Dose distribution measurements in superficial x-ray beams using NMR dosimetry

The conversion of Fe2+ to Fe3+ in a Fricke solution due to ionizing radiation can be detected using nuclear magnetic resonance (NMR) imaging. The aim of the present study was to develop a suitable system for the study of dose distributions in superficial radiation beams making use of the water equivalence of the dosimetric gel system. Agarose gels (1.5%) doped with 0.5 mM ammonium ferrous sulphate and 125 mM sulphuric acid were exposed to x-rays from a superficial radiotherapy treatment unit (HVT 1.4, 2.4 and 7.25 mm A1). In the experiments doses between 10 Gy and 20 Gy were given, and the gel surface was in direct contact with the lead glass cone applicators (diameters 1 cm and 2.5 cm) at an FSD of 10 cm. Images were obtained within 4 h after irradiation in the head coil of a 1.5 T clinical MR scanner. Using spin-echo sequences with seven different repetition times between 120 ms and 4 s, the spin-lattice relaxation time (T1) was calculated for specified regions of interest with about 1 mm spacing. The inverse of T1 was shown to be proportional to the given dose and 1/T1 maps were obtained for all three superficial radiation qualities. The depth-dose curves determined with NMR dosimetry compare well with those obtained with a thin-window parallel plate ion chamber and thermoluminescence dosimetry in the same radiation beams.