Vascular Structures



Vascular Structures


Aubrey J. Rybyinski
















This chapter stresses the importance of learning how to identify normal and abnormal abdominal vessels and how to assess normal and abnormal blood flow. The many branches of arterial flow and the confluences of venous return provide a comprehensive road map for identifying normal and abnormal anatomy and anatomic relationships. Current sonography equipment makes it possible to accurately image smaller vessels and to detect intraluminal abnormalities such as thrombi and tumors.1 Current instrumentation provides the ability to assess blood flow, to obtain hemodynamic information regarding the many factors that influence the dynamics of blood flow, and the hemodynamic consequences of vascular disease.


SONOGRAPHY EXAMINATION


Patient Preparation

It is recommended that all patients fast for at least 4 hours prior to vascular scanning and refrain from chewing gum or smoking. Fasting tends to reduce the amount of air in the abdomen, which has the potential to obscure the anatomy of interest. In emergency situations, however, scanning can be accomplished without any patient preparation.2 If Doppler interrogation is desired, studies should be performed in a consistent manner to reduce result variability. Many factors affect blood flow hemodynamics including ingestion of a meal, respiratory changes, and postural changes. In some instances, it may be helpful to perform preprandial and postprandial studies to aid in the diagnosis of abnormal blood flow states.


Equipment Considerations

With an understanding of the relationship between resolution and beam penetration, the sonographer may select either a sector or linear transducer operating at the highest clinically appropriate frequency. For adults, the common grayscale scanning frequency is 5 MHz and lower. Doppler is essential for investigating blood flow hemodynamics. The Doppler frequency may vary from the imaging frequency. Color-flow instrumentation provides rapid evaluation and visualization of blood flow and can reduce examination time.


Scanning Techniques

Most of the abdominal vasculature can be identified with the patient lying in the supine position. Standard scanning of the aorta and inferior vena cava (IVC) commences with the transducer placed in the subxiphoid position and oriented to the transverse plane. Because of their proximity, the aorta and IVC can be demonstrated in the transverse plane simultaneously, and once they are identified, gain settings should be adjusted to reveal their characteristic echo-free lumen. Some reverberation artifact may be present along the anterior aspect of each vessel. This is considered normal because of the strong reflective interface of each of the vessels’ walls. Scanning continues inferiorly until the aortic bifurcation is reached. Images are recorded at 1- to 2-cm intervals along the course of the aorta and IVC, and additional sections are recorded in any area of disease. Once transverse scanning is completed, the aorta and IVC should be imaged in longitudinal sections. Images are recorded in segments to demonstrate the entire length of each vessel.

If specific arterial branches or venous tributaries are being investigated, the examination should begin with the transducer positioned near the vessel’s origin. Transducer manipulations are then carried out in an attempt to follow the course of the vessel. Images are recorded to demonstrate vessel length as clearly as possible and any disease that may be present.

A sonographer examining the many vascular branches and pathways in the abdomen must have a working knowledge of the general course throughout. Once each vessel’s site of origin is known and the general course of the abdominal vasculature is learned, sonographic examination is made easier. Diligent Doppler sampling is the most reliable and accurate method for confirming arteries versus veins.


VASCULAR ANATOMY

Blood is distributed throughout the body by a vast network of arteries and veins. In the systemic circulation, arteries transport blood from the heart to the muscles and organs, and veins transport blood from the muscles and organs back to the heart. Typically, blood vessels are composed of three distinct layers: (1) the tunica intima, (2) the tunica media, and (3) the tunica adventitia. The tunica intima, the innermost section of a vessel wall, consists of an endothelial lining and elastic tissue. Elastic fibers and smooth muscle constitute the second layer, the tunica media. The outer portion of the vessel wall, the tunica adventitia, is composed of elastic and collagen fibers.3

Although arteries and veins are histologically similar, there are differences in the distribution of each tissue within the walls that reflect pressure differences between the two systems. For example, arterial walls are thicker and contain more elastic and smooth muscle fibers than veins. This is true especially in the tunica media, which is the thickest layer of an artery and is largely responsible for its very elastic and contractile characteristics. Because of the thickness of arterial walls, they tend to maintain a constant shape and do not readily collapse in conjunction with low blood pressure.3,4

Because veins have less smooth muscle and elastic tissue, they are unable to contract to force through blood. Venous return to the heart is, therefore, accomplished through the pressure gradient difference between the arterial and venous network, breathing, and skeletal muscle contractions. Valves are also an important part of the veins located in the extremities. The circulatory network of blood vessels, the vasa vasorum, is located within their walls.5


Abdominal Aorta and Aortic Branches


Anatomy

The aorta is the main artery of the chest and abdomen from which all other branch vessels are derived. For reference purposes, the aorta is divided into segments along its path (Fig. 6-1).

