NMR IMAGING OF SNOW

Toshihiro Ozeki*Ship Research Institute, Mitaka, Japan

Akihiro HachikuboKitami Institute of Technology, Kitami, Japan

Katsumi KoseInstitute of Applied Physics, University of Tsukuba, Tsukuba, Japan

Kouichi NishimuraInstitute of Low Temperature Science, Hokkaido University, Sapporo, Japan

KEYWORD: NMR, three-dimensional imaging, snow structure, ice sphere, dodecane

2. MATERIALS AND METHODS

which coupled with the explosive growth ctdigital computers, is playing an important role inmedical imaging over the last several years.Magnetic resonance imaging has advanced asa tomographic technique to produce images ctthe inside of the human body. Fat and water,which primarily make up the human body, havemany hydrogen atoms. MRI images an NMRsignal from the hydrogen nuclei. Thereforedevelopment of NMR imaging has enhancedour ability to diagnose and monitor diseases.Because NMR is particularly sensitive to theproton magnetic moment of water and organiccompounds, Edelstein and Schulson (1991)applied NMR imaging to visualize drainagechannels within salt-water ice. Manfres et. aI.(2000) used NMR to visualize oil diffusion intosea ice. Although there have been severalstudies on NMR imaging of sea ice, there havebeen few attempts to use NMR to visualizesnowpack. In this study, an NMR imagingsystem with a small temperature-controlledchamber was developed, and then used tovisualize snowpack samples.

In the imaging of snowpack, the NMRsignal from the ice was very weak. For tIiS

* Corresponding author address:Toshihiro Ozeki, Hokkaido University ofEducation, Iwamizawa, Hokkaido 068-8642,JAPAN; tel & fax: +81-126-32-0336;email: oze@iwa.hokkyodai.ac.jp

ABSTRACT: NMR imaging was introduced to visualize and analyze the three-dimensional structuresnowpack. We used three types of snow; ice spheres, coarsened snow grains and depth-hoar cryIn particular, because ice spheres had a uniform particle size and large enough, they was suitamaterial for the test imaging. Because the NMR signal from the ice was very weak, the air spacesnow was filled with iron-acetylaetonate doped dodecane C12H26, which had a good infiltration pr;'QPl:!rNJ1and a low freezing point (about -12°C), and we looked at the space occupied with dodecane insteadThe result of test imaging of Dodecane showed 0.5 to 2 hours were needed to obtain the one 3D imand hence we developed a specimen-cooling system using nested pipes. Cold air was sent bet ...........­

the pipes to keep the temperature below O°C. Experiments using the above snow and the system werecarried out and we succeeded to obtain 3D microscopic images; the image matrix was 1283 and vo~size was 0.2 mm 3.

1. INTRODUCTUION

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Microstructure of the snowpackshould be better understood because it affectsthe mechanical and physical properties ofsnow; for instance, heat conduction and shearstrength; and also affects the optical properties.The avalanche release mechanism alsodepends on the snowpack's microstructure.Thin sections and plane-surface sections arecurrently used to investigate the microstructureof the snowpack. However, they have onlytwo-dimensional information; therefore, a largenumber of operations are necessary toreconstruct the three-dimensional structureusing stereological methods. Several groupsare now developing new methods to get the 3Dmicrostructure of snow (Coleau et. a/., 2000;Schneebeli and KrOsi, 2000). Instead of thelatter techniques, we have developed a nuclearmagnetic resonance (NMR) imaging techniqueto visualize the 3D structure of snowpack.

Nuclear magnetic resonance imaging,

reason, the air spaces in the snow was filledwith dodecane doped with iron acetylaetonateC12H26' which has a good infiltration propertyand a low freezing point (about -12 0G). Thus,the NMR imaged the space occupied withdodecane instead of ice particles.

The three-dimensional microscopicimages, which have 128

3pixcels, were

obtained with a homebuilt NMR imaging system(Kose, 1996) using a 4.74-T, 89-mmvertical-bore superconducting magnet (OxfordInstruments) and an actively-shielded gradientcoil (Doty Scientific). The apparatus is shown inFig. 1. The phase-encoding directions are the xand y directions, and the signal readoutdirection is the z direction. Fig. 2: 20 NMR image of nylon

spheres in dodecane.

