Carstairs staining of a SIPA clot in the longitudinal direction. (A) The trumpet-like morphology of the gross clot was preserved. (B-F) Each end of the clot showed small platelet aggregates with fibrin. (C) Upstream of the throat, the thrombus had platelet aggregates protruding toward the lumen with string or fingerlike shapes alternating with trapped RBCs. Some strings were 10 times longer than they were wide. (D) The apex of stenosis was fully occluded with platelets. Some fibrin was found in the center of the lumen and pores. (E) Downstream, the clot was present at the center and near the wall; whereas the annulus between the 2 had sparce RBCs and fibrin without platelet aggregates. R1 to R5 correspond with the regions of interest shown in the schematic in Figure 2B.