize and quantify the three-dimensional structure of snowpack and tested on snow and ice samples. We studied the structure of four types of packed ice particles: ice spheres, large rounded polycrystals, small rounded monocrystals, and depth hoar. Because the nuclear magnetic resonance (NMR) signal from the ice was very weak, the air space of snow was filled with a fluid that had a strong NMR signal. By imaging the fluid, we inferred the ice shapes and positions. Both dodecane and aniline could be used, provided that they were doped with iron acetylacetonate. Test imaging of dodecane showed that 0.5–2 h were needed to obtain one 3D image; thus, we developed a specimen-cooling system to maintain the sample at a constant temperature. The chamber had a double pipe cylinder through which cold air flowed, and the temperature of the sample holder was controlled by adjusting the volume of cold airflow. Experiments using the above ice particles and the system allowed us to obtain 3D microscopic images. For an image matrix of 2563, the voxel size was 120 μm on a side, whereas image matrices of 1283 and 643 had voxel sizes of 200 and 400 μm, respectively. The imaging sequence used 3D gradient echoes. We also compared the 3D images with 2D data that was obtained using the conventional section plane method. MR microscopy is thus a very useful method to visualize the microstructure of snowpack.

Development of a compact magnetic resonance imaging system for a cold room

A compact magnetic resonance imaging (MRI) system for a cold (-5 degrees C) room has been developed to acquire MR images below the freezing point of water. The MRI system consists of a 1.0 T permanent magnet, a higher-order shim coil set, and a gradient coil probe, installed in the cold room, and a compact MRI console installed in a room at normal temperature (20-25 degrees C). The most difficult problem for the installation of the MRI system in the cold room was the degradation of the field homogeneity of the permanent magnet shimmed at 25 degrees C. To overcome this problem, higher-order shim coils were developed and the temperature variation of the magnetic field distribution was measured using a standard phantom with and without shim coil currents. As a result, it was confirmed that the homogeneity (the difference between the minimum and maximum values) of the magnetic field in the 17x17x19 mm(3) rectangular parallelepiped region was improved from 117 to 59 ppm using an appropriate combination of shim coil currents. A snowpack immersed in dodecane (C(12)H(26)) was imaged using a driven-equilibrium three-dimensional (3D) spin-echo sequence at -5 degrees C. The visualized 3D structure of the snowpack demonstrated the effectiveness of our approach.

Three-dimensional snow images by MR microscopy

MR microscopy technique was introduced to visualize and quantify the three-dimensional structure of snowpack. Since the NMR signal from the ice was week, we looked at the air space instead filling with dodecane or aniline doped with iron acetylacetonate. Four types of snow were tested: ice spheres, large rounded poly crystals, small rounded mono-crystals and depth hoar crystals. A specific specimen-cooling system was developed to keep the temperature below 0 degrees C. In the experiments 0.5 to 2 h were necessary to accumulate the signals enough to obtain a 3D micro-image; the image matrix 128(3), voxel size (200 microm)3 or 256(3) (120 microm)3. Comparison with the 2D data using the conventional section plane method was also carried out and MR microscopy is proved to be a very useful method to visualize the microstructure of snowpack.

Field Observations of Sun Crust Formation in Hokkaido, Japan

The mechanism of sun crust formation was investigated through field observations for five winters from 1991 to 1995. During the observation periods, the sun crust formed 15 times. The sun crust observed in this study was a thin ice layer, which is 1 to 2 mm thickness and composed of ice particles, with cavities formed under it. In the all cases, the snow type before the sun crust formation was wet rounded grains. The heat balance during the formation of sun crust was calculated revealing the following process. Evaporation made the surface smooth and flat. Longwave radiative flux and latent heat flux cooled the snow surface, while beneath the surface, shortwave radiation was absorbed and internal melting occurred giving rise to cavities. The longwave radiation refroze some meltwater which remained within the layer near the surface, and a thin ice layer was formed.