Magnetic resonance imaging of granular materials

Abstract

Magnetic Resonance Imaging (MRI) has become one of the most important tools to screen humans in medicine; virtually every modern hospital is equipped with a Nuclear Magnetic Resonance (NMR) tomograph. The potential of NMR in 3D imaging tasks is by far greater, but there is only "a handful" of MRI studies of particulate matter. The method is expensive, time-consuming, and requires a deep understanding of pulse sequences, signal acquisition, and processing. We give a short introduction into the physical principles of this imaging technique, describe its advantages and limitations for the screening of granular matter, and present a number of examples of different application purposes, from the exploration of granular packing, via the detection of flow and particle diffusion, to real dynamic measurements. Probably, X-ray computed tomography is preferable in most applications, but fast imaging of single slices with modern MRI techniques is unmatched, and the additional opportunity to retrieve spatially resolved flow and diffusion profiles without particle tracking is a unique feature.

High-speed x-ray tomography of silo discharge

“…The second, more sophisticated technique is to employ noninvasive tools to determine the particle arrangements and dynamics in a 3D bin. X-ray computed tomography (XR-CT) [41,42] or magnetic resonance imaging (MRI) [43] are the most promising candidates. Both methods have been successfully applied to map static granular ensembles.…”

Multi-scale mechanics of granular solids from grain-resolved X-ray measurements

“…Magnetic resonance imaging (MRI) and magnetic resonance elastography (MRE) have each permitted studies of granular solids composed of grains that are stiffer than those used for photoelasticity [21,22]. While these techniques are capable of aiding in 3D characterization of granular solids and their force networks, more effort has been devoted to using them for studying interstitial fluid migration and dynamic mixing and segregation (see [22] and citations therein).…”

Preface: Focus on imaging methods in granular physics

“…35 Finally, we can abandon the range of visible light entirely and instead use penetrating radiation to obtain the data from the bulk of the sample. A variety of such methods are extensively covered in this focus edition, covering terahertz electromagnetic radiation, 36 radar, 37 positron emission, 38 nuclear magnetic resonance, 39 and X-ray tomography. 40 Several solutions also exist for the data-bandwidth issue, which are common to the various methods.…”

“…For non-transparent materials, Magnetic Resonance Imaging (MRI) can obtain contrast-rich images within the bulk. 39 MRI reveals the edges of particles by mapping the distribution of NMR-active nuclei in liquids (or, in exceptional cases also in gases) within a sample. Two requirements have to be met.…”

“…34 Other techniques for tracking within the bulk include Positron Emission Particle Tracking (PEPT), 38 microwave radar tracking, 37 X-ray radiography, 54 or Magnetic Resonance Imaging (MRI). 39 Tracer particles (single radioactively labeled in the case of PEPT, high dielectric constant for radar tracking, steel spheres for X-ray radiography, or NMR-active nuclei for MRI) are then embedded inside the material (or spin-labeled in MRI) and the system is imaged while subjected to some excitation or loading. The knowledge of the complete trajectory of one or a few particles in 3D is then possible.…”