Nuclear Spin Relaxation Parameters of MRI Contrast Agents – Insight from Quantum Mechanical Calculations
Nuclear magnetic relaxation in the presence of paramagnetic centres has gained increasing interest in recent years partly due to its importance for contrast agents in magnetic resonance imaging. Rational design of new more efficient agents is possible as a result of a better understanding of the underlying relaxation mechanisms. Quantum chemical calculations together with molecular dynamics simulations allow obtaining fundamental parameters such as quadrupole coupling constants and hyperfine interaction tensors directly at a molecular level. Recent results are presented on gadolinium(III) ions in aqueous solution and on [Gd(DOTA)(H2O)]–, a commercial MRI contrast agent. Isotropic hyperfine coupling constants can be calculated for 17O and 1H nuclear spins of water molecules in the first and second coordination sphere of Gd3+. It is also shown that the commonly used point-dipole approximation for the dipolar interaction between the electron and the nuclear spin is in general valid for 1H spin but not for the directly bound 17O spin. The calculated quadrupole coupling parameters allow a direct determination of the rotational correlation time of complexes from the 17O nuclear spin relaxation.