Developing a New MRI Technique to Understand Changes in Brain Tumors After Treatment

This study is developing a new MRI technique that may better show how brain tumors (and brain metastases) are responding to treatment, potentially providing earlier and more accurate information than standard MRI scans. **You may be eligible if (healthy volunteers):** - You are 18 or older - You do not have a known cancer diagnosis **You may also be eligible if (cancer patients):** - You are 18 or older - You have at least one untreated brain metastasis larger than 1 cm - Some participants need to be scheduled for stereotactic radiosurgery (SRS) for the brain metastasis **You may NOT be eligible if:** - You are pregnant or breastfeeding - You have a pre-existing condition that increases the risk of seizures, claustrophobia, or heart problems during the scan - You are under 18 Talk to your doctor to see if this trial is right for you.