Because signal accumulations weredone to reduce the noise, the totalmeasurement time for one 3D image was 30 to60 minutes. As this measurement time is long,we developed a specimen-cooling system tokeep the temperature below 0 ac. This systemhas a double-pipe cylinder made of acrylicplastic with outer diameter of 40 mm and innerdiameter of 20 mm through which cold air waspassed.

Three types of snow were preparedfor the NMR imaging: ice spheres of about3-mm diameter, coarsened snow grains, anddepth-hoar crystals. In partiCUlar, because icespheres are sufficiently large and uniformlysized, they were ideal for test imaging.

50...--------------.

Fig. 3: Number of ice particles in each

frame.

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FRAME NUMBER

Fig. 1: Superconducting magnet of our

homebuilt MRI system.

The intensity of the NMR signal wasfirst tested using nylon spheres of 3.2-mmdiameter instead of ice particles, and then wefilled the interstitial pores with dodecane. Fig. 2shows a 0.2-mm slice of the complete 3Dmicroscopic image; this 2D image had a pixelsize of O.2x0.2 mm2

. The measurement time forone 30 image was 30 minutes. Because thenylon spheres did not have an NMR signal, thenylon spheres image as dark disks. This testShows that dodecane provides enough signal todistinguish dodecane from the nylon spheres,although it was necessary to improve the signalto noise ratio.

403

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Fig. 4: Porosity distribution.

3. EXPERIMENTAL RESULTS

both sid~s o~ the plot~; the thickness of thisedge regIon IS approximately the diameter Ofthe ice spheres. Although there are fluctuationsthe porosity is nearly uniform between the 30th'and 1DOth frames: the average is 0.32 and thestandard deviation is 0.04. This value is almostthe same as the porosity of closest packing Ofspheres (0.26).

Fig. 5 shows 20 cross-sectional slicesof the 3D image data of ice spheres; of the 128slices that complete the sample, (A) is the 43rd(B) the 45th, and (C) the 51st frame. There is ~gap of 0.2 mm between A and B, and C is theframe 1 mm from B. Because we couldmeasure the NMR signal from dodecane, butnot from the ice spheres, the ice spheresappear as dark disks or semicircles. The arrowsin each frame indicate how the diameter of thesame ice spheres change with frame change.The dark disks first appear, grow, and laterdisappear as we step through the sequence offrames.

Fig. 6 illustrates a volume-rendered3D display of ice spheres. Because the NMRsignal was obtained from dodecane, this is thenegative image of ice spheres; i. e., ice spheresare reconstructed as cavities. Negative imagesof the above 3D data were processed tovisualize ice spheres; this image-processed 3Ddisplay is shown in Fig. 7. This figure illustratesnot only the volume-rendered 3D image, butalso a cross-sectional image on the front side.As shown in Fig. 7, we can see the 3D structureof packed ice spheres by viewing such displaysfrom various angles and cross sections.

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Fig. 3 shows number of ice spheres ineach frame (O.2-mm thick slice) of the 3D data.The abscissa indicates the frame number from1 to 128. The signal of dodecane appearedfrom the 19th frame to the 112th frame. Thenumber of ice particles is distributed nearlysymmetrically in a bell-like curve with a peakbetween the 60th and 70th frames. Thisdistribution is primarily due to the variablecross-sectional area of our cylindrical sample'holder. This is supported by the nearly uniformporosity distribution that is plotted in Fig. 4. Inthis study, porosity was defined as the ratio ofthe dodecane area (bright area) to the wholesample area in one cross-sectional image. Anedge effect of the sample holder is shown on

(A) 43/128 (B) 45/128 (C) 51 /128

Fig. 5: 20 cross-sectional images from the 3D image of ice spheres.

A, B, and C are the 43rd, 45th, and 51st cross sections, respectively.

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Fig. 6: Volume-rendered 3D display of

ice spheres (reversed image).

Fig. 7: Negative black-white image of

ice spheres in dodecane.

