Optimised MR imaging for polyacrylamide gel dosimetry

Polyacrylamide gels are a powerful tool to measure radiation dose by quantifying the NMR T 2 relaxation times of the irradiated gel. The exploitation of these radiation sensitive gels in clinical radiotherapy requires accurate mapping of T 2 values. This paper describes the optimization strategy used to identify accurate and practical methods of measuring the range of T 2 values typical of gel dosimeters (140-700 ms). The MR imaging techniques used to measure T 2 values and the choice of image acquisition parameters are described. Four sequences are compared and the results are analysed in terms of accuracy, signal-to-noise ratio and acquisition time. A multiple spin echo sequence was found to yield the most accurate results (98.9%). Single spin echo sequences, such as Hahn spin echo and EPI spin echo, were found to measure gel T 2 values with an accuracy of 90.1%. This paper reports the importance of careful selection and optimization of the MR imaging sequences for accurate and reliable polyacrylamide gel dosimetry.