On day 1, patients will receive a single intravenous injection of AGuIX. An MRI scan will be performed 2 hours after injection to visualize the distribution of AGuIX in brain metastases and surrounding healthy tissue, and to evaluate the contrast enhancement in brain metastases. Then patients will undergo a whole brain radiation therapy, starting 4 hours after AGuIX injection, up to completion of 2 weeks, 5 days a week of treatment (30Gy, 3Gy/fraction).