Magnetic Resonance Sensors

Description:
Magnetic Resonance (MR) and Magnetic Resonance Imaging (MRI) Sensors, employ traditional and emerging "outside the magnet" MR techniques to nondestructively measure sample properties, including elastic properties of soft materials.

Broad Fields of Use:
MR sensors afford the opportunity to measure real-time at process determination of composition, water content, temperature, viscosity, and mechanical properties of fluid and solid process streams. For example, timber, an important and widely used construction material, could be used more effectively and responsibly if properties such as moisture and chemical content, density, and knot size and location, could be determined non-invasively before the tree is cut. Nuclear Magnetic Resonance spectroscopy and imaging (MR and MRI) are often the tools of choice in applications where non-invasiveness is critical. However, the requirements for magnetic resonance equipment are extremely stringent, demanding part per million field homogeneities in massive magnets; these conditions allow only for constrained sample volumes. The development of "outside the magnet" spectroscopy and imaging alleviates this problem, empowering this technique to be used in less controlled environments.

Comparison with Current Technologies:
Analytical MR studies are generally confined to the bore of a superconducting magnet, not by necessity but because most MR spectroscopists already have access to such equipment. While superconducting magnets have the advantage of producing the highest fields, they also require cryogens such as liquid nitrogen and helium to retain the necessary very low temperatures for operation; hence, they are seen as impractical in an industrial environment. A number of designs for producing magnetic fields and field gradients in an open volume are being investigated, specifically with MR application to remote sensing or studies of large objects that will not fit in such a confined space. This situation is often referred to as "inside-out" or "unilateral" MR, that is, it employs a single-sided magnet architecture. Several commercial applications of this approach have emerged recently, including analysis of water and oil in well boreholes (Schlumberger), defects in automobile tires (University of Aachen), propellant packing in solid fuel rocket motors (Lockheed-Martin), and moisture content in sheet rock (Fraunhofer Institute). These promising applications, along with our previous work on moisture content of wood chips in severe environments, encouraged us to extend these methods to the measurement of mechanical properties.

The use of MRI offers several advantages over conventional methods for acquiring the displacement and strain images required for evaluation of mechanical properties. Two popular methods for determining displacement and strain images are ultrasound and CT scans. The first relies on the heterogeneity of acoustical impedance, which combines the bulk modules and sample density. Ultrasound yields accurate results in the axial direction, but determination of the lateral displacements lack accuracy and spatial resolution. The second relies on the spatial distribution of x-ray attenuation, electron density distribution, which is related to the physical density distribution. CT scans expose samples to x-rays, which can be harmful to the test subject. MRI can produce multidimensional measurements characterized by higher spatial resolution and accuracy than either of these techniques. MRI can also be used in opaque samples, and most importantly it is a non-invasive technique.

Description of Current Application:
Previously, our group has developed magnetic resonance (MR) sensors for pulp and paper processing applications under the guidance of the American Forest Products Association and the Department of Energy's Office of Industrial Technologies. Our work in this area demonstrates the great promise the MR approach holds for determining not only bulk average % moisture content, but also fractional contributions from various water environments (pore, surface, frozen). We have further demonstrated the principle for a sensor that can quantitatively measure the Z-direction profile of water in a moving sheet of paper or other web-based product. Such analysis provides not only moisture content and thickness of the web, but may contribute to optimization of dryer operation as well.

We are conducting preliminary studies towards the use of MR sensors for the glass industry. While application of these sensors would significantly improve productivity, production facilities currently have no such tool available to them. For example, accurate on-line composition monitoring of molten glass streams will provide economic and environmental benefits across all industry segments in the form of increased production efficiency and use of cullet, or recycled glass. Furthermore, measurement of fiberglass properties, before and after coating, may result in more reproducible manufacturing.

We are initiating work on measuring spatially resolved mechanical properties of materials. We have developed an algorithm, which uses the constitutive and equilibrium equations, takes displacement data and reconstructs the elasticity modulus (Figure 1). The algorithm has been used to construct a Young's modulus map of a known ghost sample, while MRI was used to measure displacements yielding excellent results (Figure 2).

Figure 1. Young's modulus reconstruction.
 

Figure 2. Experimentally determined Young's modulus map.

We are currently working (in collaboration with the Berkeley Lab group of Alex Pine) to develop a hand held sensor that consists of a permanent magnet where MRI will be conducted outside the magnet utilizing static field gradients in combination with RF gradients. Proof of principle experiments have been conducted, producing images in a simulated "outside the magnet" environment.

Contact and Brief Bio:

Jeffrey Reimer, Faculty Scientist
Phone: (510) 642-8011
E-mail:

Professor of Chemical Engineering,
Associate Dean, Graduate Division
Department of Chemical Engineering
University of California
Berkeley, CA 94720-1462