THE COMPLETE GUIDE TO VASCULAR ULTRASOUND

Transcription

1 THE COMPLETE GUIDE TO VASCULAR ULTRASOUND PETER H. ARGER, M.D., F.A.I.U.M., F.A.C.R. Professor Emeritus Department of Radiology University of Pennsylvania Medical Center Hospital of the University of Pennsylvania Philadelphia, Pennsylvania SUZANNE DEBARI IYOOB, B.S., R.D.M.S., R.V.T. Technical Director-Vascular Laboratory Department of Radiology University of Pennsylvania Medical Center Hospital of the University of Pennsylvania Philadelphia, Pennsylvania

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3 THE COMPLETE GUIDE TO VASCULAR ULTRASOUND

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7 To Christopher, my parents Susan and Robert, my brother Chris, my grandmother Edith, and to all the rest of my family and friends (especially Susan Schultz and Bonnie Brake) for their love, guidance, and support. S.D.I. To Afento, Harry, Donald, Anastasia, Eugenia, and Nicholas, to whom I am immensely grateful, as they have profoundly influenced my whole approach to life. P.H.A.

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9 CONTENTS Preface ix Acknowledgments xi 1 Blood Vessels: Anatomy and Physiology 1 2 Abdominal Vasculature 6 I. Abdominal Aorta 6 II. Inferior Vena Cava 10 III. Hepatic Veins, Portal Veins, and Hepatic Arteries 11 IV. Superior Mesenteric Artery 17 V. Renal Arteries and Renal Veins 19 3 Peripheral Arterial Systems 26 I. Lower Extremity Arteries 26 II. Upper Extremity Arteries 41 4 Grafts 45 I. Hemodialysis Grafts 45 II. Bypass Grafts 48 5 Peripheral Venous Systems 55 I. Lower Extremity Veins 55 II. Upper Extremity Veins 69 6 Penile Vessels 75 7 Cerebrovascular System 84 8 Test Validation and Statistics A Word About Doppler Controls 109 Appendix 121 I. Review Questions 121 II. Answer Key 129 Subject Index 131

10 PREFACE Vascular ultrasound has expanded to become an integral component of nearly every aspect of diagnostic ultrasound. The complexity of vascular ultrasound has increased as the technology has increased, along with the ability of ultrasound equipment to visualize more and more vessels as well as a wider range of flow variables. The increased capability and utilization of color, power, and duplex Doppler are examples of this. Understanding both the technical and diagnostic aspects of vascular ultrasound is essential to obtaining the maximum information that can be acquired and to making the most cogent and informative diagnosis of a given problem. The Complete Guide to Vascular Ultrasound has a different approach to vascular ultrasound, that combines the technique know-how and diagnostic analysis. This approach results in a better diagnosis of pathology at multiple levels. To promote a comprehensive approach to indepth knowledge of any given vascular problem, most chapters are divided into a six-part approach: 1. Anatomy. Graphically demonstrates the general anatomy of the vascular area to be examined as well as the anatomy of individual vessels. 2. Pathology. Briefly discusses the pathologic processes, which can affect the vessels being examined. Outlines important associated pathophysiologic information necessary for good analysis. 3. History/Questions to Ask the Patient. Details symptoms associated with potential vascular disease of the vessels being evaluated. 4. Diagnostic Examinations. Details necessary technical aspects of the examination tailored to the specific vessel being evaluated. This may include commonly performed but non-ultrasound tests. 5. Diagnostic Analysis. Includes Doppler waveform images and illustrations. Discusses in a detailed outline form of the relevance of various clinical findings. 6. Other Diagnostic Tests related to the clinical problem including nonultrasound diagnostic tests. We believe the information in this book speaks to a wide audience including physicians, i.e., radiologists, vascular surgeons, and cardiologists, as well as sonographers, whose work is vital to the field of ultrasound. Peter H. Arger Suzanne DeBari Iyoob

11 ACKNOWLEDGMENTS We would like to extend our sincere thanks to Patricia Hartman whose invaluable help was a key factor in the production of this book. Her computer skills and typing of the many modifications and re-modifications were a constant source of strength. We also extend our gratitude to Philips Ultrasound, Scott Leonard, and all of the sonographers and physicians in the Ultrasound section at the Hospital of the University of Pennsylvania. We appreciate their help in acquiring the ultrasound images included in this book. We would also like to thank Steven Horii for sharing his technical expertise.

