Studies of magnetization transfer and relaxation in irradiated polymer gels - interpretation of MRI-based dosimetry

Magnetization transfer and NMR relaxation rates were measured for water pro tons in two types of polymer gels developed for radiation dosimetry with MR I in order to quantify the contributions of different relaxation processes to the radiation response in such gels. Measurements included the rate of m agnetization transfer between proton pools and the ratio of the sizes of ex changing pools, R-1 and R-2. A model of relaxation in irradiated gels is pr esented to explain their properties. The model incorporates three proton po ols: free water, macromolecular and interfacial. Two pools are insufficient to model the data. In these systems, radiation-induced polymerization appe ars to increase the size of a solid-like macromolecular proton pool but doe s not affect the rate constant of magnetization transfer per proton from ma cromolecular protons to the free water protons. The relation between R-1 an d the pool size ratio is consistent with free water exchanging with a macro molecular pool with an R-1 of similar to8 Wt. in addition, the rate of magn etization transfer is not limited by the tate of chemical exchange between the free water and the interfacial protons, and magnetization transfer most probably occurs via labile proton exchange rather than via hound water mol ecules.