Measuring viscosity with magnetism
In 2005 Bernhard Gleich and Jürgen Weizenecker of Philips Research Hamburg published a description of a new tracer-based, real-time tomographic imaging method. They called it magnetic particle imaging (MPI). In its original incarnation, MPI yielded the spatial distribution of one type of tracer particle. Now, Martin Möddel of the University Medical Center Hamburg-Eppendorf and his collaborators have generalized MPI’s underlying equations to cope with more than one tracer type simultaneously. They have also demonstrated in the lab how to measure a property, such as viscosity, that is revealed not just by the tracers’ location.
MPI relies on the S-shaped magnetization curve of ferromagnetic tracer particles and on the manipulation of strong, static magnetic fields and weaker, oscillating magnetic fields. Two strong fields are arranged with their poles antiparallel to create a tiny field-free region within the static field. When the oscillating fields are switched on, they pervade the detector volume, including the field-free region. The magnetization of a ferromagnetic tracer that happens to occupy the field-free region will oscillate in response to the weak field. Tracers outside the field-free region don’t respond to the oscillating field because the total field they experience, strong plus weak, saturates their magnetization. The tracers’ oscillating magnetization is detected by radiofrequency coils. An image is formed by the addition of electromagnets that raster the field-free region through the sample. MPI scanners like the one in the figure are now commercially available for pre-clinical applications.
How does MPI measure viscosity? In essence, a tracer’s response in a viscous medium depends on how readily the oscillating field makes the tracer wobble. After calibrating their setup on 10 different glycol-water mixtures of known viscosity, Möddel and his collaborators found they could determine an arbitrary viscosity in the range 1–51.8 millipascal-seconds with an accuracy of 6% or less. Möddel’s approach can be extended to other properties, including temperature. He anticipates applying it clinically to detect the heat associated with inflammation and the thickening associated with coagulation. (M. Möddel et al., New J. Phys. 20, 083001, 2018 ; thumbnail image credit: University Medical Center Hamburg-Eppendorf.)
