A 56-year-old male with a history of hypertension for over 10 years was admitted to the emergency department due to sudden-onset headache and hemiplegia after defecation lasting 4 hours, accompanied by vomiting twice. Physical examination: BP 26/14 kPa (195/105 mmHg), left upper and lower limb muscle strength grade 0, hypotonia, and left-sided hemihypesthesia. The most diagnostically and differentially valuable auxiliary examination for this patient is: