Primary central nervous system (CNS) lymphomas represent 5% of primary brain tumors. More than 90% of them are diffuse large B-cell lymphomas. \[18F\]-Fluorodeoxyglucose positron emission tomography (PET-\[18F\]-FDG) is the gold standard for imaging systemic lymphomas, but its application in primary CNS lymphoma is compromised by the limited specificity of brain fixations and the high uptake of \[18F\]-FDG in healthy brain tissue. \[18F\]-Fludarabine is a new radiopharmaceutical developed for PET imaging of lymphomas. Preclinical studies indicate a restricted binding specificity to lymphoid tissue compared to \[18F\]-FDG and an ability to detect residual lymphoma disease after treatment. A pilot study in humans shows good agreement of its binding with tumor sites in systemic lymphoma and superior tumor contrast to \[18F\]-FDG. Finally, a recent preclinical study shows a binding ratio in brain lymphoma 3 times higher than that of healthy brain tissue in mouse models of primary CNS lymphoma, whereas in mouse models of high-grade glial tumors, the binding level is very low, comparable to that of healthy tissue (background). Investigators hypothesize that \[18F\]-Fludarabine could be the radiopharmaceutical of choice for the diagnosis and monitoring of primary CNS lymphomas in PET. The main objective of the study is to characterize the cerebral distribution and \[18F\]-Fludarabine uptake in newly-diagnosed primary CNS lymphomas before surgery, chemotherapy or radiotherapy, using PET-MR imaging.