Binding of t-PA to non-crosslinked fibrin during perfusion. Radiolabeled, active-site blocked t-PA was perfused at a constant rate of 15 μL/min through a column of non-crosslinked fibrin. Aliquots of 100 μL were collected to calculate binding. At the arrow, a 1,000-fold molar excess of unlabeled t-PA was added to the perfusate. Results are shown for binding without ultrasound (dotted line) and in the presence of 1 MHz ultrasound at 2 W/cm²and 50% duty cycle.