Sample size is estimated to 9 patients in each group assuming a significant difference of 4.7 % for the mean values for the "time to peak" in patients with acute compared to chronic thrombosis with a significance level (alpha) of 0.05 and a power of 95 % (beta) assuming a standard deviation of 2. Sample size calculation was performed using Graphpad Prism 4.0b and StatMate 2.