Injectable compositions for the MRI detection of an analyte selected from the group consisting of reactive oxygen species, proteases and enzymes, comprisinga) a matrix material based on a responsive hydrophobic polymer capable of undergoing a chemical reaction with the analyte to be detected, such reaction leading to a disruption of the polymer chain of the responsive polymer,b) a contrast agent suitable for use in magnetic resonance imaging, embedded in or encapsulated in the polymer a),c) optionally, a functionality capable of binding a marker or probe or a probe for creating a second detection signal, andd) optionally, a non-responsive polymer not undergoing a chemical reaction with the analyte under the conditions where polymer a) undergoes a reaction leading to chain breakage
