Introduction

The development of colour flow imaging has led to an increase in the investigation of blood flow to aid diagnosis of both vascular and non-vascular disorders. An understanding of the physical properties of blood flow is essential when interpreting colour flow images and Doppler spectra. For example, the presence of reverse flow within a vessel as seen on a colour image may be due to the presence of disease or may be normal flow within the vessel. Changes in the velocity of the blood or the shape of the Doppler spectrum can help in locating and quantifying disease. A poor understanding of how blood flows in normal and diseased vessels may lead to misdiagnosis or to loss of useful clinical information. Blood flow is a complex pulsatile flow of a non-homogeneous fluid in elastic tubes; however, some understanding can be obtained by considering the simple model of steady flow in a rigid tube (Caro et al. 1978; Nichols and O'Rourke 1999).

Structure of the vessel walls

The structure of the arterial wall can change due to disease and these changes may be observed using ultrasound. Artery walls consist of a three-layer structure. The inner layer, the intima, is a thin layer of endothelium overlying an elastic membrane. The middle layer, the media, consists of smooth muscle and elastic tissue.