Dy-DTPA derivatives as relaxation agents for very high field MRI: The beneficial effect of slow water exchange on the transverse relaxivities

Proton longitudinal and transverse relaxivities of Dy(DTPA)^2- and Dy-DTPA bisamide derivatives (Dy(DTPA-BA): Dy-DTPA bisamide, Dy(DTPA-BEA): Dy-DTPA bisethylamide, Dy(DTPA-BnBA): Dy-DTPA bis-n-butylamide, and Dy(DTPA-BBMA): Dy-DTPA bisbismethylamide) were analyzed between 0.47 T and 18.8 T. Curie longitudinal relaxation was clearly observed at magnetic fields larger than 2.4 T, but the longitudinal relaxivities are limited by the fast rotation of the complexes. Rotational correlation times were separately assessed by deuterium relaxometry of the diamagnetic deuterated lanthanum analogs. Transverse relaxivity, which depends on the square of the magnetic field and on the residence time of the coordinated water molecule (τ_M), was more than 7.5 times larger at 18.8 T and 310 K for Dy(DTPA-BA) and Dy(DTPA-BEA) as compared to Dy(DTPA)^2-. This difference is mainly related to the slower water exchange of the bisamide complexes, as confirmed by the values of τ_M measured by oxygen-17 relaxometry. Such Dy-complexes, characterized by relatively long τ_M values (τ_M³¹⁰ larger than 100 ns but smaller than 1 μs), thus appear to be useful as negative T₂ (or transverse) contrast agents for high-field imaging. This was demonstrated by the spin-echo images of phantoms obtained at 4.7 T on samples containing Dy(DTPA)^2- and Dy(DTPA-BEA).