Detection of Radiation-Induced Changes in Healthy Mouse Brain Using Diffusion-weighted MRI and ¹⁸F-FDG-PET

Aim: To evaluate in vivo longitudinal PET and MRI parameter changes in images can be sensitively read out with radiation‐induced tissue changes in healthy mouse brain.
Materials and Methods: We irradiated a group of c57bl6 mice (n=6) with 5 Gy and another (n=6) with 20 Gy in the left hemisphere using an X‐ray tube (Yxlon Maxishot). Animals were imaged before, and 3‐7‐30 and 60 days post irradiation. For 18F‐Fluoro‐Deoxy‐Glucose (FDG) PET we injected 10 to 15 MBq FDG iv. PET and MR imaging was performed subsequently with a Mediso nanoScan PET/CT and a Bruker Biospec 7T MRI system. Arterial spin labeling (ASL) was probed using a FAIR‐EPI sequence. For DWI, a SE EPI‐based sequence was used. Standardized brain FDG‐PET uptake (SUV) values were determined for righ/left hemispheres and cerebellum using Rover software. ASL data and water apparent diffusion coefficients (ADC) were read out using a Matlab code after atlas coregistration. We determined statistical differences between readout results in both groups and between the time points in the same groups in these regions.
Results: There was no significant difference in ASL values neither in ADC values in the 5 Gy group compared to baseline or between time points. If both hemisphere’s respective VOI data were taken into account we could observe significant ADC differences between early (3 days) and late (30 to 60 days) changes in almost all VOIs of the brains. Using hemisphere VOI PET data we see a change at 7 days and 60 days both compared to baseline and all other time points in both groups by a decrease in SUVs of both hemispheres at Day 7 and and an increase at Day 60.
Conclusion: In our study ASL had no readout value on radiation‐induced changes. Using ADC maps, as early as 3 days and after one month post irradiation the late changes are visible throughout the brain. FDG‐PET provided us with a readable change at day 7. The direction of increased metabolism in the hemisphere 60 days read out with PET coregisters with the increase in ADC values at Day 60. These late changes are possibly due to second‐phase neuro‐inflammation and cell content increase in accordance with PET imaging results. Combined DWI MR/FDG PET is a promising means for radiation therapy side effect follow‐up.
This research reading to these results has received funding from the European Union Seventh Framework Programme FP7/2007‐2013 under grant agreement n° 305311/INSERT.