Ke. Kellar et al., MAGNETIC-FIELD DEPENDENCE OF SOLVENT PROTON RELAXATION BY SOLUTE DYSPROSIUM(III) COMPLEXES, Investigative radiology, 33(11), 1998, pp. 835-840
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.