Gadolinium( III )‐Loaded Nanoparticulate Zeolites as Potential High‐Field MRI Contrast Agents: Relationship Between Structure and Relaxivity

The effects of dealumination, pore size, and calcination on the efficiency (as expressed in the relaxivity) of Gd 3+ ‐loaded zeolites for potential application as magnetic resonance imaging (MRI) contrast agents were studied. Partial dealumination of zeolites NaY or NaA by treatment with (NH 4 ) 2 SiF 6 or diluted HCl resulted in materials that, upon loading with Gd 3+ , had a much higher relaxivity than the corresponding non‐dealuminated materials. Analysis of the 1 H NMR dispersion profiles of the various zeolites showed that this can be mainly ascribed to an increase of the amount of water inside the zeolite cavities as a result of the destruction of walls between cavities. However, the average residence time of water inside the Gd 3+ ‐loaded cavities did not change significantly, which suggests that the windows of the Gd 3+ ‐loaded cavities are not affected by the dealumination. Upon calcination, the Gd 3+ ions moved to the small sodalite cavities and became less accessible for water, resulting in a decrease in relaxivity. The important role of diffusion for the relaxivity was demonstrated by a comparison of the relaxivity of Gd 3+ ‐loaded zeolite NaY and NaA samples. NaA had much lower relaxivities due to the smaller pore sizes. The transversal relaxivities of the Gd 3+ ‐doped zeolites are comparable in magnitude to the longitudinal ones at low magnetic fields (<60 MHz). However at higher fields, the transversal relaxivities steeply increased, whereas the longitudinal relaxivities decreased as field strength increased. Therefore, these materials have potential as T 1 MRI contrast agents at low field, and as T 2 agents at higher fields.