Inclusion Criteria:

• * Two adult patient groups will be eligible for inclusion in this study:
• Group A: Patients where there is compelling evidence, based on the MRI and/or CT imaging, that a high-grade primary brain tumor is present. Pathologic confirmation will occur with biopsy or surgery.
• Patients whose tumor is felt to be inoperable and a biopsy is performed but no surgery.
• Patients with a newly diagnosed primary malignant brain tumor (WHO Grade III or IV) who will be receiving chemoradiation and who either did not undergo surgical resection or underwent incomplete resection with residual tumor \> 1.0 cm in greatest diameter by contrast MRI postoperatively.
• Group B: Patients with pathologically proven malignant brain tumor (WHO Grade III or IV glial-based tumors) who have undergone chemoradiation and have MRI-documented possible recurrence/progression versus treatment effect (pseudoprogression) within 6 months from the time of completion of chemoradiation.
• * Patients must be 18 years or older for inclusion in this study. There is little experience with the safety of \[18F\]FLT or \[18F\]Fluciclovine in children, and the risks associated with radiation exposure may be increased for children under 18 years old as well.
• * Patients must document their willingness to be followed for at least 24 months after recruitment by signing informed consent documenting their agreement to have clinical endpoints and the results of histopathologic tissue analysis (when tissue becomes available from routine care) entered into a research database.
• * All patients, or their legal guardians, must sign a written informed consent and HIPAA authorization in accordance with institutional guidelines.
• * Determination of pregnancy status: Female patients who are not postmenopausal or surgically sterile will undergo a serum pregnancy test prior to each set of \[18F\]FLT and \[18F\]Fluciclovine PET scans. A negative test will be necessary for such patients to undergo research PET imaging.
• * Pre-imaging laboratory tests must be performed within 28 days prior to the \[18F\]FLT imaging procedure. These must be less than 4.0 times below or above the upper or lower limit range for the respective laboratory test for entry into the study (unless clinically not relevant). In those instances where a laboratory value is outside of this range, then such a patient will be ineligible for enrollment unless at the discretion of the PI the abnormal lab value is of no clinical significance to the safety or integrity of the study results. . For follow-up scanning sessions after therapy has been instituted, laboratory testing will also be required due to the use of \[18F\]FLT. The patients have brain tumors and will have received chemoradiation; therefore, many routine laboratory tests may not be within the typical normal range. As such, a factor of 4.0 times above or below the upper or lower value for the normal range for any laboratory test will also be used to determine the acceptable range for the 2nd and possibly 3rd imaging time points (unless clinically not relevant as explained above). The baseline laboratory testing will include liver enzymes (ALT, AST), bilirubin (total), serum electrolytes, CBC with platelets, prothrombin time, partial thromboplastin time, BUN, and creatinine. For those patients receiving coumadin or another anticoagulant the upper limit for prothrombin time or partial thromboplastin time must not exceed 6 times the upper limit of the normal range. (Appendix E, Laboratory Study Results).

Exclusion Criteria:

• * Patients with clinically significant signs of uncal herniation, such as acute pupillary enlargement, rapidly developing motor changes (over hours), or rapidly decreasing level of consciousness, are not eligible.
• * Patients with known allergic or hypersensitivity reactions to previously administered radiopharmaceuticals. Patients with significant drug or other allergies or autoimmune diseases may be enrolled at the Investigator's discretion.
• * Patients who are pregnant or lactating or who suspect they might be pregnant. Serum pregnancy tests will be obtained prior to each set of multi-tracer PET scans in female patients that are not postmenopausal or surgically sterile.
• * Adult patients who require monitored anesthesia for PET scanning.
• * Patients who are too claustrophobic to undergo PET scanning.
• * Known HIV positive patients due to the previous toxicity noted with \[18F\]FLT in this patient group.