Magnetic La_1−x Sr _x MnO₃ nanoparticles as contrast agents for MRI: the parameters affecting ¹H transverse relaxation

Magnetic nanoparticles of the La_1−x Sr _x MnO₃ perovskite phase (x = 0.20–0.45) were synthesized by a sol–gel method followed by thermal and mechanical treatments. The particles were coated with a uniform silica shell, and differential centrifugation yielded a product with high colloidal stability in water. X-ray powder diffraction (XRD) data showed that the mechanical processing did not affect the lattice parameters of the magnetic cores but only reduced their mean size d _XRD. The magnetic properties of the bare particles were mainly controlled by the chemical composition and were also affected by the size of the particles. Subsequent silica coating led to an effective decrease in magnetization. Relaxometry measurements were focused primarily on colloids using magnetic cores of the same size (d _XRD ≈ 20 nm) and different compositions, and coated with a shell measuring approximately 20 nm in thickness. The exceedingly high transverse relaxivities [r ₂(20 °C) = 290–430 s⁻¹ mmol⁻¹ L at B ₀ = 0.5 T] of the samples exhibited pronounced temperature dependence and correlated very well with the magnetic data. Additional samples differing in the size of the cores and silica shell thickness were prepared as well to analyze the effect of the particles on ¹H transverse relaxation. The results suggest that the dominant regime of transverse relaxation is the static dephasing regime.