Fig. 8 shows a cross sectional imageof depth-hoar crystal with dodecane. Since thedepth-hoar layer was disassembled to pack intothe sample holder, the depth-hoar sample didn'tkeep the original structure. The depth-hoarcrystals were solid or striated plates, and hencewe can see the negative image of thedepth-hoar as dark lines in the cross-section.Because the efficiency of the cooling systemwas reduced by frost condensation, signalaccumulation time of coarsened snow grainswas not enough, and the 3D data was noisy.

. Continuous measurements duringmelting of the ice spheres was carried out and~D microscopic images were obtained 17 timesIn 110 minutes; the image matrix is 643 and thevoxel size is 0.4 mm 3. Fig. 9 shows the same~Iice for the 3rd (A), 6th (B), and the 15th (C)Image of the 17 3D data sets. In this seriesconductive heat from the cylinder pipe raised

405

the temperature of the sample with time.Because the ice spheres and the dodecanewere frozen in Fig. 9 (A), the NMR signals werevery weak and the entire image is dark. On theother hand, Fig. 9 (B) shows a suitablecondition for NMR imaging of snow; i.e., thedodecane has melted 'and the ice spheres arestill frozen. In Fig. 9 (C), heat conduction hasmelted the ice spheres near the pipe and themelt water has an NMR signal as well as thedodecane. These results show that NMRimaging is useful for continuousnon-destructive measurements, and could be ~powerful technique to study the structure ofsnowpack.

4. CONCLUSIONS

NMR imaging was introduced tovisualize the three-dimensional structure ofsnowpack. Because the NMR signal from theice was very weak, the air space in the snowwas filled with dodecane, and we imaged thespace occupied by dodecane instead. Because0.5 to 2 hours were needed to obtain one 3Dimage of dodecane, a specimen-cooling systemwas developed that consisted of nested pipes.We obtained 3D microscopic images using icespheres, depth-hoar crystals with a voxel sizeof 0.2 mm 3. This NMR imaging method of snowwith dodecahe appears to be a superb way tovisualize the structure inside snowpack. Wecan see the 3D structure of a bed of icespheres by viewing 3D displays from variousangles and cross sections. Also, continuousmeasurements during the melting of snowshowed that NMR imaging is useful because itis non-destructive. In future experiments, weexpect to see not only the melting, but also thegrowing and breaking of snow. Although it isnecessary to improve the resolution and toreduce the noise, we expect that shorteningdodecane's relaxation time of will allow theseimprovements, and thus we expect that the 3Dnetwork of a snowpack will be shown clearlysoon.

ACKNOWLEDGEMENTS

We are grateful to Mr. S. Nakatsubo ofInstitute of Low Temperature Science,Hokkaido University for his work in assemblingand operating the specimen-cooling system.

We also thank Mr. T. Haishi of Institute ofApplied Physics, University of Tsukuba for hiswork in operating the NMR imaging system.

5. REFERENCE

Coleou C., B. Lesaffre, J. B. Brzoska, W.LUdwig, E. Boller, 2000. 3-D snowimages by X-rays micro-tomography.Abstracts of Int. Sym. Snow,Avalanches and Imp. Forest Cover,paper No. 64.

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

177-180.Kose, K., 1996. 3D NMR imaging of foam

structures. J. Mag. Resonance A118195-201. . ,

Manfred. A, U. Buschmann, B. Pfleiderer, 2000.OHand Sea Ice : NMR Tomography.Abstracts of Int. Sym. Sea Ice and ItsInteractions, paper No. 33A202.

Schneebeli, M. and G. Kri.isi, 2000.Three-dimensional reconstruction Ofsnow. Abstracts of Int. Sym. SnowAvalanches and Imp. Forest Cov~paper No. 97. '

Fig 8: 2D cross-sectional image of

depth-hoar crystals.

(A)3/17 (B)6/17 (C)15/17

Fig 9: 2D cross-sectional images of ice spheres during

melting. (A) Dodecane and ice are frozen, (B)

Dodecane melted with ice frozen, and (C) Ice

spheres are melting.

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