\>= 18 years of age

High grade brain glioma (Grade 3 and 4)

No prior radiation or systemic treatment for high grade glioma

Planned to undergo radiation therapy with the following regimens:

* Grade 3: 54-60 Gy over 30 fractions with temozolomide

* Grade 3: 40Gy over 15 fractions with temozolomide

* Grade 4: 60 Gy over 30 fractions, with or without temozolomide o Grade 4: 40 Gy over 15 fractions, with or without temozolomide

Able to tolerate PET/MRI scan with intravenous contrast

Willing to provide informed consent.

18F-FET-PET/MRI vs Standard MRI Alone for Stereotactic RadioTherapy Planning for High Grade Brain Gliomas