Which of the following is the investigation of choice in diagnosing a case of acute Subarachnoid hemorrhage?

The history and physical examination, especially the neurologic examination, are essential components in the diagnosis and clinical staging of SAH. The diagnosis is confirmed radiologically urgent computed tomography (CT) scan without contrast. Traditionally, a negative CT scan is followed with lumbar puncture. However, noncontrast CT followed by CT angiography (CTA) of the brain can rule out SAH with greater than 99% sensitivity. CT without contrast is the most sensitive imaging study in SAH. When carried out within 6 hours of headache onset, CT has 100% sensitivity and specificity. Sensitivity is 93% within 24 hours of onset, 80% at 3 days, and 50% at 1 week. Thin (3 mm) cuts are necessary to properly identify the presence of smaller hemorrhages.