Flow in porous metallic materials: A magnetic resonance imaging study

Abstract

Purpose:To visualize flow dynamics of analytes inside porous metallic materials with laser-detected magnetic resonance imaging (MRI). Materials and Methods:We examine the flow of nuclearpolarized water in a porous stainless steel cylinder. Laserdetected MRI utilizes a sensitive optical atomic magnetometer as the detector. Imaging was performed in a remotedetection mode: the encoding was conducted in the Earth's magnetic field, and detection is conducted downstream of the encoding location. Conventional MRI (7T) was also performed for comparison.Results: Laser-detected MRI clearly showed MR images of water flowing through the sample, whereas conventional MRI provided no image. Conclusion:We demonstrated the viability of laser-detected MRI at low-field for studying porous metallic materials, extending MRI techniques to a new group of systems that is normally not accessible to conventional MRI. MAGNETIC RESONANCE IMAGING (MRI), conventionally performed in a strong homogenous magnetic field, is a versatile imaging modality for materials research (1). Its advantage of noninvasiveness allows imaging of opaque porous materials not possible by optical methods. Sederman et al (2) used MRI to study the structureflow correlation in packed beds. Seymour et al (3) studied biofouling of a homogeneous model porous media. Callaghan and Khrapitchev (4), and Blü mich et al (5) developed various pulse sequences to investigate timedependent flow velocities in porous media. Swider et al (6) employed high-resolution MRI to study both localized and global flow in porous biomaterials. Recently, Granwehr et al (7) developed a time-of-flight MRI scheme that utilized remote detection to study the flow behavior of xenon gas in porous rocks. In addition, different fluids can be selectively encoded based on their respective chemical shifts (8). The samples involved in these studies with conventional MRI are limited to electrically nonconductive materials.Flow dynamics of liquids in porous metallic materials is of extensive practical interest, as these materials are widely used in filtration, catalysis, and biomedicine (9 -12). Conventional MRI techniques, however, are not applicable to these materials for two fundamental reasons. First, the penetration depth of radiofrequency radiation, which is inversely proportional to the square root of the frequency, is only several micrometers in metals for the strong magnetic fields (Ͼ1 T) used in conventional MRI scanners (13). The nuclear spins within an electrically conductive sample that reside deeper than the penetration depth will not be sufficiently excited, and thus, will not contribute to the MR images. Even if some nuclear spins could be excited within a region sufficiently below the penetration depth of the metallic sample, these spins would not be detected at the pickup coil, because their oscillating field would be similarly screened out. Second, the large magnetic-susceptibility gradients intrinsically associated with porous metallic materials significantly distort the field homogeneity a...

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