Proximal MCA occlusion is recognized using MR or CT angiography performed before inclusion. All patients are treated with alteplase according to current guidelines. In addition, patients randomized in the active group are continuously exposed to transcranial 2-MHz ultrasound for 1 hour combined with intravenous administration of microbubbles. Controls are only treated with alteplase. The ultrasound beam is positioned at the thrombus/blood flow interface using color-coded sonography. Galactose-based microbubbles are administered via continuous intravenous infusion over 40 min.