A 69-year-old woman with a 10-year history of hypertension (HTN) on regular antihypertensive medication. Six hours before admission, while watching TV, she suddenly developed numbness and weakness in the right upper and lower limbs, dysarthria, drooling, headache, and vomiting once, followed by somnolence, prompting emergency admission. On arrival, blood pressure (BP) was 26/14 kPa (195/105 mmHg). Which investigation is most diagnostic at this stage?