Dosimetric Characterization Using Quantitative Magnetic Resonance Imaging of PAGAT and Gd-PAGAT Polymer Gel Dosimeters at High Gd Concentration

Gadolinium-doped polymer gel dosimeters enable experimental evaluation of dose enhancement in low-energy radiotherapy, although the high Gd concentrations required substantially alter their magnetic relaxation properties, making the optimal Magnetic Resonance Imaging (MRI) readout uncertain. This study characterized the dosimetric response of PAGAT and Gd-PAGAT (9 mg/mL Gd) irradiated with a 150 kVp X-ray beam from 1 to 10 Gy using quantitative 1.5 T MRI. Quantitative R1 and R2 maps were obtained from variable flip-angle spoiled gradient echo (SPGR) and multi-echo spin echo (MSE) sequences, respectively, and validated by UV-Vis spectrophotometry. R1 showed no significant dose dependence in Gd-PAGAT because of severe T1 shortening. In contrast, R2 exhibited a robust response, with second-order polynomial fits (R2 = 0.9984 for PAGAT and 0.9969 for Gd-PAGAT) and linear behavior between 1 and 7 Gy. The dose sensitivity of Gd-PAGAT was 2.84 ± 0.19 times higher than that of PAGAT (0.5328 vs. 0.1949 s−1·Gy−1). Spectrophotometry confirmed the PAGAT calibration but showed markedly lower sensitivity for Gd-PAGAT, particularly at low doses. These findings demonstrate that R2 mapping overcomes the limitations imposed by extreme T1 shortening, extending the applicability of Gd-PAGAT dosimeters for three-dimensional dose enhancement studies in low-energy radiotherapy.