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13 1 BLOOD VESSELS: ANATOMY AND PHYSIOLOGY I. ANATOMY OF BLOOD VESSELS A. Three Layers (Tunicae) of Blood Vessels (Fig. 1.1) 1. Tunica interna or intima. This is the innermost layer and is composed of endothelial cells. 2. Tunica media. This is the middle layer and is composed of smooth muscle and elastic fibers. It is thicker in arteries, can change the size and shape of arteries, gives arteries their rigidity and round shape, and is influenced by hormones and other chemicals. 3. Tunica externa or adventitia. This is the outer layer and is composed of collagenous and elastic fibers. It protects and anchors the vessel to surrounding tissues. B. Circulatory System (Fig. 1.2) 1. Systemic circulation refers to the flow of blood from the left ventricle of the heart through the body (except for the lungs) and back to the right atrium of the heart. The blood carries oxygen and nutrients to the tissues of the body. It also removes wastes, carbon dioxide, and heat from the tissues of the body. The blood leaves the left ventricle of the heart, and goes through the aorta, arteries, arterioles, venules, veins, and vena cava to enter the right atrium of the heart: Heart Aorta Arteries Arterioles Capillaries Venules Veins Vena cava Heart 2. Pulmonary circulation refers to the flow of blood from the right ventricle of the heart, through the right and left pulmonary arteries, to the alveoli (air sacs) in the lungs, then from the alveoli of the lungs, through the right and left pulmonary veins, and back to the left atrium. The blood is deoxygenated when it enters the alveoli from the right ventricle (as it has already gone through the rest of the body through the systemic circulation) and is oxygenated when it leaves the alveoli of the lungs to go into the left atrium. Heart Pulmonary arteries Alveoli Pulmonary veins Heart C. Types of Blood Vessels 1. Arteries are blood vessels that transport blood from the heart to the tissues of the body. They contain all three layers of tunicae. The tunica interna and media is thicker than in veins. The tunica externa is thinner than in veins. These divide into smaller and smaller branches, eventually dividing into arterioles. 2. Arterioles. These small vessels are regulators of blood flow from the arteries into the capillaries. As they get closer to the capillaries, the layers of arteriole decrease to consist only of an endothelial layer surrounded by a few smooth muscle fibers. Vasoconstriction (when the smooth muscle constricts) decreases blood flow into the capillaries. Vasodilation (when the smooth muscle relaxes) increases blood flow into the capillaries. Arterioles have the highest resistance in the circulatory system. They account for one half of the total resistance to blood flow. 3. Capillaries. These microscopic vessels only have a single layer of endothelium and a basement membrane. They allow exchange of nutrients and waste products between the blood and the cells of tissue. Capillaries (sometimes extensive networks of capillaries) usually connect arterioles and venules. 4. Venules. These vessels drain blood from the capillaries into the veins. Close to the capillaries, venules may only consist of an endothelial layer surrounded by the tunica externa. Closer to the veins, venules consist of all three layers. 5. Veins. Veins are blood vessels that transport blood from the tissues of the body back to the heart. They are composed of all three layers of tunicae, although the tunica intima and tunica media are thinner than in arteries. The tunica externa is thicker than in arteries. Veins contain valves to prevent backflow of the blood, which has lower pressure at this point. 6. Vasa vasorum. This is a network of minute blood vessels that perfuse the tissues of blood vessels themselves.

