Effect of pH on the nuclear magnetic resonance relaxivity of encapsulated solid paramagnetic substances.

Encapsulation, by acid-stable materials, of solid paramagnetic metal complexes based on sulphonated polystyrene ion-exchange resin, prevents the significant demetallation which otherwise occurs when the complexes are subjected to an acidic environment equivalent to that of the human stomach (pH 1-2). In turn, this protects the ability of those complexes to enhance the nuclear magnetic resonance (NMR) spin-lattice relaxation rate of water. Application of the results in the development of contrast agents for MR imaging of the gastrointestinal (GI) tract, in the evaluation of coatings used in drug formulations, and in the measurement of GI transit times of drugs is suggested.