Muller RN, Vander Elst L, Rinck PA, Vallet P, Maton F, Fischer H, Roch A, Van Haverbeke Y. The importance of nuclear magnetic relaxation dispersion (NMRD) profiles in MRI contrast media development. Invest Radiol 1988;23(Suppl 1):S229-S231. Observation of the relaxivity of MRI contrast media over a wide range of magnetic fields is not only necessary for predicting their efficiency at any field but also compulsory for understanding and improving their mechanisms of action. The best experimental approach to this problem is the field cycling method, which allows the exploration of nuclear relaxation over a broad interval of magnetic field intensity but requires a specially dedicated instrument called a relaxometer. Particularly relevant are the relaxivity profiles of the two chelates Gd-DOTA and Gd-DTPA. Both show an important decrease from low to high fields within the current imaging range (0.02 T to 1.5 T). Although high field relaxivities of these chelates are similar, Gd-DTPA becomes less efficient in facilitating water protons relaxation at fields lower than 0.15 T. This behavior has to be related to different electronic relaxation times due to a different chelate symmetry.