MR images of the skull-shaped Fricke gel phantom (a) axial image before irradiation, (b) axial image at the same position after irradiation, (c) difference image obtained by subtracting (a) from (b). Note, the six fiducial marks [three on each side of (a) and (b)] disappeared after image subtraction. This is an important indication to make sure a good alignment was upheld when the phantom was placed back for the second MR scanning after it was removed for irradiation.
The relationship of MR image intensity change (ΔS) versus the absorbed dose (D). The regression correlation coefficient (R2) is 0.9946, exhibiting a strong correlation between the regression curve and the data points.
Dose-response curves for the different choices of TR values. TR = 600 ms gave the best initial sensitivity (19.9 Gy−1) and the highest overall latitude (448 in arbitrary units). It is noted that better linearity occurred for TR = 320 ms. The error bars represent one standard deviation at the marked doses.
Dose-response curves obtained with the two different choices of effective TE (eTE). Both the initial sensitivity (22.6 Gy−1) and the overall latitude (518 in arbitrary units) for eTE = 8.2 ms are better than eTE = 16.33 ms case.
Dose-response curves for the different choices of ETL (N) values. The initial sensitivities for ETL = 8 and 16 appeared similar and are slightly better than ETL = 32 case.
The 90%, 70%, 50%, and 30% isodose contours. Two separate gel dosimetry experiments were performed. We used the dose-response curve calculated from the first experiment to calculate the dose distributions in the second experiment. The solid curves are calculated by applying the dose-response regression curve for TR/eTE/ETL = 600 ms/8.2 ms/16. The dashed curves are computed from the GammaPlan treatment planning system. Good matches were observed for the 70%, 50%, and 30% paired contours.
Dose-response curves computed from different imaging planes (z = 98, 99, 100, 101 mm). No apparent difference is observed between them.
Gamma index maps for imaging planes (a) z = 98, (b) z = 99, (c) z = 100, and (d) z = 101 mm for 3%/3 mm criteria between the experimentally measured and GammaPlan computed dose distributions. The largest gamma value was 1.2 located at (x, y, z = 98, 93, 101 mm). The center of these gamma maps correspond to the center of the MR image in Fig. 1 . For easy demonstration of the points that failed the gamma criteria, we have interpolated the dose distribution matrix size from 31 × 31 to 93 × 93.
“Two-point method” was used to measure the T2 of our dosimeter gel. Total 1/T2 change was 2.91 s−1, from 12.67 s−1 before irradiation to 15.58 s−1 after 40 Gy irradiation.
Schematic diagrams of the fast spin-echo sequence.
3D-FSE imaging parameters in three experiments.
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