Core-shell particles with tailored magnetic susceptibility for signal-efficient magnetic resonance imaging of...

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 3.85ms, 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.