In vitro characterization of the Gd complex of [2,6-pyridinediylbis(methylene nitrilo)] tetraacetic acid (PMN-tetraacetic acid) and of its Eu analogue, suitable bimodal contrast agents for MRI and optical imaging.

Gd and Eu complexes of PMN-tetraacetic acid show interesting properties either for MRI or for optical imaging; that is, for the Gd-complex, a high proton relaxivity with favorable water residence time; for the Eu-complex, a luminescence lifetime of 400 micros at room temperature compatible with the use of time-resolved luminescence technique. Both complexes have a good stability in physiological medium.