Are nanocubes more efficient than nanospheres to enhance the nuclear magnetic relaxation of water protons? A Monte Carlo simulation study

Iron oxide superparamagnetic nanoparticles have been extensively studied as T2 contrast agents in magnetic resonance imaging. The theory of nuclear magnetic relaxation induced by superparamagnetic nanoparticles has been validated by numerous experimental studies in the case of spherical particles. Recently, several studies focused on the synthesis of cubic nanoparticles. Some of them reported significantly higher relaxivities compared to their spherical counterpart and attributed this increase to their specific shapes. This work investigates the impact of cube-shaped nanoparticles on nuclear magnetic relaxation through Monte Carlo methods. Transverse relaxation at high static magnetic field is simulated by modeling the proton diffusion in the magnetic field generated by a cubic or a spherical nanoparticle. The results indicate that, in the case of magnetite nanoparticles, there is no significant difference between both shapes for sizes above 30 nm when particles are compared at equal volumes and magnetization. Below this size, a -40%-15% variation of the relaxation rates is predicted for the cubic case compared to the spherical case. These results are explained using general relaxation models that incorporate the distribution of the magnetic field generated by the nanoparticles. The simulation predictions are compared to some experimental results from the literature, revealing that, in some cases, the magnetic field specific to the nanoparticle shape alone cannot explain the observed increase in the relaxation rate of cubic nanoparticles.