The trial was performed at 38 centers in the United States. Neurointerventionalists were preapproved to participate on the basis of training and experience. (For approval requirements, see the Supplementary Appendix, available with the full text of this article at NEJM.org.) Enrolled patients or their surrogates provided written informed consent. Patients were enrolled if they met clinical and imaging eligibility requirements and could undergo initiation of endovascular therapy between 6 and 16 hours after the time that they had last been known to be well, including if they had awakened from sleep with symptoms of a stroke. Perfusion imaging had to be performed at the trial-site hospital in which endovascular therapy was planned.
Patients were eligible if they had an initial infarct volume (ischemic core) of less than 70 ml, a ratio of volume of ischemic tissue to initial infarct volume of 1.8 or more, and an absolute volume of potentially reversible ischemia (penumbra) of 15 ml or more. Estimates of the volume of the ischemic core and penumbral regions from CT perfusion or MRI diffusion and perfusion scans were calculated with the use of RAPID software (iSchemaView), an automated image postprocessing system. The size of the penumbra was estimated from the volume of tissue for which there was delayed arrival of an injected tracer agent (time to maximum of the residue function [Tmax]) exceeding 6 seconds.8 (link) (An example is given in Fig. 1.) Patients were required to have an occlusion of the cervical or intracranial internal carotid artery or the proximal middle cerebral artery on CT angiography (CTA) or magnetic resonance angiography (MRA). Detailed inclusion and exclusion criteria for the trial are provided in the Supplementary Appendix.