Abdominal Vasculature Review PDF

SECTION III Abdominal Sonography CHAPTER 13 Abdominal Vasculature MARSHA M. NEUMYER OBJECTIVES Describe the anatomy of the vasculature of the liver, spleen, mesenteric, and renal systems. Define the role of duplex scanning and color-flow imaging for evaluation of abdominal vascular disease. Des...

SECTION III Abdominal Sonography CHAPTER 13 Abdominal Vasculature MARSHA M. NEUMYER OBJECTIVES Describe the anatomy of the vasculature of the liver, spleen, mesenteric, and renal systems. Define the role of duplex scanning and color-flow imaging for evaluation of abdominal vascular disease. Describe the sonographic appearance of the hepatoportal, mesenteric, and renal vascular systems. Define the hemodynamic patterns and spectral waveforms found in the normal abdominal vasculature. KEY WORDS Doppler Spectral Waveform — Provides information about blood flow velocity, flow direction, presence of flow disturbance or turbulence, and vascular impedance. Duplex sonography — Real-time imaging and pulsed Doppler capabilities used either simultaneously or sequentially. Hepatofugal — Flow direction away from the liver. Hepatopetal — Flow direction toward the liver. High-resistance vessels — Arteries with low or reversed flow in diastole that supply organs that do not demand constant blood perfusion. Low-resistance vessels — Arteries supplying organs that demand constant forward blood flow or perfusion. Spectral Broadening — An increase in returned echoes proportional to an increase in turbulence or flow disturbance. Systolic Window — Relatively signal-free area between the arterial Doppler shift signal and the baseline during the systolic portion of a Doppler spectral display. NORMAL MEASUREMENTS Anatomy Measurement Average Diameter of Abdominal Arteries Aorta Celiac artery SMA IMA Renal arteries 2.0 to 2.5 cm 0.70 cm 0.60 cm 0.30 cm 0.40 to 0.50 cm Anatomy Anatomy Anatomy Measurement Average Diameter of Abdominal Veins IVC Renal veins Hepatic veins SMV Splenic vein Portal vein 25 to 35 mm 4.0 to 6.0 mm 4.0 to 7.0 mm 6.0 to 7.0 mm 4.0 to 6.0 mm 13 mm Images in this chapter are courtesy Penn State Hershey Vascular Noninvasive Diagnostic Laboratory, Hershey, Pennsylvania. 192 Chapter 13 Since its introduction more than 40 years ago as a fairly complex technology that combined gray-scale imaging for characterization of tissues and blood vessels and pulsed Doppler for assessment of flow dynamics, vascular duplex sonography has evolved dramatically. Initially used for noninvasive evaluation of the superficial arteries and veins, outstanding technical advancements have facilitated extension of this modality into the deep vessels of the abdomen. In current clinical practice, duplex technology is complemented by color, power, and harmonic and real-time compound imaging for examination of the hepatoportal, mesenteric, and renal vascular systems. In extended, advanced practice, investigators are exploring the use of 3- and 4-dimensional and volume imaging for use in selected vascular cases. Abdominal Vasculature 193 THE ABDOMINAL ARTERIAL SYSTEM Location The abdominal arterial system consists of the segment of the aorta from the level of the diaphragm to the aortic bifurcation and its branches, the celiac axis/artery (and its branches, the common hepatic, splenic, and left gastric arteries), and the superior mesenteric, inferior mesenteric, renal (and renal parenchyma vessels), and common iliac arteries (Figure 13-1). The abdominal aorta commences at the aortic opening of the diaphragm, lying slightly to the left and anterior to the vertebral column (Table 13-1). It terminates on the body of the fourth lumbar vertebra, at which point it bifurcates into the common iliac arteries. The vessel diameter tapers slightly from its proximal to distal segments. Aortic arch Inferior vena cava Common hepatic artery Abdominal aorta Celiac axis Splenic artery Right renal artery Left kidney Right kidney Left renal artery Left renal vein Superior mesenteric artery Inferior mesenteric artery Aortic bifurcation FIGURE 13-1 Abdominal arterial system. 194 Section III ABDOMINAL SONOGRAPHY Table 13-1 Location of Abdominal Aorta and Branches Routinely Visualized With Ultrasound Aorta (can be tortuous) Celiac Artery Left Gastric Artery (very tortuous) Splenic Artery (tortuous) Anterior to Spine Aorta Celiac artery, splenic artery, CHA Posterior to Lt renal vein, SMA, splenic vein, pancreas body/tail, celiac artery, splenic artery, CHA, lt gastric artery, inferior duodenum, stomach, peritoneum, liver Lt gastric artery, peritoneum, liver Liver, peritoneum Celiac artery, pancreas tail, superior pole lt kidney Liver, lt gastric artery stomach, peritoneum SMA, pancreas body, splenic vein Diaphragm, EGJ Splenic artery, cardiac end of stomach Celiac artery, splenic artery, SMA, CHA Pancreas body, splenic