Studies of the design, use, and characteristics of methacrylic acid-based polymer gel dosimeters
Whitney, Heather Marie
Polymer gel dosimeters are three-dimensional radiation-sensitive materials comprised of monomers and other chemicals distributed in an aqueous gelatin matrix. Upon irradiation by high energy X or gamma rays, free radicals formed within the water initiate polymerization of the monomers, resulting in distributions of polymer that reflect the distribution of radiation dose. The polymers in turn affect the local nuclear magnetic resonance (NMR) properties of water, so that complex, integrated radiation dose distributions can be measured with high spatial resolution using magnetic resonance imaging. Previous studies have demonstrated the use of polymer gels for applications in dosimetry for clinical radiation therapy. However, there are several aspects of polymer gel dosimetry that remain unresolved. In this thesis some of these problems are addressed. In particular, the design and composition of gels for optimal dose response, the characterization of their dose responses for different NMR properties, the development of improved imaging methods, and the underlying mechanisms of dose response, are each considered. Methacrylic acid-based dosimeters have been optimized for measurements of dose based on transverse relaxation rates. In addition, measurements of other NMR parameters, such as the rates that govern magnetization transfer, are made, are considered and a new magnetization transfer parameter, the magnetization transfer proportion, is introduced as a simplified measure of dose response that is less susceptible to imaging errors than more traditional measures. A simple model is introduced to explain the dose response in terms of an increase in the number of efficiently relaxing protons through a chemical exchange relaxation mechanism, and the parameters of this model are derived from a series of appropriate NMR experiments.