  Magnetic particles are very efficient Magnetic Resonance Imaging (MRI)
contrast agents. In the recent years, chemists have unleashed their imagination
to design multi-functional nanoprobes for biomedical applications including MRI
contrast enhancement. This study is focused on the direct relationship between
the size and magnetization of the particles and their nuclear magnetic
resonance relaxation properties, which condition their efficiency. Experimental
relaxation results with maghemite particles exhibiting a wide range of sizes
and magnetizations are compared to previously published data and to
well-established relaxation theories with a good agreement. This allows
deriving the experimental master curve of the transverse relaxivity versus
particle size and to predict the MRI contrast efficiency of any type of
magnetic nanoparticles. This prediction only requires the knowledge of the size
of the particles impermeable to water protons and the saturation magnetization
of the corresponding volume. To predict the T2 relaxation efficiency of
magnetic single crystals, the crystal size and magnetization obtained through a
single Langevin fit of a magnetization curve is the only information needed.
For contrast agents made of several magnetic cores assembled into various
geometries (dilute fractal aggregates, dense spherical clusters, core-shell
micelles, hollow vesicles), one needs to know a third parameter, namely the
intra-aggregate volume fraction occupied by the magnetic materials relatively
to the whole (hydrodynamic) sphere. Finally a calculation of the maximum
achievable relaxation effect and the size needed to reach this maximum is
performed for different cases: maghemite single crystals and dense clusters,
core-shell particles (oxide layer around a metallic core) and zinc manganese
ferrite crystals.
