Accurate positioning of magnetic microparticles beyond the spatial resolution of clinical MRI scanners using susceptibility artifacts

Susceptibility-based negative contrast in magnetic resonance imaging (MRI) provides a mean to visualize magnetic microparticles. In the presence of a number of micropaticles in the field of view (FOV), the shape of the artifact is affected by the dipole-dipole interaction between the particles. Due to the limited spatial resolution of the clinical MR scanners, the exact positioning of the particles in MR images is not possible. However, the shape of the artifact can shed light on how the particles are distributed within the FOV. In this work, a simulation model and in-vitro experiments were used to study the shape and the amount of the susceptibility artifact for various spacing and angulations between the microparticles. The results showed that for a pair of identical particles with a diameter of D, the signal loss starts to change when particles are separated ~15×D and they become fully distinguishable when their distance reaches ~ 40×D.