For this study, your standard of care daily CT scans will be used to help researchers adjust participants' radiation therapy, if necessary, on a weekly basis. In particular, the changes in normal tissue that are being studied are any changes in the salivary glands. An additional research related MRI scan will also be performed in this study, but the MRI scan will not be used to adjust participants' therapy. Researchers want to learn if MRI scanning may be able to help doctors in the future to adjust someone's treatment based on changes in a tumor (its features and size) in response to radiation.