vein, SMA Diaphragm Diaphragm Spleen Celiac artery Celiac artery, lt gastric artery, CHA, aorta Superior to Inferior to Medial to Lt lateral to Diaphragm Splenic artery, lt renal artery, lt kidney, lt ureter, lt adrenal gland, pancreas tail, ascending duodenum, lt crus of diaphragm Spine, IVC, rt renal artery, rt kidney, rt adrenal gland, rt crus of diaphragm, CHA, PHA, GDA Liver, caudate lobe, CHA Rt lateral to Common Hepatic Artery Proper Hepatic Artery Gastroduodenal Artery Superior Mesenteric Artery Anterior to Celiac artery, IVC Portal vein Aorta, lt renal vein Posterior to Liver, lt gastric artery, peritoneum Common duct, peritoneum IVC, CBD, pancreas neck/ head Liver, peritoneum Superior to Pancreas neck/head, SMA GDA PHA, common duct, portal vein Rt and lt hepatic arteries, porta hepatis Common duct, rt and lt hepatic arteries, porta hepatis, cystic duct Celiac artery, splenic artery CHA Inferior to Medial to Lt lateral to Rt lateral to PHA Duodenum Splenic vein, pancreas body, liver, SMV, peritoneum Renal arteries, renal veins, common iliac arteries, common iliac veins Diaphragm, celiac artery, splenic artery, lt, gastric artery Splenic vein, pancreas tail, lt ureter SMV, IMV, PSC, IVC CHA, SMA, SMV, splenic vein, pancreas head Chapter 13 Abdominal Vasculature 195 Aorta (can be tortuous) Celiac Artery Routinely Visualized Left Gastric Artery (very tortuous) Splenic Artery (tortuous) Table 13-1 Location of Abdominal Aorta and Branches With Ultrasound—cont’d Rt Renal Artery Lt Renal Artery Rt Common Iliac Artery Lt Common Iliac Artery Anterior to Rt crus of diaphragm Posterior to IVC, rt renal vein, peritoneum, PSC, pancreas head/uncinate Rt common iliac vein, proximal IVC, spine Peritoneum, small intestine, rt ureter Lt common iliac vein, spine Peritoneum, small intestines, lt ureter Superior to Common iliac arteries, common iliac veins, rt ureter Inferior to SMA, celiac artery, rt adrenal gland Medial to Rt kidney Lt crus of diaphragm Lt renal vein, peritoneum, pancreas tail, splenic vein Common iliac arteries, common iliac veins, lt ureter SMA, celiac artery, lt adrenal gland, splenic artery Lt kidney Lt lateral to Rt lateral to Aorta, SMA SMA, renal arteries and veins Rt common iliac vein, proximal IVC, psoas major muscle Aorta, SMA Lt common iliac vein, lt common iliac artery SMA, renal arteries and veins Psoas major muscle Lt common iliac vein, rt common iliac artery, rt common iliac vein CBD, Common bile duct; CHA, common hepatic artery; EGJ, esophageal gastric junction; GDA, gastroduodenal artery; IMV, inferior mesenteric vein; IVC, inferior vena cava; PHA, proper hepatic artery; PSC, portal splenic confluence; SMA, superior mesenteric artery; SMV, superior mesenteric vein. Celiac artery Anterior Superior mesenteric artery Superior Inferior Posterior Abdominal aorta FIGURE 13-2 Gray-scale image of a longitudinal section of the abdominal aorta and the origins of the celiac and superior mesenteric arteries from the anterior wall of the aorta. The abdominal aorta is bordered anteriorly by the stomach, pancreas, celiac axis, splenic vein, and superior mesenteric artery and vein. It is bordered on its right by the inferior vena cava (IVC) and on its left by the splenic vein and tail of the pancreas. The celiac and superior mesenteric arteries originate from the anterior wall of the aorta, and the inferior mesenteric artery (IMA) originates from the left anterolateral wall (Figure 13-2). The celiac artery is bordered on its left side by the cardiac end of the stomach and rests on the superior border of the pancreas. It is located 1 to 3 cm below the diaphragm at about the level of the twelfth thoracic and first lumbar vertebrae. The celiac divides into three major branches—the 196 Section III ABDOMINAL SONOGRAPHY Right renal vein Anterior IVC Left renal vein Kidney Right Left Right renal artery Posterior Aorta FIGURE 13-3 Transverse gray-scale image of the right kidney demonstrating the length of the right renal artery as it courses behind and inferior to the inferior vena cava. common hepatic, splenic, and left gastric arteries— approximately 1 to 2 cm from its origin. The celiac artery and its branches supply blood to the stomach, liver, spleen, and small intestine. From its origin at the celiac axis, the common hepatic artery courses along the superior border of the pancreatic neck and head. Between the duodenum and the anterior surface of the head of the pancreas, it gives rise to the gastroduodenal artery. Coursing superiorly, it becomes the proper hepatic artery and gives rise to the right gastric artery before entering the porta hepatis. Interrogation of the hepatic artery and its branches is best achieved using color-flow imaging and a coronal oblique image plane. Within the liver, the proper hepatic artery branches into the right and left hepatic arteries, which divide into