What constitutes the M1 segment of the middle cerebral artery?

Intravenous tissue plasminogen activator has limited efficacy in fibrinolysis of large proximal intracranial thrombi. Thus, recent endovascular acute stroke trials restricted their selection criteria to patients with proximal occlusions in the anterior circulation. Although the terminal internal carotid artery occlusion is relatively easy to identify, there is often a debate as to what constitutes a proximal (involving the M1 segment) versus a distal (involving the M2 segment and beyond) middle cerebral artery occlusion. In light of overwhelming evidence demonstrating superiority of endovascular treatment in patients with proximal occlusion, this distinction has significant practical implications in patient selection. Here we present a brief review of the proximal (M1) segment of the middle cerebral artery anatomy and provide practical methods to recognize and separate the M1 and M2 segments. In keeping with the belief that CT angiography (CTA) (preferably multiphase CTA) is the ideal screening test for patients with emergent large vessel occlusion, we have provided tips for expeditious and accurate vascular imaging interpretation.