This study was approved by Institutional Review Boards at Johns Hopkins and the National Institutes of Health, and all patients provided informed consent. Eligible patients were ≥ 18 years old with a diagnosis of recurrent high grade glioma suspected by MRI findings. All patients had a clinically indicated need for surgical intervention. Patients were required to have: Karnofsky performance status (KPS) of ≥ 60%; normal liver and kidney function; absolute neutrophil count ≥ 1500 cells/mm3; and a platelet count ≥ 100,000 cells/mm3. Patients were excluded if they were currently receiving chemotherapy or radiation therapy, allergic to temozolomide, pregnant or breast-feeding, had a serious medical or psychiatric illness or social situation that could interfere with catheter placement/monitoring. Patients with a prior use of VEGF or VEGFR-targeted therapy, use of investigational agents within the past 4 weeks, NCI CTC grade 3 or greater baseline neurologic symptoms, history of cardiac, bronchospastic lung disease, or a contraindication to adenosine were all excluded from study participation. Additionally, patients were asked to refrain for caffeine use at least 24 h prior to regadenoson administration, secondary to its ability to blunt the effect of regadenoson.