This pilot clinical trial studies how well voxel based diffusion tensor imaging in predicting response in patients with brain metastases undergoing whole-brain radiation therapy or stereotactic radiosurgery. Voxel based diffusion tensor imaging (VB-DTI) may allow doctors to measure response to whole brain radiation therapy or stereotactic radiosurgery earlier than is possible with a standard magnetic resonance imaging. The earlier ability to measure response may allow for consideration of alternative therapies at an earlier stage.