18F-FluorThanatrace (PET/CT) in Glioblastoma

Participants will undergo up to three \[18F\]FTT PET/CT scans at the following time points: * Baseline: Subjects from either cohort may agree to undergo a baseline \[18F\]FTT PET/CT prior to the start of TTFields therapy to measure baseline FTT uptake. This scan is not required and may be omitted at the discretion of the investigator or based on subject availability. * Post-TTFields: Subjects from both cohorts will undergo an \[18F\]FTT PET/CT scan 3-7 days after initiation of TTFields therapy and before surgical resection or initiation of niraparib. This scan will measure the FTT uptake after TTFields therapy is initiated. This scan is required. * Post-PARPi: Subjects from Cohort A may agree to undergo an post-therapy \[18F\]FTT PET/CT scan 1- 21 days after the initiation of niraparib to collect pilot data on the changes in FTT uptake after TTFields and niraparib combination therapy. This scan is not required and may be omitted at the discretion of the investigator or based on subject availability. For each \[18F\]FTT PET/CT imaging session, participants will undergo approximately 60 minutes of dynamic brain scanning starting nearly simultaneously with the injection of \[18F\]FTT. A second optional brain scan will be done at approximately 90 minutes post-injection. Each imaging session will include an injection of up to 12 mCi (approximate range for most studies is anticipated to be 8-12 mCi) of \[18F\]FTT.