Initial investigation on the use of MR spectroscopy and micro-MRI of GAFCHROMIC® EBT radiotherapy film

Purpose: This article presents an initial investigation of the efficacy of using H 1 MRS and micro-MRI as analysis techniques for irradiated GAFCHROMIC EBT® radiotherapy films. Methods: GAFCHROMIC® EBT radiotherapy film was irradiated with 6 MV x rays to known doses ranging from 5 to 1000 cGy. 24 h following irradiation H 1 MRS measurements were performed to access the degree of post-irradiation polymer cross-linking. 2D H 1 micro-MRI experiments were also performed for film irradiations of 0 and 300 cGy. Results: Linear response of the H 1 MRS linewidth to dose in the range from 0 to 400 cGy ( R 2 = 0.98 ) was observed. Such linearity is not seen when analyzed under conventional light analysis. The sensitivity of the film, as measured by the slope of the curve between 0 and 400 cGy, is 0.0042 ± 0.0003 kHz / cGy , demonstrating the sensitivity of the H 1 MRS technique used to analyze the film. The film saturates at a dose of approximately 900 cGy. Broadline H 1 MRS provides a quantitative measure of the degree of polymerization of the film. Conclusions: A quantitative measurement of the degree of polymerization of GAFCHROMIC® EBT film has been presented using H 1 MRS. The saturation of the film at approximately 900 cGy is corroborated by that observed with light analysis. Further MR spectroscopic experiments are needed to investigate the response of the film to dose, allowing for a better understanding of the relationship between polymer cross-linking in the active layer.