18F-Fluciclovine PET-MRI in High-grade Glioma

Inclusion Criteria * 1\. Histopathology-proven HGG (WHO grade III-IV) or DMG (WHO grade IV) or, in the case of DMG of the pons, imaging that is characteristic of Diffuse intrinsic pontine gliomas (DIPG) (diffusely infiltrating \>=2/3 of the pons). * 2\. Measurable disease, measuring at least 1x1 cm. * 3\. Life expectancy of greater than 8 weeks. * 4\. Age \> 1 years but \< 21 years of age at enrollment. For those without planned surgery: * 1\. Participants with clinical and/or radiographic suspicion of True progression (TP) or Pseudoprogression (PsP) during radiation but yet to have the initial post-radiation MRI scan. or * 2\. Participants with suspicion for TP or PsP on first post-radiation MRI For those with planned surgery: * 1\. Clinical or radiographic suspicion of tumor progression with plan to undergo surgery or biopsy. Exclusion Criteria: * 1\. Inability to tolerate imaging procedures in the opinion of an investigator or treating physician. * 2\. Pregnant or breastfeeding participants. * 3\. Participant who would require sedation or anesthesia for imaging beyond standard of care (SOC). * 4\. Participants who weigh less than 8 kg. * 5\. Participants who cannot avoid contact with a pregnant woman or infant for at least 12 hours following injection. * 6\. Participants with a history of abnormal kidney function or creatinine \>= CTCAE v5.0 grade 2 at time of study registration. 7\. Participants with primary tumors of the spinal cord.