Phase interference-based portable magnetic resonance techniques for elastometry and flow

Abstract

Magnetic resonance imaging (MRI) is one of the most effective methods for non-invasive material characterization, and has been widely applied since its invention. Despite its utility, MRI has limitations—conventional instruments are large, expensive, and confined to laboratories. In recent years, significant advances have been made in compact, portable magnetic resonance systems tailored for specific applications.

This work focuses on using a constant gradient portable magnetic resonance instrument to characterize material properties in elastometry and flow applications. Although various experiments are discussed, all rely on the fundamental concept of motion-induced phase interference in the sensitive region and its effect on the measured magnetic resonance signal magnitude and phase.

Beginning with elastometry experiments, we examine bulk longitudinal sinusoidal excitation. This type of excitation produces an approximately uniform velocity distribution within the sensitive region, allowing phase measurements to be converted into velocity. By measuring velocity at various points throughout the vibration cycle, the resulting waveforms provide insight into the sample’s viscoelastic properties.

Building on this work, we consider shear wave excitation. In this case, the shear wavelength is comparable to the size of the sensitive region, making phase interference a significant factor. Due to the complexity of the sample response, we analyze changes in signal magnitude resulting from the arrival of the shear wave in the sensitive region to characterize the shear wave speed.

We then apply these concepts to flow systems, beginning with circular Couette flow, which has a well-defined analytical solution for the velocity distribution. Using this solution, we derive expressions for the MR signal response that depend on the fluid’s non-Newtonian properties.

Finally, we consider a rotating cylinder of fluid in cases where inducing open flow is impractical, relevant to applications involving sensitive samples, such as pharmaceuticals. While the equilibrium state is independent of fluid properties, the time to reach solid-body rotation depends on viscosity. We examine how MR signal magnitude evolves following rotation, exploring the influence of viscosity, cylinder height, and rotation speed.