Alteplase concentrations measured by ELISA over 2, 4, and 6 hours. (A) Intact alteplase present, with or without ultrasound exposure, was functionally active (approximately 99%) at 2 hours. (B) Intact alteplase present, with or without ultrasound exposure, was functionally active (approximately 99%) at 4 hours. (C) Intact alteplase present, with or without ultrasound exposure, was functionally active (approximately 99%) at 6 hours.