Hypotheses In this study we will assess 40 patients with STEMI. Primary PCI will be performed according to our hospital standard working protocol with thrombus aspiration and balloon dilatation of the culprit lesion if necessary prior to stent deployment. IVUS imaging will be undertaken prior to, and following, passage of the aspirations catheter to assess (1) initial and (2) residual thrombus volume. A third and final IVUS imaging run will be performed after stenting of the culprit lesion. IVUS analysis will be performed off-line to assess the lumen enlargement and volume of residual thrombus at each stage. All studies will be undertaken with the approval of the local Clinical Research Ethics.