A 58-year-old man from Delhi presents to the emergency department with acute onset left-sided weakness and speech difficulty that began 2 hours ago. His wife reports he was fine while watching television. On examination, he has left hemiparesis (grade 3/5) and expressive aphasia. Blood pressure is 156/92 mmHg, heart rate 88/min, regular. Non-contrast CT head is normal. What is the next best imaging modality to confirm the diagnosis and guide thrombolytic therapy?