Additional Study Information: This study is for patients who have been diagnosed with Multiple System Atrophy (MSA). The main goals of this study are to determine how safe and well tolerated AZD3241 is in patients who have MSA, and whether AZD3241 has certain effects in the brain of patients with MSA. These effects in the brain are measured with positron emission tomography (PET) scanning, and involve changes in brain cells called microglia. The reason for studying effects on microglia is to determine whether AZD3241 has the activity that is predicted based on other research. Showing that AZD3241 has these effects may suggest that AZD3241 has the potential to be a treatment for MSA, but does not prove that AZD3241 is an effective treatment for MSA.