Page 58 - Libro vascular I

Chap-05.qxd 29~8~04 13:25 Page 49
Chapter 5
Blood flow and its appearance on color flow imaging
49
INTRODUCTION
Arterial blood flow is complex and consists of pulsatile flow of an inhomogeneous fluid through viscoelastic arteries that branch, curve and taper. However, a useful understanding of hemodynam- ics can be gained by first considering simple models, such as steady flow in a rigid tube. Factors affecting venous flow will also be considered. This will allow us to interpret spectral Doppler and color Doppler images of blood flow more easily. However, when interpreting color flow images it is important to remember that the color represents the mean Doppler frequency obtained from the sample volumes and that this will depend on the angle between the ultrasound beam and blood flow. The pulse repetition frequency (PRF) and filter setting used and the length of time over which the image is created may also affect the appearance of the image. Artifactual effects also have to be consid- ered carefully before drawing conclusions about the blood flow.
STRUCTURE OF VESSEL WALLS
The arterial and venous systems are often thought of as a series of tubes that transport blood to and from organs and tissues. In reality, blood vessels are highly complex structures that respond to nervous stimula- tion and interact with chemicals in the blood stream to regulate the flow of blood throughout the body. Changes in cardiac output and the tone of the smooth muscle cells in the arterial walls are crucial factors that affect blood flow. The structure of a
CHAPTER CONTENTS
Introduction 49
Structure of vessel walls 49
Why does blood flow? 50 Resistance to flow 51
Velocity changes within stenoses 52 Flow profiles in normal arteries 53
Pulsatile flow 54
Flow at bifurcations and branches 56 Flow around curves in a vessel 57
Flow through stenoses 58
Transition from laminar to turbulent flow 59
Venous flow 60
Changes in flow due to the cardiac cycle 60 Effects of respiration on venous flow 60 Changes in venous blood pressure due to
posture and the calf muscle pump 61 Abnormal venous flow 62