Longitudinal and transverse NMR relaxivities of Ln(III)-DOTA complexes: a comprehensive investigation

Longitudinal and transverse 1H NMR nuclear relaxivities of Ln(III)-DOTA complexes (with Ln = Gd, Dy, Tb, Er; DOTA = 1,4,7,10-tetraazacyclododecane-N,N',N',N''-tetraacetic acid) and Mn(II) aqueous solutions were measured in a wide range of frequencies, 10 kHz - 700 MHz. The experimental data were interpreted by means of models derived from the Solomon-Bloembergen-Morgan theory. The data analysis was performed assuming the orbital angular momentum L = 0 for Gd-DOTA and the aqua ion [Mn(H2O)6]2+ and L ≠ 0 for Dy-, Tb-, Er-DOTA. A refined estimation of the Zero-Field-Splitting barrier Δ and of the modulation correlation time τv was obtained for [Mn(H2O)6]2+ by extending the fitting of NMRD profiles to the low-field regime. The Gd-DOTA fitting parameters resulted in good agreement with the literature, and the fit of transverse relaxivity data confirmed the negligibility of the scalar interaction in the nuclear relaxation mechanism. Larger transverse relaxivities of Dy-DOTA and Tb-DOTA (~10 mM-1s-1) with respect to Er-DOTA (~1 mM-1s-1) were observed at 16 Tesla. Such higher values are suggested to be due to a shorter residence time τm, that is possibly linked to the fluctuations of the hyperfine interaction and the different shape of the magnetic anisotropy. The possible employment of Dy-DOTA, Tb-DOTA, and Er-DOTA as negative MRI contrast agents for high-field applications was envisaged by collecting spin-echo images at 7 Tesla. Particularly in Dy- and Tb- derivatives the transverse relaxivity at 16 Tesla is of the order of the Gd- one at 1.5 Tesla.