the segmental and subsegmental hepatic artery branches. These branches course parallel to the bile ducts and portal vein branches. The left gastric artery courses along the lesser curvature of the stomach, sending branches to the anterior and posterior segments of the stomach and esophagus. The splenic artery is tortuous as it courses along the anterosuperior margin of the pancreas body and tail to terminate within the hilum of the spleen. The splenic artery is best visualized from a transverse scanning plane superior to the body of the pancreas. Using the spleen as an acoustic window and a lateral approach, the distal segment of the artery can be interrogated in the splenic hilum. The superior mesenteric artery (SMA) originates from the anterior wall of the aorta 1 to 2 cm inferior to the origin of the celiac axis. At its origin, the SMA lies between the aorta (posteriorly) and the splenic vein and body of the pancreas (anteriorly). Its proximal segment runs between the pancreas and the transverse portion of the duodenum. This artery anastomoses to the celiac artery by way of the superior and inferior pancreaticoduodenal arteries, which serve as major collateral pathways in the presence of occlusive disease of the SMA or celiac artery. The SMA supplies blood to the jejunum, ileum, cecum, ascending and transverse colon, portion of the duodenum, and the pancreatic head. The inferior mesenteric artery (IMA) originates from the aortic wall approximately 4 cm superior to the aortic bifurcation. The IMA lies anterolateral to the distal abdominal aorta at its origin. It then descends to the left iliac fossa, anterior to the left common iliac artery, to enter the pelvis as the superior hemorrhoidal artery. This vessel lies in close proximity to the aorta along the first several centimeters of its course. The artery supplies blood to the descending and sigmoid flexures of the colon and the greater part of the rectum. The left and right renal arteries originate from the lateral wall of the aorta approximately 1 to 1.5 cm below the SMA, just posterior to the renal veins. In their proximal segment the renal arteries follow the crus of the diaphragm. The right renal artery is longer than the left because it must pass behind the IVC and right renal vein to enter the hilum of the right kidney (Figure 13-3). The left renal artery originates from the aortic wall somewhat higher than the right, posterior to the left renal vein. Before entering the hilum of the kidney, each renal artery divides into four or five branches, the greater number of which most often lie between the renal vein and the ureter. The vessels further branch to form the interlobar and arcuate arteries, which pass between the Chapter 13 197 Abdominal Vasculature Lateral LT Kidney RT Superior Inferior Renal arteries Aorta Left renal vein Medial FIGURE 13-4 Color-flow image demonstrating multiple renal arteries arising from the aortic wall and entering the renal hilum. medullary pyramids of the renal parenchyma (see Urinary System, Chapter 17). Duplicated and/or accessory renal arteries are noted, originating from the aortic wall in approximately 20% of the population (Figure 13-4). The accessory renal arteries usually enter the upper or lower poles of the kidney rather than entering the organ at the hilum. Duplicate and accessory polar renal arteries are found more often on the left side than the right. Size The abdominal arteries normally appear as anechoic structures with bright, echogenic walls. This linear wall reflectivity is attributed to the acoustic properties of collagen fibers found in the tunica intima and tunica media. The aorta often displays significant pulsatility, making it easy to distinguish from adjacent structures. The branches of the abdominal aorta consistently demonstrated with ultrasound are the celiac artery (its splenic and common hepatic artery branches), the superior mesenteric artery, renal arteries, and common iliac arteries. The average diameters of the aorta and its branch arteries are as follows: Hemodynamic Patterns NORMAL MEASUREMENTS Anatomy Measurement Aorta Celiac artery SMA IMA Renal arteries 2.0 to 2.5 cm 0.70 cm 0.60 cm 0.30 cm 0.40 to 0.50 cm Sonographic Appearance In addition to the following discussion, the sonographic appearance of the abdominal aorta and its branch arteries is covered in Chapter 10. The suprarenal abdominal aorta supplies the largest portion of its blood flow to low-resistance vessels that supply the liver, spleen, and kidneys. These organs all have high metabolic rates and demand constant forward blood flow. In contrast, in a fasting patient, the SMA is a high-resistance vessel supplying the muscular tissues of the small intestine, cecum, and colon. Blood flow through the suprarenal aorta therefore meets little resistance to runoff, and forward flow is noted throughout