• Histologically proven intracranial glioblastoma or gliosarcoma at initial surgery - Patients will be eligible if the original histology was low-grade glioma and a subsequent diagnosis of glioblastoma or gliosarcoma is made (high-grade transformation) - Karnofsky performance status >= 70 - Women must not be pregnant or breast-feeding - Progression of disease assessed by local site using Revised Assessment in Neuro-Oncology (RANO) criteria, with plan to give whole-dose bevacizumab therapeutically, either as single therapy or in conjunction with other chemotherapeutic regimens; patients getting bevacizumab to support additional radiation therapy or immunotherapy, or primarily for reduction of edema rather than for tumor treatment, are excluded; this must be the patient?s initial recurrence - Patient must not have been treated previously with immunotherapies (vaccines, checkpoint inhibitors, T-cells) - Intratumoral hemorrhage (acute, subacute, or chronic) as seen on hemosiderin-sensitive (gradient-echo) MRI may preclude patient inclusion because of anticipated limited evaluation due to magnetic susceptibility artifact on the heavily T2-weighted DSC-MRI images; if the region of enhancing tumor not affected by blooming artifact on the hemosiderin-sensitive images does not meet the 10 x 10 x 10 mm ?measurable enhancement? threshold specified elsewhere, the patient is ineligible - Progressive enhancement (> 25% increase in contrast enhancing volume compared to nadir) on MRI within 14 days of registration, >= 42 days since completion of radiation/temozolomide therapy, and >= 28 days since surgical resection or cytotoxic chemotherapy; measurable enhancement is defined as two perpendicular in-plane diameters of at least 10 mm and at least 10 mm in the 3rd orthogonal direction - Patients must be able to tolerate brain MRI scans with dynamic intravenous gadolinium-based contrast agent injections - Ability to withstand 22 gauge intravenous (IV) placement - No history of untreatable claustrophobia - No magnetic resonance (MR) incompatible implants/devices or metallic foreign bodies - No contraindication to intravenous contrast administration - Adequate organ function, including adequate renal function defined as estimated glomerular filtration rate (eGFR) >= 40 mL/min/1.73 m^2 as calculated per institution standard of care, and meeting local site requirements for intravenous administration of gadolinium-based MRI contrast agents - No known allergy-like reaction to gadolinium or moderate or severe allergic reactions to one or more allergens as defined by the American College of Radiology (ACR); patient may be eligible if willing to undergo pre-treatment as defined by the institution's policy and/or ACR guidance - Weight compatible with limits imposed by the MRI scanner table - Patient must be scheduled to receive treatment with a standard dose regimen of bevacizumab (bevacizumab infusion on days 1 and 15 of a 28-day treatment cycle); patient can be treated with bevacizumab alone or in combination with other chemotherapies

(see Inclusion Criteria)