Complex arterial wall motion across a plaque with a severe 80% stenosis. This shows an example of arterial wall motion across a plaque with a severe 80% stenosis (diameter reduction). The high velocity jet flow hits against the posterior wall which shows marked wall displacements distal to the plaque. Within the plaque there is significant vessel wall constriction due to the pressure drop within the lumen. This is an example of the Venturi effect, well known in fluid dynamics in which the increased kinetic energy of blood flow through the constriction corresponds to a pressure drop and consequent constriction of the vessel walls. See Additional file 5 for the real-time movie of the top right image. The lower right graph displays the dilation data as a surf plot with dilation plotted as a function of the spatial position and the time.