the cardiac cycle (Figure 13-5, A). The peak aortic systolic velocity decreases with age, perhaps as a result of decreased vessel wall compliance. The infrarenal aortic blood supply is principally to the high-resistance peripheral arterial system of the lower extremities and lumbar arteries. The pressure wave noted in this segment of the aorta therefore 198 Section III ABDOMINAL SONOGRAPHY 150 100 A 50 150 100 cm/s A 50 cm/s B FIGURE 13-5 A, Doppler spectral waveform from the suprarenal abdominal aorta. Note forward diastolic flow. B, Doppler spectral waveform from the infrarenal aorta, demonstrating the triphasic velocity waveform consistent with a vessel feeding a high-resistance vascular bed. B 250 200 150 100 50 cm/s FIGURE 13-7 A, High-resistance velocity spectral waveform recorded from the fasting superior mesenteric artery. B, Postprandially, the superior mesenteric artery diastolic flow component increases in response to the metabolic demands imposed by digestion. 80 60 40 20 cm/s FIGURE 13-6 Doppler velocity waveform from the celiac axis. Note constant forward diastolic flow. FIGURE 13-8 Doppler spectral waveform from a normal renal artery. The high diastolic flow component is consistent with a vessel feeding a low-resistance end organ. resembles the velocity waveforms recorded from peripheral arteries (Figure 13-5, B). (See Vascular Technology, Chapter 32). The celiac axis supplies low-resistance end organs— the liver and spleen—through its branch vessels—the hepatic, left gastric, and splenic arteries. Like the flow patterns seen in the suprarenal aorta, constant forward flow is documented throughout the vascular tree supplied by the celiac artery (Figure 13-6). The SMA supplies the tissues of the pancreas, small intestine, and colon. Flow in the SMA varies, depending on the activity of these organs and their metabolic status. In the fasting state, there is relatively high resistance to arterial flow to the tissues of the gut (Figure 13-7, A). After ingestion of a meal, remarkable changes occur in the flow patterns in the SMA, reflecting the metabolic demands imposed by the digestive process. Increases occur in the diameter of the SMA, peak systolic and end-diastolic velocities, and volume flow to the small bowel. Constant forward flow should be observed throughout the cardiac cycle, reflecting the flow demands of the postprandial vascular bed (Figure 13-7, B). The kidneys, like the brain, eyes, liver, and spleen, are low-resistance organs that demand constant blood flow to moderate their metabolic activity. Hemodynamic flow patterns in normal renal arteries that supply healthy kidneys demonstrate high diastolic flow (Figure 13-8). In patients with chronic renal disease, the vascular resistance of the kidney increases. This increase in renovascular resistance of the end organ may be expressed in the flow patterns from the renal artery as a decrease in the diastolic flow component. Doppler Velocity Spectral Analysis The Doppler velocity waveform from the suprarenal abdominal aorta normally demonstrates an absence of reversed diastolic flow, reflecting the low vascular resistance of its end organs (see Figure 13-5, A). In contrast, the signals from the infrarenal aorta are multiphasic, which is consistent with a vessel feeding a high-resistance Chapter 13 peripheral arterial tree (see Figure 13-5, B). Occasionally, a biphasic flow pattern is present. The absence of a forward diastolic flow cycle reflects the relative decrease in arterial wall compliance or elasticity. This flow pattern may be seen in the elderly population and in patients with medial calcification of the arterial wall. Peak systolic velocity normally ranges from 40 to 100 cm/sec. The Doppler spectral waveforms from the celiac, hepatic, and splenic arteries demonstrate forward diastolic flow compatible with high flow demands of the liver and spleen (see Figure 13-6). Peak systolic velocity in the celiac artery normally ranges from 98 to 105 cm/ sec, whereas the common hepatic and splenic arteries demonstrate velocity ranges that are slightly lower. The splenic artery is frequently tortuous, and spectral broadening may be noted in the quasi-steady waveform recorded from this vessel. In the fasting state the Doppler spectral waveform from the SMA demonstrates low diastolic flow; there may be a brief period of reversed flow during early diastole (see Figure 13-7, A). Peak systolic velocity normally ranges from 97 to 142 cm/sec in the fasting state. Postprandially, the peak systolic velocity increases in the normal artery, and a twofold to threefold increase in end-diastolic flow may be documented (see Figure 13-7, B). Because of the collateral potential expressed in the mesenteric arterial system, disease in one of the three major mesenteric arteries can result in increased flow and velocity in the others. The inferior