pH-responsive nanogels (NGs) were used to prepare high-efficiency magnetic resonance imaging dual T1/T2 contrast agents for pH imaging. The polymeric NG matrix acts as a strong polydentate ligand that chelates the Mn cations in its inner cavity generating a hybrid NG structure. The Mn chelate NG is sensitive to pH changes, such that protonation induces a change of the polymer hydration state and consequent swelling. The swollen nanogel allows water molecules to enter and interact with the Mn chelate, shortening the relaxation time (switch ON) and giving rise to positive or negative contrast on T1- or T2-weighted magnetic resonance images.