At 3-month follow-up , primary venous patency was 100 % in the ultrasound-assisted catheter-directed thrombolysis group and 96 % in the conventional catheter-directed thrombolysis group ( P = 0.33 ) , and there was no difference in the severity of the post-thrombotic syndrome ( mean Villalta score : 3.0 3.9 [ range 0-15 ] versus 1.9 1.9 [ range 0-7 ] ; P = 0.21 ) , respectively .