mesenteric artery may be difficult to accurately identify by duplex or color-flow imaging. In the fasting state, its Doppler spectral waveform mimics that of the fasting SMA, exhibiting low diastolic flow. Age-matched peak systolic velocities have not been well validated for the inferior mesenteric artery but most often range from 93 to 189 cm/sec. Postprandially, little immediate change occurs in the diastolic flow value. The signature Doppler waveform from the renal arteries resembles that from other vessels that feed organs with high flow demand (see Figure 13-8). The normal waveform from the proximal renal artery may demonstrate a clear systolic window, with minimal spectral broadening of velocities evident in the mid to distal segments of the vessel. This increase in spectral bandwidth occurs because the sample volume size used to monitor the flow is normally large in relation to the lumen of the vessel, or the sample volume may have been increased in size during the study to encompass the entire lumen of a poorly visualized artery. Normally, the renal artery peak systolic velocity is less than 100 cm/sec. Because the normal kidney has high metabolic demands and low vascular resistance, the Doppler spectral waveform from the interlobar and arcuate arteries of the renal medulla and cortex should demonstrate significant diastolic flow (Figure 13-9, A). Abdominal Vasculature 199 A B FIGURE 13-9 A, Doppler velocity waveform recorded from normal renal parenchyma. B, Diastolic flow component of the renal parenchymal Doppler velocity signal decreases as renovascular resistance increases due to intrinsic renal pathology. With increased renovascular resistance caused by intrinsic renal pathology, the end-diastolic flow component decreases throughout the vascular tree of the kidney, and the velocity waveform becomes markedly pulsatile (Figure 13-9, B). THE ABDOMINAL VENOUS SYSTEM Location The abdominal venous system consists of the IVC from the level of its origin at the union of the common iliac veins to the diaphragm, and its tributaries and the portal venous system (Figure 13-10). Because the hepatic artery shares a partnership with the venous circulatory supply of the liver, it will be considered in the discussion of the abdominal venous system. The IVC is formed by the confluence of the common iliac veins, which drain the lower extremities and pelvis. It normally courses superiorly through the retroperitoneum, lying on the right side of the body just anterolateral to the vertebral processes. The IVC lies medial to the right kidney and posterior to the liver before coursing through the diaphragm to enter the right atrium of the heart (Table 13-2). Although the normal IVC diameter is less than 2.5 cm, there is often a slight increase in diameter above the entry level of the renal veins because of the increased volume of blood returned from the kidneys. Body habitus, respiration, and right atrial pressure influence the diameter of the IVC. A number of anatomic anomalies have been recognized. The most common of these include duplication 200 Section III ABDOMINAL SONOGRAPHY Hepatic veins Right suprarenal vein Right renal vein Left suprarenal vein Left renal vein Left gonadal vein Iliac veins A Right portal vein Left portal vein Main portal vein Splenic vein Superior mesenteric vein B Inferior mesenteric vein FIGURE 13-10 A, Abdominal venous system showing major branches. B, Portal venous system. of the entire length or short segments of the IVC, segmental absence of portions of the vessel, and anatomic relocation of the suprarenal segment, infrarenal segment, or entire length of the IVC to the left of the aorta. Although the IVC gives rise to multiple tributaries, only those accessible to sonographic evaluation and included as part of the vascular ultrasound examination of the hepatoportal and renal systems will be discussed. The renal veins return blood from the kidneys to the systemic circulation, emptying into the IVC immediately superior to the level of the renal arteries. The left renal vein is longer than the right renal vein, coursing anterior to the aorta to lie between the aortic wall and the SMA (Figure 13-11). The left renal vein receives the left gonadal and suprarenal veins (see Figure 13-10, A). These smaller veins are not included in the routine evaluation of the renal venous system. The right renal vein is shorter than the left renal vein and may receive the right suprarenal vein. The hepatic veins empty into the IVC superior to the location of the renal veins. Normally, there are three major hepatic veins—right, middle, and left— that give rise to multiple branches within the parenchyma of the liver (Figure 13-12). The right and left hepatic veins empty the right and left lobes of the liver, respectively, and the middle hepatic vein drains the caudate lobe. The portal vein