Inclusion Criteria:
* Adult patients (greater than 18 years of age) with a known or suspected intracranial tumor.
* Able to provide informed written consent and/or acceptable surrogate capable of providing consent on the patient's behalf.
* Legally authorized representative (LAR)-signed informed consent and assent obtained for those subjects identified as decisionally impaired
* Intracranial lesion known or suspected to be neoplastic greater than 10 mL as assessed by T2/fluid attenuated inversion recovery (FLAIR) MR imaging.
* Karnofsky performance score \> 60 or Eastern Cooperative Oncology Group (ECOG) \< 3 as assessed by referring clinician.
* Planning to undergo or previously received therapeutic intervention for the intracranial tumor.
Exclusion Criteria:
* Pregnant or breast feeding.
* Contraindication to PET, MRI, FMISO, or intravenous gadolinium based contrast agents.
* Claustrophobia.
* Weight greater than modality maximum capacity.
* Presence of metallic foreign body or implanted medical devices in body not documented as MRI safe according to the Oregon Health \& Science University (OHSU) Department of Radiology guidelines (including but not limited to cardiac pacemaker, aneurysm clips, surgical clips, prostheses, artificial hearts, valves with steel parts, metal fragments, shrapnel, tattoos near the eye, or steel implants).
* Sickle cell disease.
* Reduced renal function, as determined by glomerular filtration rate (GFR) \< 45 mL/min/1.73 m\^2 based on a serum creatinine level obtained per OHSU Department of Radiology and Advanced Imaging Research Center (AIRC) clinical criteria.
* History of allergic reactions attributed to compounds of similar chemical or biologic composition to FMISO. An allergic reaction to nitroimidazoles is highly unlikely.
* Unsure of pregnancy status as assessed by Department of Radiology and AIRC guidelines.
* Subjects for whom supplemental oxygen could be harmful such as people with potential for hypoventilation (end-stage COPD, OSA on CPAP/Bi-PAP, etc).
* Subjects with a relative contraindication to supplemental oxygen administration will not be provided oxygen but may still participate in the study.
* Presence of any other co-existing condition that, in the judgment of the principal investigator, might increase the risk to the subject (i.e., plans for hospice or end of life care).
* Poor peripheral intravenous access evaluated by patient history.
* Presence of other serious systemic illnesses, including: uncontrolled infection, other uncontrolled malignancy, uncontrolled diabetes type II, or psychiatric/social situations which might impact the endpoint of the study or limit compliance with study requirements.
