CHARACTERIZATION AND EVALUATION OF A NIPAM POLYMER GEL MRI DOSIMETER SYSTEM

Sammanfattning: Purpose: The normoxic polymer gel dosimeter based on N-isopropyl acrylamide (NiPAM) is a promising full 3D-dosimeter with high spatial resolution and near tissue equivalency. However, limited work have been done in vestigating this dosimeter. The dose response and reproducibility of a NiPAM dosimeter was investigated. The dose rate dependence and the effect of sequential irradiation was studied. Furthermore, the homogene ity of the R2 values across the MRI field of view was examined. Theory: Polymer gel dosimeters are mostly composed of water and when exposed to ionizing radiation, radicals are formed in the process of radiolysis which in turn polymerize the gel via propagation reactions. The gels dose response is related to the degree of monomer polymerization. As the gel polymerize, the spin-lattice relaxation rate changes, making it possible to evaluate the 3D dose distribution within the gel with magnetic resonance imaging. Method: The chemicals for the gel were mixed under controlled conditions and was left to set over night. The NiPAM gel samples were irradiated to different doses using a TrueBeam™ linear accelerator. The absorbed dose was evaluated using a FSE-based MRI sequence and statistical significance of the analyzed data was calculated. The analysis of the DICOM images was carried out using an in house developed software for image processing and data handling. Result: The gel dosimeter was found to respond linearly to the absorbed dose. The reproducibility analysis of the NiPAM gel did not generate a conclu sive result and need to be investigated further. The gel exhibited a dose rate dependence, as well as a dependence on the sequential irradiation scheme. A higher dose rate as well as a higher per sequence dose resulted in a lower dose response. Homogeneity analysis of the calculated R2 val ues across the MRI field of view demonstrated a maximum difference of 4.3% between calculated R2-values.