• Inclusion Criteria for Healthy Participants:
• * No history of cerebrovascular disease.
• * No cognitive impairments.
• * Able to provide informed consent.
• Inclusion Criteria for Participants with Brain Tumors:
• * Biopsy-proven cases of developed recurrent tumor or radiation necrosis,
• OR:
• * a. PET identified with developed recurrent tumor or radiation necrosis. OR
• * b. Highly suspicious case with developed recurrent tumor or radiation necrosis confirmed by tumor board, attending physician or surgeon.
• * ECOG performance status 0-2.
• * Life expectancy \> 6 months.
• * Participant with other sites of extracranial metastatic disease or any prior or current systemic therapies will be considered and evaluated by the investigators of the study on a subject basis.

Exclusion Criteria:

• * Pregnant women OR lactating women
• * Participants with ferromagnetic or otherwise non-MRI compatible aneurysm clips.
• * Participants who cannot go into the MRI scanner due to metal implants and other medical conditions.
• * The presence of an implanted medical device that is not MRI-compatible, including, but not limited to: pacemaker, defibrillator.
• * Participants with contraindications for MRI due to embedded foreign metallic objects such as bullets, shrapnel, metalwork fragments, or other metallic material.
• * Known history of severe claustrophobia.
• * Participants unable to lay still in the scanner for 30 minutes at a time.