and its branches are intraabdominal vessels. The portal vein is formed by the confluence of the superior mesenteric and splenic veins (see Figure 13-10, B). It is located posterior to the neck of the pancreas, where the splenic vein can be found just medially (Table 13-3). The splenic vein may receive the inferior mesenteric vein before emptying into the portal vein. The superior mesenteric vein (SMV) returns blood from the small intestine and segments of the large intestine, where it courses superiorly, parallel with the inferior mesenteric vein. The main portal vein courses superiorly and laterally to the right for several centimeters before entering the liver through the porta hepatis. It divides into the left and right portal veins (Figure 13-13). The left portal vein courses horizontally to supply the left lobe of the liver, giving rise to several primary medial and lateral branches. The right portal vein courses to the right lobe of the liver and gives rise to anterior and posterior branches. The hepatic artery is one of the three primary branches of the celiac trunk. From its origin, it courses superiorly and right laterally to enter the porta hepatis (Figure 13-14, A) with the portal vein and common bile duct (Figure 13-14, B). It branches into the right and left trunks, which have multiple subdivisions that carry arterial blood flow to the right and left lobes of the liver. Size The size of the abdominal veins varies with respiration. The diameters indicated below are those associated with expiration: NORMAL MEASUREMENTS Anatomy Measurement IVC Renal veins Hepatic veins SMV Splenic vein Portal vein 25 to 35 mm 4.0 to 6.0 mm 4.0 to 7.0 mm 6.0 to 7.0 mm 4.0 to 6.0 mm 13 mm Chapter 13 201 Abdominal Vasculature Table 13-2 Location of Inferior Vena Cava and Tributaries Routinely Visualized With Ultrasound Anterior to Posterior to Superior to Inferior to Medial to Inferior Vena Cava (IVC) Hepatic Veins Rt Renal Vein Spine, rt crus of diaphragm, rt psoas major muscle, rt renal artery, rt adrenal gland Pancreas head/uncinate process, transverse duodenum, portal vein, CBD, posterior surface of liver, hepatic veins IVC Quadratus lumborum muscle, rt renal artery Diaphragm Rt renal vein, rt kidney, rt ureter, rt adrenal gland Lt lateral to Rt lateral to Pancreas head Renal veins Diaphragm Common iliac veins SMA, rt adrenal gland Rt kidney Lt renal vein, aorta, caudate lobe liver IVC Lt Renal Vein Rt Common Iliac Vein Lt Common Iliac Vein Anterior to Posterior to Superior to Inferior to Medial to Aorta, lt renal artery Pancreas body/tail Common iliac veins SMA, lt adrenal gland Lt kidney Psoas major muscle Rt common iliac, artery Psoas major muscle Lt common iliac artery Renal veins Lt common iliac artery Rt common iliac vein and artery Lt lateral to Rt lateral to IVC Renal veins Lt common iliac vein and artery, rt common iliac artery IVC, Inferior vena cava; SMA, superior mesenteric artery. Anterior Superior mesenteric artery Left renal vein Right Left Aorta Posterior Left renal artery FIGURE 13-11 Transverse image of an axial section of the abdominal aorta demonstrating its posterior relationship to the long section of left renal vein and axial section of the superior mesenteric artery. Sonographic Appearance In addition to the following discussion, the sonographic appearance of the abdominal veins is covered in Chapter 11, The Inferior Vena Cava, and Chapter 12, The Portal Venous System. The IVC normally appears on ultrasound images as an anechoic structure whose diameter varies with changes in respiration. Deep inspiration causes increased abdominal pressure and impedes venous return from the abdomen. This results in dilation of the IVC. Dilation can also occur in the presence of congestive heart failure, tricuspid regurgitation, or any condition that results in increased right atrial pressure. 202 Section III ABDOMINAL SONOGRAPHY Hepatic veins Anterior Right Left Liver Inferior vena cava Posterior FIGURE 13-12 Subcostal longitudinal gray-scale image of the right, middle, and left hepatic veins at their confluence with the inferior vena cava. Table 13-3 Location of the Portal Venous System Routinely Visualized With Ultrasound Main Portal Vein Splenic Vein Superior Mesenteric Vein Inferior Mesenteric Vein Anterior to IVC, pancreas uncinate process Lt kidney, lt renal vein, lt renal artery, SMA, rt renal artery Lt common iliac vessels, lt psoas major muscle Posterior to Pancreas neck, superior duodenum, CHA, PHA, common duct, peritoneum Pancreas head, SMV, IMV, splenic vein Porta hepatis, rt portal vein branch, lt portal vein branch Cystic duct Pancreas neck/body/ tail, peritoneum Pancreas/inferior head/ uncinate process, IVC, rt ureter, inferior duodenum Pancreas neck, peritoneum Superior to Inferior to Medial to Lt lateral to Rt lateral to SMV, IMV, splenic vein, pancreas head, aorta