The tests under study for diagnosing PCNSL or toxoplasmic encephalitis are measurement of Epstein Barr virus (EBV) DNA in cerebrospinal fluid (CSF) and FDG-PET scan of the brain. EBV is often found in the CSF of people with PCNSL. The study also will compare the accuracy of two imaging techniques-TI-SPECT and FDG-PET-in distinguishing between toxoplasmosis and PCNSL.