DOSE-RESPONSE CHARACTERISTICS OF A FERROUS-SULFATE-DOPED GELATIN SYSTEM FOR DETERMINING RADIATION ABSORBED DOSE DISTRIBUTIONS BY MAGNETIC-RESONANCE-IMAGING (FE MRI)

The nuclear magnetic resonance (NMR) longitudinal relaxation rate R1 dose-response characteristics of a ferrous-sulphate-doped chemical dosimeter system (Fe MRI) immobilized in a gelatin matrix were explored. Samples containing various concentrations of the FeSO4 dosimeter were irradiated to absorbed doses of 0-150 Gy. R1 relaxation rates were determined by imaging the samples at a field strength of 1.5T(H-1 Lamor frequency of 63.8 MHz). The response of the system was found to be approximately linear up to doses of 50 Gy for all FeSO4 concentrations studied (0.1-2.0 mM). Changing concentrations in the range of 0.1-0.5 mM affected both the slope and intercept of the dose-response curve. For concentrations of 0.5-2.0 mM, the slope of the dose-response curves remained constant at approximately 0.0423 s-1 Gy-1 in the dose range of 0-50 Gy. However, the intercept of the curve continued to increase in that region, as expected, because of the additional paramagnetic ions. The reproducibility of the absorbed dose estimates for measurements made over a 22 cm field of view was found to be 5% in the range of 20-50 Gy (an uncertainty of 0.81 Gy on average), decreasing to approximately 10% in the dose range of 5-10 Gy.