Core–shell particles with tailored magnetic susceptibility for signal-efficient magnetic resonance imaging of granular systems

Abstract

Magnetic Resonance Imaging (MRI) has been recently applied to decipher the complex flow of dry granular materials, which play an important role in a variety of chemical engineering applications. In these materials, the short apparent transverse relaxation time of the constituent grains, T₂^∗, leads to rapid signal decay and hence limits the choice of suitable pulse sequences. While oil-rich agricultural seeds and oil-filled core–shell particles containing substantial amounts of liquids have been used to generate MRI signals, these particles still suffer from T₂^∗ values much shorter than the transverse relaxation time T₂ of the contained liquid. This work investigates the effect of magnetic susceptibility on T₂^∗ through numerical simulations and experiments. Numerical results demonstrate that matching the magnetic susceptibility of the particles, χ_p, to that of the air between them, χ_air, reduces dipolar magnetic field inhomogeneities, theoretically enabling T₂^∗=T₂. We also found that common imperfections in core–shell particles — such as asphericity, non-concentricity of core and shell, and uneven shell thickness — cause significant field inhomogeneities. These inhomogeneities can only be mitigated if both the core and shell materials have a magnetic susceptibility matching that of air. Based on these findings, we designed and manufactured core–shell particles with doped cyclooctane (CO) encapsulated in doped gelatin with χ_core≈χ_shell≈χ_air. These particles exhibited a T₂^∗ of 3.85 ms, more than twice that of previously available materials, thus improving the signal-to-noise ratio and enhancing pulse sequence flexibility. This advancement opens up new possibilities for applications in engineering and the study of granular physics.