Magnetic field dependence of solvent proton relaxation by solute dysprosium (III) complexes.
OBJECTIVE
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 effects on 1/T1 and 1/T2 of water protons with no shift in the water-proton resonance frequency. 1/T1 and 1/T2 nuclear magnetic relaxation dispersion (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 the electronic relaxation time 100-fold, and can have a large effect on proton 1/T2 and resonance frequency. Relaxation theory is well-developed and applicable to both ions but, for Dy(III), the phenomena are more wide-ranging. Recent interpretations have suggested that the data are anomolous, requiring a new mechanism for their explanation. The authors explain published Dy(III) data in terms of known theory, guided by experience with Gd(III) agents.


METHODS
For fields below 1 T, the authors incorporate the shortened electronic relaxation time into the usual low-field theory for magnetic dipolar interactions between water protons and Dy(III) magnetic moments. Both inner- and outer-sphere relaxations 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" causes a quadratic dependence of 1/T1 on field, and--through dipolar-induced shifts--an even greater quadratic dependence of 1/T2.


RESULTS
All published data can be explained by magnetic dipolar interactions. For Dy(III), the Curie term has a longer correlation time than the low-field term, namely, the rotation of solute for 1/T1 and the even longer water exchange lifetime tau M for 1/T2. This exchange modulates the shift, producing phenomena not seen with Gd(III).


CONCLUSIONS
Relaxation by Dy(III) chelates can be explained by the same well-established theory of dipolar interactions used for their Gd(III) analogs. Interestingly, for MRI applications, tau M should be long for Dy(III)-based agents and short for Gd(III)-based agents.
Kellar
K E
KE
Nycomed Amersham Imaging, Wayne, Pennsylvania, USA.
Fossheim
S L
SL
Koenig
S H
SH
D003287:Contrast Media; D004419:Dysprosium / Q000737:chemistry; D005682:Gadolinium / Q000737:chemistry; D006801:Humans; D008279:Magnetic Resonance Imaging / Q000379:methods*
D003287:Contrast Media; D004419:Dysprosium; D005682:Gadolinium