SU‐G‐TeP3‐12: Retrospective Assessment of R2star Using Ultra‐High Field MRI in a Rodent Model of Radiation Necrosis

Purpose: To establish a quantitative MRI method that would be capable of predicting radiation necrosis without using a contrast agent. Methods: Healthy male Fischer 344 rats were irradiated using an animal irradiator capable of delivering 2.3 Gy/min with a kVp of 225 V. A dose of 40 Gy was given to half the brain in a single session. Rats were scanned using a 9.4 T animal MRI before irradiation and every two weeks following radiation until either necrosis developed or they were sacrificed for health reasons. A multi‐echo gradient‐echo sequence was acquired at every time point and the apparent transverse relaxation rate R 2 * was calculated based on the measured signal decay. At the last time point, an ROI mask with an R 2 * value of greater than 45 s −1 was applied in the area of the external and internal capsule where radiation necrosis was confirmed. A retrospective analysis was performed to determine whether R 2 * values would be able to predict where radiation necrosis would occur. Results: Radiation necrosis was morphologically visible between weeks 22–24 following treatment. Gadolinium MRI and histology confirmed radiation necrosis in the area of MRI enhancement. Our data suggests that there is a trend towards significance in the lesion as early as 12 weeks prior to morphological changes on MRI with significance occurring 6 weeks prior (p≤0.05, p≤0.01 at week 24). Measurements of the R 2 * in the hippocampus did not show any significant difference; however, there are areas of visible R 2 * change within sub‐hippocampal regions. Conclusion: R 2 * is a promising method that could be able to predict an underlying disease process that occurs prior to radiation necrosis. The constant increase in R 2 * values suggests a possible neuroinflammatory mechanism rather than an acute vascular event where R 2 * would tend to decrease in the area.