A 68 year-old retired pilot is brought to hospital by a friend because of abnormal behaviour and poor memory which started around 4 months ago. Over the past month, he has been having difficulty sleeping and has become more disinhibited. He has become progressively more apathetic. On examination, he is mildly rigid in all 4 limbs. His MOCA score was 18/30. MRI FLAIR and DWI of the brain are shown below. What is the most likely diagnosis?