Metal ions are often used for diagnostic medical imaging. Metal complexes can be used either for radioisotope imaging (from their emitted radiation) or as contrast agents, for example, in magnetic resonance imaging (MRI). Such imaging can be enhanced by manipulation of the ligands in a complex to create specificity so that the complex will be taken up by a certain cell or organ type.
Examples of metals used for diagnosis include:
* Technetium. Tc is the most commonly used radioisotope agent for imaging purposes. It has a short half-life, emits only gamma ray photons, and does not emit beta or alpha particles (which are more damaging to surrounding cells), and thus is particularly suitable as an imaging radioisotope. 
* Gadolinium(III), Iron(III), Manganese(II): For MRI imaging paramagnetic metals are needed for contrast imaging. Gadolinium(III), Iron(III), and Manganese(II) are all paramagnetic metals that are able to alter the tissue relaxation times and produce a contrast image. 
* Gallium-68 is useful as a positron source for Positron emission tomography.
* Cobalt(III): Cobalt(III) is used with the compound bleomycin (BLM) (Figure 1), which is an antibiotic, to selectively be taken up by tumor cells. The use of cobalt results in the best blood-to-tumor distribution ratio, but its half-life is too long to be conducive for imaging purposes. A solution has been proposed to attach an EDTA moiety to the terminal thiazole ring of bleomycin, radiolabeled so that the entire complex can then be traceable. This system could provide tumor locations accurately, leading to earlier detection and more non-invasive procedures in the future.