14 2 The Complete Guide to Vascular Ultrasound FIGURE 1.1. Three layers of a vessel wall. II. PHYSIOLOGY AND CHARACTERISTICS OF BLOOD FLOW A. Blood flow is the amount of blood that passes through a vessel during an episode of time. Blood flows in a laminar flow pattern in most vessels (Fig. 1.3). A laminar flow pattern is a stable pattern consisting of many laminae (layers) that are concentric. Each layer is thought to flow at a different velocity. The velocity of each layer increases as they approach the center of the lumen. The center is where the highest velocity of blood flow is thought to exist. The two primary factors that determine blood flow are blood pressure and resistance. B. Blood pressure. The pressure that the blood exerts on the vessel walls is considered the blood pressure. 1. Blood pressure is directly proportional to blood flow. When one increases, so does the other. 2. Blood always flows from areas with higher pressure to areas with lower pressure. 3. Blood pressure starts off high as the blood leaves the left ventricle to go into the systemic circulation (mean pressure of 100 mm Hg) as it progresses down to 0 mm Hg when the blood returns to the heart in the right atrium. 4. Principal factors that affect arterial blood pressure: a. Cardiac output. Cardiac output is determined by multiplying the stroke volume (which is the amount of blood ejected from either ventricle in one systole, typically 70 ml) by the heart rate. This is 5.25 L/min in a normal, resting adult. Cardiac output is directly proportional to blood pressure. When one increases, so does the other. When one decreases, so does the other. b. Blood volume. The volume of the blood is also directly proportional to the blood pressure. When one increases, so does the other. When one decreases, so does the other. Normally, the volume of blood in an adult is about 5 L. Hemorrhage decreases blood volume and thus the blood pressure also decreases. High salt intake (water retention) increases blood volume, and thus the blood pressure also increases. c. Peripheral resistance is defined as all the factors that oppose blood flow in the circulatory system. Arterioles change their diameters to affect the resistance, which affects blood flow and blood pressure. As peripheral resistance increases, so does the arterial blood pressure. This is a directly proportional relationship as well. 5. Resistance. Opposition to blood flow. This occurs as a result of friction between the blood vessel walls and the blood. It also occurs due to the viscosity that is created by the plasma proteins and the red blood cells. It is directly proportional to blood pressure and as a result blood flow. An increase in resistance would result in an increase in both blood flow and blood pressure. FIGURE 1.2. Simplified circulatory system.

15 1/Blood Vessels: Anatomy and Physiology 3 FIGURE 1.3. Laminar flow. 6. Principal factors that affect resistance to blood flow: a. Blood viscosity. The viscosity of the blood is how thick the blood is. It takes more energy to move blood that is more viscous. The viscosity is directly proportional to resistance and thus blood pressure. Dehydration, severe burns, and polycythemia (an increase in the number of red blood cells) all cause an increase in the viscosity of the blood, which increases resistance and increases blood pressure. A condition such as anemia or hemorrhage can cause a decrease in the viscosity of the blood, which decreases the resistance and the blood pressure. b. Blood vessel radius. The fourth power of the radius of a blood vessel is inversely proportional to the resistance. When the radius decreases, the resistance and blood pressure increase. Smaller vessels obviously have higher resistances. c. Blood vessel length. The longer the blood vessel, the higher the resistance. There is a direct proportional relationship. C. Poiseuille s Law helps to determine how much fluid is moving through a vessel. It is an equation that describes the relationship between resistance, pressure, and volume flow. It demonstrates that the change in the diameter of a blood vessel affects resistance the most. As the radius decreases, the resistance increases. The velocity of the blood flow then must increase to keep the same amount of blood moving through the vessel. Therefore, there is an inversely proportional relationship between the velocity of a blood vessel and the site of the blood vessel. (P 1 P 2 ) π r 4 Poiseuille s Law = Q = 8ηL Q = volume flow P 1 = pressure at the proximal end of the vessel P 2 = pressure at the distal end of the vessel r = radius of the vessel L = length of the vessel π = η = viscosity of the fluid An abbreviated pressure/volume flow relationship can also be used: Q = P/R Q = volume flow P = pressure R = resistance D. Reynolds Number. As pressure increases, volume flow increases as well. This occurs only to a point. When the stable laminar flow becomes disturbed, the smooth streamlines break up and form small circular currents called eddy currents and vortices. Osborne Reynolds discovered that when the flow pattern changes from smooth to disturbed, flow volume was no longer increased by increased pressure. Instead, the flow disturbance increased. Reynolds number is a dimensionless number that reveals at what point flow becomes turbulent. When the number exceeds 2,000, the flow becomes turbulent. Other factors such as irregularities of the vessel wall and plaque, pulsatility of blood flow, and body movement can cause blood flow to become turbulent at lower values of the Reynolds number. Vq2r Reynolds Number (Re) = Re = η Re = Reynolds number V = velocity q = density of the fluid (since density and viscosity are usually constant, turbulent flow) r = radius of the tube (develops mainly as a result of changes in velocity or radius) η = viscosity of the fluid E. The Bernoulli Equation. This demonstrates that there is an inversely proportional relationship pressure and velocity. When there is high velocity, there is low pressure and vice versa. Flow separations are pressure gradients (the difference in pressure from one area of the vessel to another). These can be caused by a change in direction of the vessel (curve or bend) or a change in the geometry of the vessel (due to widening such as in the carotid bulb or narrowing due to stenosis or plaque) (Fig. 1.4). Velocity decreases in an area of a flow separation, and the pressure increases. Because blood flows from high pressure to low pressure, the direction of the blood flow changes in this area.

