A series of hydrogel nanoparticles incorporating MRI contrast agents (GdDOTP and MS325) as potential cross-linkers were elaborated by an easy and robust ionotropic gelation process. By this process, high Gd loadings were obtained (between 1.8 and 14.5 × 10⁴ Gd centres per NP). By tuning the cross-linker ionization degree and the nature of the polymer matrix it was possible to boost the r₁ relaxivity per Gd centre up to 22-fold. The greatest gains in relaxivity were observed for nanogels for which the polymer matrix was constituted of chitosan and hyaluronan. Relaxivities per Gd centre as high as 100 s⁻¹ mM⁻¹ at 30 MHz can be reached, which highlighted the fact that molecular motion of the Gd chelate was effectively restricted and water access to the inner core of these nanogels was not limited.