celiac artery Pancreas body, peritoneum SMV, IMV Splenic artery, portal vein Spleen, portal vein IVC, pancreas head SMA, aorta Splenic vein, portal vein Pancreas head, duodenum Splenic vein, portal vein SMV, portal vein, pancreas head Splenic vein IMV, splenic vein, SMA CHA, Common hepatic artery; IMV, inferior mesenteric vein; IVC, inferior vena cava; PHA, proper hepatic artery; SMV, superior mesenteric vein. The hepatic veins normally appear sonographically as anechoic structures, which lack echogenic walls. Their diameter may appear small within the parenchyma of the liver, but it increases in the region of the caval confluence. Several approaches can be used to interrogate the hepatic veins sonographically. Most often, these veins can be insonated from a subcostal approach or from a right intercostal approach. Most often all three branches can be visualized; occasionally, only two branches are seen from the subcostal approach (sometimes referred to as the “bunny sign”). The main portal vein can be interrogated from a right intercostal approach with the transducer angled toward the porta hepatis. Within the porta hepatis, the portal vein is closely associated with the hepatic artery and the common bile duct. Color-flow imaging will facilitate definition of the course of the vein, its branches, and the direction of flow. It should be noted that the hepatic veins are boundary formers coursing longitudinally toward the IVC, whereas the portal veins branch horizontally and are oriented toward the porta hepatis. The walls of the main portal vein and its branches are echogenic because of the collagen content found in Chapter 13 Right portal vein branch Abdominal Vasculature Anterior 203 Left portal vein branch Main portal vein Right Left Liver Posterior FIGURE 13-13 Oblique, longitudinal, intercostal, contrast-enhanced gray-scale image of main, right, and left portal veins. (Courtesy Edward Grant, M.D.) the intimal and medial layers of the vein wall. This feature is in sharp contrast to the sonographic appearance of the hepatic vein walls. The diameters of the left and right portal veins are greater at their origin in the region of the porta hepatis; minimal changes in diameter are noted during respiration. Normally, the diameter of the main portal vein is less than 13 mm in the segment just anterior to the IVC. The diameter increases during expiration and decreases during inspiration as a result of variation in the volume of blood entering the visceral arterial system and the volume outflow through the systemic venous channels. Longitudinal sections of the renal veins appear sonographically as anechoic tubular structures extending from the renal hila to the posterolateral walls of the IVC. Patency and flow direction in the renal veins is facilitated by using a combination of color and spectral Doppler. Hemodynamic Patterns and Doppler Spectral Display The IVC and its tributaries drain the lower extremities, large and small intestines, kidneys, and liver. In contrast to other systemic veins, the portal venous system supplies, rather than empties, a major organ system. The IVC demonstrates complex flow patterns in its proximal segment as a result of variations in intraabdominal pressure associated with respiration and regurgitation of blood from the right atrium during atrial systole (Figure 13-15, A). Distally, the IVC flow pattern reflects the phasic flow patterns seen in the peripheral veins (Figure 13-15, B). Color-flow imaging reveals directional variations associated with respirophasicity in the distal segment of the vein and reflected right atrial pulsations proximally. Velocities are variable but remain low throughout the length of the vessel. In contrast, the hepatic veins exhibit pulsatility, a reflection of cardiac and respiratory activity (Figure 13-16). Characteristically, the normal hepatic venous flow pattern is similar to that seen in the proximal IVC. The right, middle, and left hepatic veins should demonstrate three phases of flow. The first two are toward the heart and represent reflections of right atrial and ventricular diastole. The third phase is represented by systolic flow reversal and is caused by contraction of the right atrium. This flow pattern yields a W-shaped waveform as a result of changes in central venous pressure, respiration, and compliance of the liver parenchyma. Normal flow direction is hepatofugal, or away from the liver. Flow should be found throughout all segments of the right, middle, and left hepatic veins without significant disturbance at the hepatocaval confluence. Intraabdominal pressure effects associated with respiration are transmitted through the liver to the portal and splanchnic veins, causing an undulating flow pattern in the portal venous system (Figure 13-17). The portal vein and its tributaries are responsible for approximately 70% of