16 4 The Complete Guide to Vascular Ultrasound FIGURE 1.4. Effects of stenotic plaque on flow pattern, velocity, and pressure. F. Other Characteristics of Blood Flow 1. Flow that is low-resistance is continuous and steady throughout systole and diastole. It is usually found feeding a dilated vascular bed, such as in the internal carotid, renal, vertebral, hepatic, splenic, and celiac arteries. 2. Flow that is high resistance is pulsatile (tri- or biphasic) (Fig. 1.5). It can usually be found in arteries that supply high-resistance peripheral vascular beds such as the external carotid, aorta, iliac, subclavian, fasting superior mesenteric, and extremity arteries. 3. Diastolic flow reversal is normally found in arteries supplying high-resistance peripheral vascular beds. This flow reversal increases with vasoconstriction and decreases with vasodilation (which can be produced by exercise, body heating, and stenosis). For example, a normally high-resistance arterial Doppler signal becomes low resistance after exercise. The diastolic flow reversal portion of the waveform is now seen as a forward reflection. 4. Diastolic flow reversal disappears distal to a stenosis. 5. Diastolic flow reversal can also disappear proximal to a significant stenosis. 6. Vasoconstriction causes an increase in pulsatility in small and medium-size arteries and a decrease in pulsatility in minute arteries, arterioles, and capillaries. FIGURE 1.5. Doppler tracing of the midaorta, demonstrating high-resistance, pulsatile flow.

17 1/Blood Vessels: Anatomy and Physiology 5 7. Vasodilation causes a decrease in pulsatility in small and medium-size arteries and an increase in pulsatility in minute arteries, arterioles, and capillaries. 8. Total blood flow may be normal in an extremity at rest even when there is a significant stenosis or occlusion of a vessel. This is due to development of a collateral network. BIBLIOGRAPHY Rumwell C, McPharlin M. Arterial evaluation in vascular technology an illustrated review Pt. 1, 2nd ed. Pasadena, CA: Davies Publishing, 2000:1 33. Tortora GJ, Anagnostakos NP. The cardiovascular system: vessels and routes. In: Principles of anatomy and physiology, 6th ed. New York: Harper & Row, 1990:

18 2 ABDOMINAL VASCULATURE I. ABDOMINAL AORTA A. Anatomy (Fig. 2.1) 1. Abdominal aorta extends from the twelfth thoracic vertebra to the fourth lumbar vertebra. 2. Portions of the Abdominal Aorta (Fig. 2.2): a. Proximal superior to or at the level of the celiac axis, measures 2 to 3 cm b. Middle below the celiac axis, above the renal arteries, measures 1.6 to 2.5 cm c. Distal just above the bifurcation, measures 1.1 to 2.0 cm d. Iliac arteries measure 0.6 to 1.4 cm 3. Main branches off the aorta: a. Celiac axis or celiac artery: first branch off the abdominal aorta, divides into the left gastric artery, splenic artery, and common hepatic artery. These feed the stomach, spleen, liver, pancreas, and duodenum. b. The superior mesenteric artery is approximately 1 cm distal to the celiac axis. This artery feeds the small intestine, ascending colon, cecum, and part of the transverse colon. c. The renal arteries are the next branches. They supply the kidneys, ureters, and adrenal glands (Fig. 2.3). d. The inferior mesenteric artery is approximately 3 to 4 cm above the aortic bifurcation. It supplies the descending, iliac, and sigmoid colon, as well as the left half of the transverse colon and part of the rectum. e. The common iliac arteries are the terminal branches of the abdominal aorta. They divide into the internal iliac arteries and the external iliac arteries. 4. Other branches off the abdominal aorta but not commonly seen on ultrasound are: a. The paired inferior diaphragmatic or inferior phrenic arteries. b. The paired middle suprarenal arteries. c. The paired gonadal arteries. d. The paired first to fourth lumbar arteries. e. The middle sacral artery. 5. Above the umbilicus, all paired arteries course posterior to their related vein. Below the umbilicus, all paired arteries course anterior to their related vein. B. Pathology 1. Ectasia when the abdominal aorta does not taper as it normally does, but is not dilated to the point of aneurysm. 2. Atherosclerosis or arteriosclerosis thickening, hardening, and deposition of plaque in the intimal wall of arteries, which can cause stenosis. a. This is associated with smoking, hypertension, sedentary lifestyle, diabetes mellitus, and increased levels of low-density lipoprotein (LDL) levels of cholesterol. b. Because of its large size and high flow rate, the abdominal aorta is sensitive to plaque. Two common sites of plaque formation are the origin of the renal arteries and the bifurcation into the common iliac arteries. The most common site is the infrarenal portion of the aorta. c. More men than women are affected with atherosclerosis, and the incidence increases with age. d. Atherosclerosis may be associated with aneurysm and wall weakening. 3. Coarctation is the narrowing of the aorta. There are two clinical findings associated with this: a. Hypertension resulting from decreased kidney perfusion. b. Manifestation of lower extremity ischemia decreased lower extremity pulses. 4. Aneurysm increase in arterial diameter (Fig. 2.4). a. Types of aneurysms (Fig. 2.5): (1) Fusiform symmetric swelling, most common, usually found below the level of the renal arteries (90% of all aneurysms) and may extend into the common iliac arteries. These also may contain thrombus. (2) Saccular focal outpouching, least common, usually affects the left lateral portion of the distal abdominal aorta (where the least support is), causes marked alteration in the pattern of blood flow. These also may have thrombus or plaque. (3) Dissecting tear in intimal lining of the wall of the vessel. A false lumen is created between the

