Detection of radiation effects in polymer gel dosimeters using 129Xe NMR.

Polymer gel dosimeters consist of monomers, with or without cross-linking agents, dispersed in a gel. Upon exposure to ionizing radiation, polymerization proceeds within the gel matrix, thereby changing several measurable physical properties that can then be related quantitatively to absorbed dose. Several previous studies have examined how various nuclear magnetic resonance (NMR) properties, such as the relaxation rates of water protons, change with dose, and magnetic resonance imaging (MRI) has been used successfully to measure three-dimensional dose distributions in irradiated polymer gels. Here we report our first observations of the manner in which the chemical shift of xenon gas (129Xe) dissolved in a gel changes with absorbed dose, and we introduce the potential use of high resolution xenon NMR spectra for understanding better the dose response of gels. 129Xe possesses a large chemical shift range and xenon spectra are sensitive to subtle changes in the physical and chemical environments in which the gas is dissolved. For doses ranging from 0 Gy to 40 Gy we found that the mean chemical shift of 129Xe was linearly related to dose, and that the gel dosimeter could be described in terms of a two-component model undergoing fast exchange. We found no evidence of radiation damage to the gelatin matrix at doses between 0 Gy and 40 Gy. At 40 Gy, the fast-exchange model begins to break down, and distinct gelatin and poly(methacrylate) resonances are observed at higher doses. High resolution NMR measurements of xenon provide a novel method for probing radiation dose effects in irradiated polymer gels.

The relationship of problems in biomedical MRI to the study of porous media.

The NMR methods that are used to characterize inanimate porous media measure relaxation times and related phenomena and material transport, fluid displacement and flow. Biological tissues are comprised of multiple small, fluid-filled compartments, such as cells, that restrict the movement of the bulk solvent water and whose constituents influence water proton relaxation times via numerous interactions with macromolecular surfaces. Several of the methods and concepts that have been developed in one field of application are also of great value in the other, and it may be expected that technical developments that have been spurred by biomedical applications of MR imaging will be used in the continuing study of porous media. Some recent specific studies from our laboratory include the development of multiple quantum coherence methods for studies of ordered water in anisotropic macromolecular assemblies, studies of the degree of restriction of water diffusion in cellular systems, multiple selective inversion imaging to depict the ratios of proton pool sizes and rates of magnetization transfer between proton populations, and diffusion tensor imaging to depict tissue anisotropies. These illustrate how approaches to obtain structural information from biological media are also relevant to porous media. For example, the recent development of oscillating gradient spin echo techniques (OGSE), an approach that extends our ability to resolve apparent diffusion changes over different time scales in tissues, has also been used to compute surface to volume measurements in assemblies of pores. Each of the new methods can be adapted to provide spatially resolved quantitative measurements of properties of interest, and these can be efficiently acquired with good accuracy using fast imaging methods such as echo planar imaging. The community of NMR scientists focused on applications to porous media should remain in close communication with those who use MRI to study problems in biomedicine, to their mutual benefits.