Factors determining the proton T1 relaxivity in solutions containing Gd-DTPA

The relaxation times T1 and nuclear Overhauser effects were measured in the 13C nuclear magnetic resonance (NMR) spectra of the diamagnetic calcium (Ca) and lanthanum (La) analogs of Gd-DTPA. From these data the correlation times for the molecular reorientation tau R were calculated; it is suggested that this term is the dominant one in the evaluation of the paramagnetic relaxation equation. The similar tau R values obtained in water and blood serum ruled out the occurrence of any adduct formation between Gd-DTPA complex and the macromolecular framework in the latter system. The difference in relaxivity shown by Gd-DTPA in the two media is interpreted in terms of influence of the serum components on the dissociation equilibrium of Gd-DTPA.