This first-in-human study will establish the human safety and radiation dosimetry of the system A amino acid transport substrate, (R)-3-[F-18]fluoro-2-methyl-2-(methylamino)propanoic acid ([F-18]MeFAMP), for positron emission tomography (PET) imaging of primary and metastatic brain tumors. This study will include 3 cohorts: healthy volunteers for whole body dosimetry estimates (n=6-8, Dosimetry Cohort), patients undergoing evaluation for recurrent high grade glioma after radiation therapy (n=10, high grade glioma (HGG) Cohort), and patients with brain metastases from extra-cranial solid tumors before and after radiation therapy (n=10, Metastasis Cohort). Exploratory assessment of the diagnostic accuracy of MeFAMP for distinguishing recurrent/progressive brain tumors from radiation-related treatment effects will also be performed for subsequent trial design. The study will complete accrual and safety assessment in the Dosimetry Cohort before recruiting for the HGG and Metastasis Cohorts.

Inclusion Criteria for all cohorts:

1. 18 years of age or older at the time of enrollment
2. Females with childbearing potential must have a negative urine human chorionic gonadotropin (hCG) test on the day of procedure or a serum hCG test within 48 hours prior to the administration MeFAMP.
3. Must have a life expectancy greater than 12 weeks.

Exclusion Criteria for all cohorts:

1. Use of an investigational drug for any indication within 3 months prior to the imaging study.
2. Pregnancy or breast feeding
3. Inability to complete the PET scans.
4. Significant renal or hepatic dysfunction (estimated glomerular filtration rate (GFR) < 60 mL/min)
5. Any condition which may interfere with ability to participate in or complete all study-related activities as assessed by the study team.

6.4.9.3. Inclusion criteria specific to Dosimetry Cohort

1. Normal complete metabolic profile (CMP) and cell blood count (CBC) with differential at baseline.
2. Normal ECG at baseline.

Exclusion criteria specific to Dosimetry Cohort

1) Major medical problems (e.g. renal, hepatic, inflammatory) that could interfere with biodistribution of MeFAMP as assessed by the study team.

Inclusion Criteria specific to HGG Cohort

1. Grade III or Grade IV glioma previously treated with radiation therapy
2. Standard of care contrast-enhanced MRI showing an enhancing lesion at least 1-cm in maximum dimension that is equivocal or suspicious for recurrent glioma.
3. Eastern Cooperative Oncology Group (ECOG) performance score of 2 or better

Inclusion Criteria specific to Metastasis Cohort

1. At least one brain metastasis from melanoma, lung cancer (small or non-small cell), or breast cancer measuring at least 1-cm in maximum dimension on contrast-enhanced MRI
2. Plan for stereotactic radiation therapy within 2 weeks of initial MeFAMP-PET/MRI scan.
3. ECOG performance score of 2 or better

Inclusion of Women and Minorities

Patients 18 years of age or older will be eligible for study participation. No other discriminatory factors, including age, sex, or ethnic background will be used to determine eligibility. Every effort will be made to ensure that minorities are recruited for study participation.