Abstract: The invention relates to a complex between block polymers comprising a first hydrophilic block and a second block comprising at least one ionised function, and optionally carrying at least one additional chemical or biochemical group or comprising a third polymer block, and mixtures of said polymers, and ions selected from the elements Cu, Ga, Sr, Bi, Se, Y, lanthanide, Pb, Te, Zn, Zr at different degrees of ionisation and the mixtures thereof, more particularly for the use thereof in medical imaging, and to a production method and a physiologically acceptable composition containing said complex.