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 R2 * was calculated based on the measured signal decay. At the last time point, an ROI mask with an R2 * 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 R2 * 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 R2 * in the hippocampus did not show any significant difference; however, there are areas of visible R2 * change within sub-hippocampal regions.

CONCLUSION: R2 * is a promising method that could be able to predict an underlying disease process that occurs prior to radiation necrosis. The constant increase in R2 * values suggests a possible neuroinflammatory mechanism rather than an acute vascular event where R2 * would tend to decrease in the area.