The present invention relates to the field of diagnostics and, more in particular, to MRI activatable contrast agents and compositions thereof for the detection of nuclease activity, wherein said nuclease activity is caused by microbial infection or by nuclease activity related to cancer, particularly colon cancer or pancreatic cancer. Activatable contrast agents for MRI have been developed, wherein the oligonucleotide is flanked by a paramagnetic and a superparamagnetic agent, and thus providing magnetic quenching. Moreover, the oligonucleotide has regions that confer resistance to mammalian endonucleases and sensitivity to microbial endonucleases. When the activatable contrast agent of the invention is in the presence of microbial nuclease activity or a tumour cell nuclease activity, the oligonucleotide is cleaved, agents are unquenched, and the signal derived from the activated contrast agent is detected by MRI.
