Investigating the performance of susceptibility-weighted images in Fricke gel dosimeter reading and optimizing the related imaging parameters

Abstract

Fricke gel dosimeters are especially useful in small-field dosimetry and validation of treatment delivery in
three-dimensional space with features such as tissue equivalence, non-toxicity, high spatial resolution, nondependence
on energy, and dose rate. The use of basic MRI imaging protocols (T1W and T2W) for reading
Fricke gel dosimeters has always been considered the dominant method in many studies. However, the
development and application of advanced MRI techniques for more accurate readings of Fricke gel
dosimeters can be useful. Considering that in the main structure of Fricke gel, there are conversions of iron
ions to each other, this study aimed to investigate the performance of susceptibility-based images (SWI and
QSM) based on magnetic susceptibility in the reading of Fricke gel dosimeters and to optimize the related
imaging parameters. For this purpose, a Fricke-Xylenol orange-gelatin was made at five concentrations of
iron ammonium sulfate. To obtain gel dosimeter calibration curves, vials containing gel were subjected to
irradiation at three different doses by a linear accelerator. The reading of gel dosimeters was performed
using MRI imaging in three protocols, T1W, T2W, and SWI, and analyzed with a method unique to each
one. Finally, the results obtained from the three protocols were compared with each other. The comparison
of calibration curves in three imaging protocols shows that the sensitivity of calibration curves in SWI was
about three times its value in T2W, and on the other hand, the reported sensitivity in T1W was very small
compared to the other two protocols. The linearity factor was similar between SWI and T1W protocols and
higher in T2W. Therefore, it is concluded that in addition to the relaxometry techniques that have been used
as a conventional method for reading Fricke gel dosimeter, SWI imaging has high sensitivity and specificity
for reading dosimeter gel based on iron.