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

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, T 2 ∗ , 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 T 2 ∗ values much shorter than the transverse relaxation time T 2 of the contained liquid. This work investigates the effect of magnetic susceptibility on T 2 ∗ 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 a i r , reduces dipolar magnetic field inhomogeneities, theoretically enabling T 2 ∗ = T 2 . 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 c o r e ≈ χ s h e l l ≈ χ a i r . These particles exhibited a T 2 ∗ 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. • Investigation of the effect of the susceptibility of particles in packings. • Simulations of the effect of doping on field homogeneity within the packing. • Manufactured core–shell particles show a two-fold T 2 ∗ increase to 3.85 ms.