Employing paramagnetic shift for responsive MRI probes

Paramagnetic metal ions with anisotropic magnetic susceptibilities can shift the proton NMR signals of chelating ligands beyond the diamagnetic range of endogenous proton resonances. Such large shifts, along with enhanced longitudinal relaxation rates, allow paramagnetic complexes to be exploited as molecular imaging probes for MRI. Paramagnetically-shifted imaging probes are detected directly against zero background, as opposed to the indirect induced relaxation enhancement of surrounding water molecules, and are reviewed herein. ‘Smart’ probes that are sensitive to their surrounding environment are also being developed, and the some of the challenges faced for in vivo imaging are discussed, including issues of sensitivity and dose, biodistribution and clearance. Several examples of lanthanide complexes have been investigated, and more recently certain paramagnetic transition metal complexes are being considered as potential imaging agents.