MAGNETIC-FIELD DEPENDENCE OF SOLVENT PROTON RELAXATION BY SOLUTE DYSPROSIUM(III) COMPLEXES

RATIONALE AND OBJECTIVES. Many magnetic resonance imaging (MRI) agents are Gd(III)-based; its half-filled f-shell has an S-ground state and hence a long electronic relaxation time, leading to comparably large e ffects on 1/T1 and 1/T2 of water protons with no shift in the water-pr oton resonance frequency. 1/T1 and 1/T2 nuclear magnetic relaxation di spersion (NMRD) profiles of the Dy(III) aquo ion and its chelates have been reported recently, Dy(III) ions differ magnetically from Gd(III) ; the large spin-orbit interaction of its non-S-ground state reduces t he electronic relaxation time 100-fold, and can have a large effect on proton 1/T2 and resonance frequency, Relaxation theory is well-develo ped and applicable to both ions but, for Dy(III), the phenomena are mo re wide-ranging. Recent interpretations have suggested that the data a re anomolous, requiring a new mechanism for their explanation, The aut hors explain published Dy(III) data in terms of known theory, guided b y experience with Gd(III) agents. METHODS. For fields below 1 T, the a uthors incorporate the shortened electronic relaxation time into the u sual low-field theory for magnetic dipolar interactions between water protons and Dy(III) magnetic moments. Both inner- and outer-sphere rel axations are included, At higher fields (and unusual for small single- ion agents) one must include dipolar interactions of protons with the magnetization of the Dy(III) moments. This ''Curie magnetization'' cau ses a quadratic dependence of 1/T1 on field, and-through dipolar-induc ed shifts-an even greater quadratic dependence of 1/T2. RESULTS. All p ublished data can be explained by magnetic dipolar interactions. For D y(III), the Curie term has a longer correlation time than the low-fiel d term, namely, the rotation of solute for 1/T1 and the even longer wa ter exchange lifetime tau(M) for 1/T2, This exchange modulates the shi ft, producing phenomena not seen with Gd(III). CONCLUSIONS. Relaxation by Dy(III) chelates can be explained by the same well-established the ory of dipolar interactions used for their Gd(III) analogs, Interestin gly, for MRI applications, tau(M) should be long for Dy(III)-based age nts and short for Gd(III)-based agents.