International Snow Science Workshop (2002: Penticton, B.C.)

Three-dimensional MR microscopy of snowpack structures

Toshihiro Ozeki* (1), Katsumi Kose (2), Tomoyuki Haishi (3),

Seitarou Hashimoto (2l, Shun-ichi Nakatsubo (4), Kouichi Nishimura (4)

(1) Hokkaido University ofEducation, lwamizawa, Japan(2) Institute ofApplied Physics, University ofTsukuba, Tsukuba, Japan(3) MR Technology Co., Tsukuba, Japan(4) Institute ofLow Temperature Science, Hokkaido University, Sapporo, Japan

Abstract: A magnetic resonance microscopy technique was developed to visualize and quantify thethree-dimensional structure of snowpack. Because the NMR signal from the ice was very weak, we looked at theair space in the snow that was filled with dodecane doped with iron acetylaetonate. Four types of snow weretested: ice spheres, large rounded poly-crystals, small rounded mono-crystals and depth hoar crystals. Amongthese materials, the depth hoar crystals were the best for the test imaging, since they were sufficiently large andhave anisotropic structures. A specific specimen-cooling system was developed to keep the temperature below 0°C. In the experiments 0.5 to 2 hours were necessary to accumulate the signals enough to obtain a 3Dmicro-image; the image matrix 1283, voxel size 200 1Jlll3 or the image matrix 2563, voxel size 120 1Jlll3. The 3Dstructures of snow were investigated viewing from various angles and cross sections. Comparison with the 2Ddata using the conventional thin section method was also carried out and it was attested that the MR microscopycould be a powerful tool to reveal the microstructure of snowpack.

Keywords: snow crystal structure, magnetlc resonance imaging, three-dimensional structure, dodecane

1. Introduction

Microstructure of the snowpack should be betterunderstood because it determines the physicalproperties of snow. The avalanche releasemechanism also depends on the three-dimensionalnetwork structure of snowpack. Although thinsections and plane-surface sections have beenapplied to reveal the snow structures, they havegiven us only two-dimensional information;therefore, a large number of operations arenecessary to reconstruct the three-dimensionalstructure. Recently, several researches are made todevelop new technique getting 3D microstructure ofsnow. Schneebeli (2000) developed a smart systemthat consisted of slicing, segmentation andreconstruction of snow samples filled withdimethylphthalate. The 3-D structure of snow couldbe reconstructed and visualized on the computerwithin 2 hours. Coleou et. al. (2001) used X-rayabsorption tomography to build 3-D high-resolutionimage of snow (10 1Jlll3) and obtained the porosityand discrete local curvature of snow.

• Corresponding author address: Toshihiro Ozeki,Hokkaido University of Education, Midorikaoka2-34-1, Iwarnizawa, Hokkaido 068-8642, JAPAN;tel: +81-126-32-0336; fax: +81-126-32-0251;email: oze@iwa.hokkyodai.ac.jp

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On the other hand, nuclear magnetic resonance(NMR) imaging, which coupled with the explosivegrowth of digital computers, is playing an importantrole in medical imaging over the last several years.The development of NMR imaging has enhancedability to diagnose and monitor diseases. BecauseNMR is particularly sensitive to the protonmagnetic moment that occurs in water and organiccompounds, the mixture of liquid and solid watergives good contrast of image. Edelstein andSchulson (1991) and Eicken et. al. (2000) appliedNMR imaging to visualizing of drainage channelwithin salt-water ice accordingly. Ozeki et. al.(2000) applied the NMR imaging system to snowstructures and succeed to visualize the packing ofice spheres 3 mm in diameter. They tried to look atthe depth hoar crystals as well, but failed toreproduce the original structure since the depth hoarlayer was disassembled when it was set into thesample holder.

In this paper we introduce the improved NMRimaging technique so as to visualize and quantifythe 3D structure of natural snowpack.

2. Materials and methods

Magnetic resonance imaging has advanced as atomographic technique to medical imaging over thelast several years. MRI images a NMR signal from

Mountain Snowpack

AIR COMPo ETYLEN GLYCOL SAMPLE HOLDER

Figure 1: Apparatus for MR imaging of snow

structure.

the proton magnetic moment. NMR is sensitive tothe proton magnetic moment in water and fat, whichprimarily make the human body. In MR microscopy,the sample of interest is set in a strong, uniformmagnetic field with gradient magnetic field thatvaries linearly. Radio-frequency coils are used toapply an RF magnetic field to the sample and toreceive NMR signals from the sample.

In this study three-dimensional microscopicimages were obtained with a homebuilt NMRimaging system using a 4.74 Tesla, 89 mmvertical-bore super-conducting magnet and anactively shielded gradient coil (Kose, 1996). Theproton resonance frequency was 202 :MHz. Thephase-encoding directions were the x- and y­directions and the signal readout direction was thez- direction.

Since the NMR signal from the ice was veryweak, we filled the air space of snow with ironacetylacetonate C15H210tYe doped aniline C&H7N ordodecane C12Hz6, both of which give NMR signalstrong enough, a good infiltration property and alow freezing point (about -6°C and -12°C). Thus,the NMR system images the space occupied withdodecane instead of ice particles. The spin-latticerelaxation time (Tl) of the iron acetylacetonatedoped dodecane was about 400 ms.

Figure 2: 2D cross-sectional MR image ofdepth hoar crystals.

Signal accumulations were necessary in order toreduce the noise, and hence it took more than halfan hour to obtain one 3D image of snow. So a smallacrylic chamber with a double pipe cylinder wasconstructed to keep the temperature of sample coldenough. A sample holder with dimensions of 24mm inner diameter and lOO rom long was set intothe inner cylinder. Cold air, which was kept about-25°C through frozen ethylene glycol, was led intothe inner pipe (Figure 1) by the air compressor. Thetemperature of the sample holder was kept betweeno°C and the melting point of the dodecane duringthe measurements.