the oxygenated blood supply of the liver. Normally the high-volume portal venous flow pattern is characterized by minimally phasic, slightly disordered flow with low peak and mean velocities (20-30 cm/sec) in the supine, fasting patient. Flow direction should be hepatopetal, or toward the liver. Portal venous flow normally accelerates during 204 Section III ABDOMINAL SONOGRAPHY Common hepatic artery Anterior Splenic artery Right Left A Posterior Celiac artery Anterior Liver Portal vein Hepatic artery Right Left Kidney B Posterior FIGURE 13-14 A, Color-flow image of a longitudinal section of the hepatic artery at its origin from celiac artery bifurcation. B, Color-flow image of hepatic artery as it courses with portal vein in the porta hepatis. expiration and decelerates during inspiration. Portal venous flow is affected by posture, exercise, and dietary state. Exercise and postural changes usually cause a decrease in portal venous flow, whereas eating will increase flow as a result of splanchnic vasodilation and hyperemia. To control variations in flow, patients should be examined in the supine or left lateral decubitus position after an 8-hour fast. The renal veins carry blood from tributaries within the renal medulla and cortex and empty into the IVC. Their flow patterns are influenced by the systemic circulation. For this reason, they do not exhibit pulsatility associated with atrial or ventricular contraction (Figure 13-18). The hepatic artery is normally responsible for approximately 30% of the oxygenated blood supply to the liver. Because the liver is a low-resistance end organ, the Doppler spectral waveform pattern from the hepatic artery is characterized by constant forward flow throughout the cardiac cycle (Figure 13-19). Peak systolic velocity is normally less than 100 cm/sec. When portal venous flow is compromised, velocity most often increases in the hepatic artery as a result of collateral compensatory mechanisms. Chapter 13 10 cm/s -10 -20 A IVC DIST -10 -5 cm/s INVERT 5 10 15 B FIGURE 13-15 A, Doppler spectral waveform from the proximal inferior vena cava. B, Doppler spectral waveform from the distal inferior vena cava. 10 cm/s -10 FIGURE 13-16 Doppler spectral waveform from a hepatic vein exhibiting the influence of cardiac and respiratory activity on its waveform pattern. 40 cm/s FIGURE 13-17 Doppler spectral waveform from the portal vein demonstrating the minimally phasic flow pattern associated with this vessel. 20 cm/s -20 FIGURE 13-18 Doppler spectral waveform from a renal vein exhibiting the influence of the systemic venous circulation. 80 60 40 20 cm/s FIGURE 13-19 Doppler spectral waveform pattern from the low-resistance hepatic artery. Abdominal Vasculature 205 REFERENCE CHARTS ASSOCIATED PHYSICIANS • Vascular surgeon: Specializes in the surgical and/or endovascular treatment of abdominal vascular disorders. • Gastroenterologist: Specializes in the treatment of disorders involving the gastrointestinal system. • Nephrologist: Specializes in treatment of disorders involving the kidneys. • Interventional vascular radiologist: Specializes in the endovascular treatment of abdominal vascular disorders. COMMON DIAGNOSTIC TESTS • Vascular angiography: A contrast medium is injected into an artery or vein, and radiographic films are taken at specific intervals to observe blood flow patterns in vessels and organ vasculature. Performed by interventional vascular radiologists and vascular surgeons, assisted by radiologic technologists. Interpreted by interventional vascular radiologists and vascular surgeons. • Computed tomography angiography: A contrast medium is injected intravenously while x-ray data are acquired continuously during a single breath hold or as a bolustracking method. The acquired data are reconstructed and displayed as axial slices or in 3-dimensional format. Performed by interventional vascular radiologists and vascular surgeons, with assistance by radiologic technologists. Interpreted by interventional vascular radiologists and vascular surgeons. • Magnetic resonance angiography: There are three types of magnetic resonance angiography (MRA). The first type is unenhanced, meaning it uses no contrast agent. The second type, an enhanced MRA, employs the contrast agent gadolinium and is not useful for imaging vessels less than 1 mm in diameter. The third type of MRA is referred to as phase-sensitive imaging. This method acquires paired images in either 2 or 3 directions. Each pair has a different sensitivity to flowing blood. The collected images are combined to create a 3-dimensional image. Performed and interpreted by interventional vascular radiologists. LABORATORY VALUES Nonapplicable VASCULATURE See Chapters 10 through 12 for discussions of the abdominal aorta, inferior vena cava, portal vein, and related structures. AFFECTING CHEMICALS Nonapplicable