A 67 year-old lady attends the emergency department accompanied by her husband. He is worried that she appears confused. According to him, she has been asking the same questions repeatedly. She is aware which month and year it is. She is able to recognise her son and daughter-in-law when they come to visit her in hospital. She has a MRI of the brain the next morning, by which time she is back to her normal self. FLAIR, DWI and ADC images are shown below. What is the most likely diagnosis?