Four types of snow were tested: ice spheres ofabout 3-rom in diameter, large roundedpoly-crystals, small rounded mono-crystals anddepth hoar crystals. The ice spheres were made ofwater dropping into the liquid nitrogen. Others weretaken from the natural snowpack in Hokkaido,Japan. Among these materials the depth hoarcrystals were utilized to improve the NMR imagingsystem, since they were sufficiently large and haveanisotropic structures.

Figure 3: Section planes of depth hoar. Intervals of (A) to (B) and (B) to (C) are 0.1 rom.

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International Snow Science Workshop (2002: Penticton, B.C.)

Figure 4: Volume-rendered 3D display ofdepth hoar (reversal image).

Images were obtained with the following threekinds of sequences; the image matrix 2563

, voxelsize 120 !UJl3, the image matrix 1283, voxel size 200!UJl3 and the image matrix 643, voxel size 400 !UJl3.The first one was for the imaging of the smallrounded mono-crystals and the last one was used forthe continuous measurement aimed to look at snowmetamorphism.

3. Experimental results

Figure 2 shows horizontal cross-sectional imagecut from 3D MR image data of depth hoar layer;field-of-view (FOV) =25.6 mID x 25.6 mm, imagematrix = 128x 128, and slice thickness = 0.2 mID.

The depth hoar specimens were filled with ironacetylacetonate doped aniline. Because NMR signalis obtained from aniline and not from the ice, thedepth hoar crystals are shown as dark particles orsegments in the figure. On the other hand, Figures 3(A), (B) and (C) show the section planes dyed withsudan-black B using the same sample in Figure 2.The procedure of making thin section is introducedin Hachikubo et. al (2000). Dark area in Figure 3 isthe pour space of the depth hoar specimens.Intervals of (A) to (B) and (B) to (C) are 0.1 mID.

Since the NMR signal was gained from the voxel of0.2 mm thick, the MR image in Figure 2 isreflecting the structure shown in the three sectionplanes in Figure 3. We see the pictures in Figure 3and 2D image in Figure 2 resemble closely, whichindicates that MRl reconstructs the distribution ofice grains accurately. Comparison of the images inFigures 2 and 3 is also useful to determine thethreshold brightness, which distinguishes the icefrom the filling in NMR images.

Figure 4 illustrates a volume rendered 3Ddisplay of depth hoar specimen. Because NMRsignal was obtained from dodecane, this was

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1 cm

Figure 5: Volume~rendered 3D display ofdepth hoar (voxel size 200 !UJl3).

reversal image of the crystals: i.e. depth hoarcrystals correspond to void area in the figure.Negative images of the above 3D data wereprocessed and 3D display of the depth hoar isshown in Figure 5 (voxel size 0.2 mID x 0.2 mm x0.2 mID). The classical thin section of the depthhoar specimen is also shown in Figure 6. Figure 7illustrates a volume rendered 3D image of largerounded poly-crystals, and the classical thin sectionis shown in Figure 8. In Figure 7, we can recognizethe 3D structure of large rounded poly-crystals fromvarious viewing angles and cross sections. Thegrain size of small rounded mono-crystals wassmaller than others (Figure 9), then MR image wasgained using the sequence of image matrix 2563

•

Figure 10 illustrates a volume rendered 3D image ofsmall rounded mono-crystals. Although the higherresolution seems preferable to analyze the bondstructure between grains, it is certain the NMRimaging system developed in this study will be apowerful tool to investigate the snowpackstructures.

1 mmFigure 6: Thin section of depth hoar crystals.

1 em

Figure 7: 3D display of large roundedpoly-crystals (voxel size 200 /Ull3).

Acknowledgements

We are grateful to Assoc. Prof. A. Hachikubo ofKitami Institute of Technology for his support ofsample preparation. We also thank Assoc. Prof. HNarita of the Institute of Low Temperature Science,Hokkaido University for his useful technicalsuggestions for processing the thin section of snow.

4. References

Coleou C., B. Lesaffre, J. B. Brzoska, W. Ludwig, E.Boller, 2001. 3-D snow images by X-raysmicro-tomography. Ann. Glaciology, 32, 75-81.

Edelstein, W. A., E. M. Schulson, 1991. NMRimaging of salt-water ice. J. Glaciol., 37,177-180.

Eicken, H, C. Bock, R. Wittig, H. Miller, H. O.

1 mmFigure 9: Thin section of small roundedmono-crystals.

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1 mm

Figure 8: Thin section of large roundedpoly-crystals.

Poertner, 2000. Magnetic resonance imaging ofsea-ice pore fluids: methods and thermalevolution of pore microstructure. Cold Reg. Sci.Technol., 31, 207-225.

Hachikubo, A., H. Arakawa, K Nishida, T.Fukuzawa, E. Akitaya, 2000. Section planes ofsnow specimens visualized with sudan black B.Polar Meteorol. Glaciol., 14, 103-109.

Kose, K, 1996. 3D NMR imaging of foamstructures. J. Mag. Resonance, A1I8, 195-201.

Ozeki, T., A. Hachikubo, K Kose, K Nishimura,2000. NMR imaging of snow. Proceedings ofInternational Snow Science Workshop 2000, BigSky, Montana, 402-406.

Schneebeli, M., 2000. Three-dimensional snow:How snow really looks like. Proceedings ofInternational Snow Science Workshop 2000, BigSky, Montana, 407-408.

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Figure 10: 3D display of small roundedmono-crystals (voxel size 120 /Ull3).