  Elevated-temperature polyol-based colloidal-chemistry approach allows for the
development of size-tunable (50 and 86 nm) assemblies of maghemite iso-oriented
nanocrystals, with enhanced magnetization. 1H-Nuclear Magnetic Resonance (NMR)
relaxometric experiments show that the ferrimagnetic cluster-like colloidal
entities exhibit a remarkable enhancement (4 to 5 times) in the transverse
relaxivity, if compared to that of the superparamagnetic contrast agent
Endorem, over an extended frequency range (1-60 MHz). The marked increase of
the transverse relaxivity r2 at a clinical magnetic field strength (1.41 T),
which is 405.1 and 508.3 mM-1 s-1 for small and large assemblies respectively,
allows to relate the observed response to the raised intra-aggregate magnetic
material volume fraction. Furthermore, cell tests with murine fibroblast
culture medium confirmed the cell viability in presence of the clusters. We
discuss the NMR dispersion profiles on the basis of relaxivity models to
highlight the magneto-structural characteristics of the materials for improved
T2-weighted magnetic resonance images.
