Glioblastoma is the most common primary malignant neoplasm of the adult brain. Even after multimodal therapy, outcomes remain poor, with a median survival of one year. Although advanced imaging methods have been suggested as molecular markers of prognosis and therapeutic response, these methods have not been validated for clinical use. In this exploratory, imaging-based, trial, thirty patients with a pathological diagnosis of glioblastoma will be followed prospectively for two years. The study examines how PET and MR imaging signals change following administration of a standard radio-chemotherapy treatment regimen to determine whether these imaging modalities can provide early indicators of response to therapeutic intervention. The investigators hypothesize that decreases in uptake of an investigational 18F-FLT PET tracer following treatment with radiation and chemotherapy will be a reliable predictor of glioblastoma response. In a more exploratory fashion, the investigators also will identify changes in diffusion and hypoxia MR imaging that may also correlate well with treatment response.