The correction of time and temperature effects in MR-based 3D Fricke xylenol orange dosimetry

Abstract

Previously developed MR-based three-dimensional (3D) Fricke-xylenol orange (FXG) dosimeters can provide end-to-end quality assurance and validation protocols for pre-clinical radiation platforms. FXG dosimeters quantify ionizing irradiation induced oxidation of Fe ions using pre- and post-irradiation MR imaging methods that detect changes in spin-lattice relaxation rates (R = [Formula: see text]) caused by irradiation induced oxidation of Fe. Chemical changes in MR-based FXG dosimeters that occur over time and with changes in temperature can decrease dosimetric accuracy if they are not properly characterized and corrected. This paper describes the characterization, development and utilization of an empirical model-based correction algorithm for time and temperature effects in the context of a pre-clinical irradiator and a 7 T pre-clinical MR imaging system. Time and temperature dependent changes of R values were characterized using variable TR spin-echo imaging. R-time and R -temperature dependencies were fit using non-linear least squares fitting methods. Models were validated using leave-one-out cross-validation and resampling. Subsequently, a correction algorithm was developed that employed the previously fit empirical models to predict and reduce baseline R shifts that occurred in the presence of time and temperature changes. The correction algorithm was tested on R -dose response curves and 3D dose distributions delivered using a small animal irradiator at 225 kVp. The correction algorithm reduced baseline R shifts from -2.8 × 10 s to 1.5 × 10 s. In terms of absolute dosimetric performance as assessed with traceable standards, the correction algorithm reduced dose discrepancies from approximately 3% to approximately 0.5% (2.90 ± 2.08% to 0.20 ± 0.07%, and 2.68 ± 1.84% to 0.46 ± 0.37% for the 10 × 10 and 8 × 12 mm fields, respectively). Chemical changes in MR-based FXG dosimeters produce time and temperature dependent R values for the time intervals and temperature changes found in a typical small animal imaging and irradiation laboratory setting. These changes cause baseline R shifts that negatively affect dosimeter accuracy. Characterization, modeling and correction of these effects improved in-field reported dose accuracy to less than 1% when compared to standardized ion chamber measurements.

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

“…The linearity coefficient of the calibration curve obtained for all TRs and all concentrations was above 0.9. In a study by Welch et al [44], the direct dependence of the dose and the changes in the R1 relaxation rate were obtained with a linearity of 0.99, which confirms the results of the present study. The linearity coefficient of the calibration curve had no significant relationship with the amount of TR or the slice number.…”

Does the gelation temperature or the sulfuric acid concentration influence the dosimetric properties of radiochromic PVA-GTA Xylenol Orange Fricke gels?

“…with volume of one liter or more). Indeed, for such volumes the cooling rate at the edge of the dosimeter might be much faster than at its center, causing a non-uniform performance across the volume (De Deene et al, 2007;Welch et al, 2017). PVA-GTA matrix has the advantage of not requiring temperature gradients for gelation.…”

Temperature behavior of radiochromic poly(vinyl-alcohol)-glutaraldehyde Fricke gel dosimeters in practice

“…Generally, any change occurring in a chemical dosimeter as effect of changes in temperature, can modify the dosimetric properties [16]. We recently demonstrated that, in case of PVA-GTA-FG dosimeters, changes in A c c e p t e d M a n u s c r i p t 4 gelation temperature during the manufactory process does not influence either the dose response curve, or the Fe 3+ diffusion rate within the gel matrix [39].…”

“…The recording of three-dimensional dose distribution is intrinsic in gel dosimetry [13]. Gel dosimeters are soft-tissue equivalent chemical dosimeters [14,15] that undergo localized chemical changes upon the absorption of ionizing radiation [16]. Fricke gel dosimeters are a type of gel dosimeter based on an aqueous ferrous (Fe 2+ ) sulphate solution dissolved in a gel matrix [17].…”