A 72 year old woman with a history of hypertension presents with sudden onset right sided weakness and slurred speech which started 1 hour ago. On examination she has right facial droop and power 3/5 in the right arm and leg. Capillary blood glucose is 6.0 mmol/L. Her blood pressure is 178/96 mmHg and she is in sinus rhythm. What is the single most appropriate next investigation?