New trends in ultrasonic diagnostics: measurements and applications in blood velocity evaluation by correlation functions

Shear stress is considered to be an important parameter in diagnostic evaluation of the state of arteries; levels too low induce a rapid increase in plaque, while levels too high can damage vessel walls. It would, therefore, be extremely useful to have a non-invasive method to measure this parameter throughout the cardiac cycle. Shear stress is the relation between the dragging force exerted by blood on the vessel wall and the surface on which this force acts. It is, however, difficult to measure this force by non-invasive means; the most convenient approach is to measure the shear rate, i.e. the relative flow of layers of blood, near to the vessel wall. The shear stress can then be obtained from this when the viscosity characteristics of the blood are known. Laboratory measurements on samples of blood allow a sufficiently precise curve relating shear rate and shear stress to be constructed.