Abstract: The present invention relates to a contrast agent suitable for ex vivo imaging, particularly of vascular structures and renal tubular structures, and a method for ex vivo imaging. The contrast agent is a polymer comprising monomers M. The monomer comprises a backbone having 2 to 6 elements, wherein at least one element is —CH(R)— or —N(R)—. R is a moiety -E-H, -L-(NH2)m or a moiety of formula 1, with E, L, R1 and R2 being defined as described in the present specification. The monomer comprises at least one —I to allow detection via X-ray and at least one —NH2 to allow crosslinking. Furthermore, the monomer comprises polar functional groups that contribute to water solubility. To avoid extravasation and glomerular filtration, the polymers are pre-crosslinked before the vasculature of a tissue, an organ or whole animal is perfused. After perfusion, the pre-crosslinked contrast agent is further crosslinked to be retained within the tissue, organ or animal permanently.