Background Paramagnetic species such as O2 and free radicals can enhance T1 and T2 relaxation times. If the change in relaxation time is sufficiently large, the contrast will be generated in magnetic resonance images. Since radiation is known to be capable of altering the concentration of O2 and free radicals during water radiolysis, it may be possible for radiation to induce MR signal change. Purpose We present the first reported instance of x‐ray‐induced MR signal changes in water phantoms and investigate potential paramagnetic relaxation enhancement mechanisms associated with radiation chemistry changes in oxygen and free radical concentrations. Methods Images of water and 10 mM coumarin phantoms were acquired on a 0.35 T MR‐linac before, during, and after a dose delivery of 80 Gy using an inversion‐recovery dual‐echo sequence with water nullified. Radiation chemistry simulations of these conditions were performed to calculate changes in oxygen and free radical concentrations. Published relaxivity values were then applied to calculate the resulting T1 change, and analytical MR signal equations were used to calculate the associated signal change. Results Compared to pre‐irradiation reference images, water phantom images taken during and after irradiation showed little to no change, while coumarin phantom images showed a small signal loss in the irradiated region with a contrast‐to‐noise ratio (CNR) of 1.0–2.5. Radiation chemistry simulations found oxygen depletion of −11 µM in water and −31 µM in coumarin, resulting in a T1 lengthening of 24 ms and 68 ms respectively, and a simulated CNR of 1.0 and 2.8 respectively. This change was consistent with observations in both direction and magnitude. Steady‐state superoxide, hydroxyl, hydroperoxyl, and hydrogen radical concentrations were found to contribute less than 1 ms of T1 change. Conclusion Observed radiation‐induced MR signal changes were dominated by an oxygen depletion mechanism. Free radicals were concluded to play a minor secondary role under steady‐state conditions. Future applications may include in vivo FLASH treatment verification but would require an MR sequence with a better signal‐to‐noise ratio and higher temporal resolution than the one used in this study.

PurposeCBCT‐guided online adaptive radiotherapy (oART) plans presently utilize daily synthetic CTs (sCT) that are automatically generated using deformable registration algorithms. These algorithms may have poor performance at reproducing variable volumes of gas present during treatment. Therefore, we have analyzed the air mapping error between the daily CBCTs and the corresponding sCT and explored its dosimetric effect on oART plan calculation.MethodsAbdominopelvic air volume was contoured on both the daily CBCT images and the corresponding synthetic images for 207 online adaptive pelvic treatments. Air mapping errors were tracked over all fractions. For two case studies representing worst case scenarios, dosimetric effects of air mapping errors were corrected in the sCT images using the daily CBCT air contours, then recalculating dose. Dose volume histogram statistics and 3D gamma passing rates were used to compare the original and air‐corrected sCT‐based dose calculations.ResultsAll analyzed patients showed observable air pocket contour differences between the sCT and the CBCT images. The largest air volume difference observed in daily CBCT images for a given patient was 276.3 cc, a difference of more than 386% compared to the sCT. For the two case studies, the largest observed change in DVH metrics was a 2.6% reduction in minimum PTV dose, with all other metrics varying by less than 1.5%. 3D gamma passing rates using 1%/1 mm criteria were above 90% when comparing the uncorrected and corrected dose distributions.ConclusionCurrent CBCT‐based oART workflow can lead to inaccuracies in the mapping of abdominopelvic air pockets from daily CBCT to the sCT images used for the optimization and calculation of the adaptive plan. Despite the large observed mapping errors, the dosimetric effects of such differences on the accuracy of the adapted plan dose calculation are unlikely to cause differences greater than 3% for prostate treatments.