19 2/Abdominal Vasculature 7 FIGURE 2.1. Abdominal vasculature. intima and the media. These may extend from the aortic valve to the abdominal aorta. They can be very dangerous. Blood flow may push the flap across the lumen and completely (or almost completely) obstruct flow. Two flow lumens with markedly different flow characteristics would be seen. These are most common in the thoracic aorta (Fig. 2.6). (4) Pseudoaneurysms or false aneurysms. Small pocket of moving blood connected to an artery through a small opening or neck and may be partly surrounded by thrombus. These form due to interventional radiology procedures (e.g., cardiac catheterization using the common femoral artery), trauma, surgery, or infection. They can be felt as a pulsatile mass. A turbulent flow pat- FIGURE 2.2. Sagittal view of the middle and distal aorta. FIGURE 2.3. Transverse view of the renal arteries coming off the aorta.

20 8 The Complete Guide to Vascular Ultrasound FIGURE 2.4. Abdominal aortic aneurysm. Note the irregular dilatation of the distal aorta. tern would be visualized. Compression with a transducer may clot them off. An alternative therapy would be thrombin injection. b. Abdominal aortic aneurysm is considered if the diameter is greater than 3 cm or greater than 50% than the original diameter. c. A true aneurysm contains all the layers of the artery (fusiform and saccular.) A false aneurysm or pseudoaneurysm leaks through a hole in the intima but is contained by the external layer or by the body. d. There is an increased risk for abdominal aortic aneurysm in close relatives. e. The incidence of the disease is highest among men, particularly those over 65 years of age. f. The identification of an abdominal aortic aneurysm increases the risk for aneurysms in the common iliac arteries, common femoral arteries, and popliteal arteries. g. Only 4% of abdominal aortic aneurysms actually affect flow into the renal arteries. Stenosis of the renal arteries and hydronephrosis are potential complications of abdominal aortic aneurysms. FIGURE 2.6. Dissection of the abdominal aorta. There appear to be parallel hypoechoic lumens within the aorta. The posterior portion is the false lumen, separated from the true lumen by the flap (arrow). (Image courtesy of Philips Medical Systems.) h. Aneurysms over 6 cm in diameter are considered surgical emergencies, and 60% of all aneurysms over 7 cm will rupture within 1 year. Abdominal aortic aneurysms can have small tears and leak into the body cavity. i. The main complication of abdominal aortic aneurysm is rupture; the main complication of peripheral aneurysm is distal embolization. j. Atherosclerotic abdominal aortic aneurysms may become inflammatory, and the wall will become thickened and surrounded by fibrosis. k. Marfan syndrome is associated with weakness of the arterial wall and may result in aneurysm of the first portion of the aorta leading up to the aortic valve. This may lead to dissection of this portion of the aorta. FIGURE 2.5. Types of aneurysms.