The aorta originates from the left ventricle. As it leaves the heart, systemic circulation begins. The aorta arises from the left ventricular outflow tract and then courses slightly
posterior, a short distance medial, and then superior to form the ascending aorta. It then curves lateral and posterior to form the aortic arch. As the aorta completes its curve at the arch, it begins to descend inferiorly into the chest. This portion, the descending aorta, soon gives rise to the thoracic aorta. Once the aorta penetrates the diaphragm, it is termed the abdominal aorta until it bifurcates into the common iliac arteries prior to entering the pelvic cavity. It is the abdominal aorta that is most accessible to sonographic examination (Fig. 6-2).






As it courses through the abdomen, several major vessels branch off of the abdominal aorta. The first branch is the celiac axis (CA; also known as the celiac trunk). It originates from the anterior aspect of the aorta and is usually found within the first 2 cm. The CA is a short vessel, approximately 1 cm long. It divides into three branches: (1) the hepatic artery, (2) the left gastric artery, and (3) the splenic artery.3,4 The vessel may present with anatomic variations on the number of branches (Fig. 6-3A-C).

The hepatic artery leaves the CA at approximately a 90-degree angle, and it crosses the midline and courses toward the right side of the abdomen, following the upper border of the pancreatic head. At the duodenum, the hepatic artery turns anteriorly to enter the liver hilum, following the course of the main portal vein. Intrahepatically, the artery then divides into left and right branches at the portal fissure to supply the left and right hepatic lobes, respectively.3,4,6,7 The left gastric artery initially has an anterior and a superior course from the CA. It then turns lateral to the left side of the abdomen to supply the stomach and esophagus with blood.3,4,6,7 The splenic artery takes a horizontal course from the CA and follows the upper margin of the pancreatic body posteriorly. Along its route to the spleen, it generates arterial branches to the stomach and pancreas.3,4,6,7






The second major branch vessel, the superior mesenteric artery (SMA), also originates from the anterior surface of the aorta approximately 1 to 2.5 cm distal to the CA (although this distance varies), and occasionally it may branch off of the CA. As it begins to course inferiorly, it travels posterior to the pancreatic body and anterior to the uncinate process. It then continues inferiorly, paralleling the aorta. The left renal vein, duodenum, and uncinate process pass between the SMA anteriorly and the aorta posteriorly. The nutcracker phenomenon refers to the compression of the left renal vein between the aorta and the SMA (like a nut in a nutcracker) (Fig. 6-4A). Several branches arise along the length of the SMA and are responsible for supplying the small and large bowel with blood (Fig. 6-3A-C).

The renal arteries are located just inferior to the SMA. The right renal artery tends to arise from the right lateral aspect of the aorta, whereas the left renal artery tends to arise from the left lateral or posterolateral aspect of the aorta. Both then course posterolaterally to enter the respective kidneys.3,4,6,7

The inferior mesenteric artery (IMA) is the last major branch to arise from the abdominal aorta before it bifurcates. It originates from the anterior aspect of the aorta and runs slightly inferiorly and to the left side of the abdomen. It is responsible for supplying the distal portion of the colon with blood.













At about the level of the umbilicus, the aorta bifurcates into the left and right common iliac arteries. The common iliac arteries are about 5 cm in length (right somewhat longer than the left) and course inferiorly and posteriorly until they branch into the external and internal (hypogastric) iliac arteries. As a result of their fairly deep location in the pelvis, the iliac arteries can be very difficult to image. Full bladder techniques as well as left and right lateral decubitus position may be necessary to visualize them (Fig. 6-4B).3,4,6,7


Sonographic Appearance

Sonographically, the lumen of the aorta and other vascular structures appear anechoic. It is important that the sonographer optimizes gain settings to demonstrate normal vessels as anechoic structures. In the longitudinal plane, slightly to the left of midline, the proximal aorta can be seen as an anechoic tubular structure following a somewhat anterior and inferior course within the abdomen. The spine lies immediately posterior to it, providing a highly reflective echo boundary. As the aorta courses inferiorly, it tapers and becomes smaller in caliber. In the proximal aspect of the aorta, both the CA and the SMA can be seen as they arise anteriorly. A longitudinal section provides the best scanning plane to image the proximity of the CA and SMA (see Figs. 6-3C and 6-4).

In transverse scan planes, the aorta takes on a more rounded appearance and again can be seen lying anterior to the spine. Because the transducer is moved inferiorly from the xiphoid process, the first aortic branch to be encountered is the CA. It appears as an anechoic tubular structure that divides into the hepatic artery and the splenic artery. If viewed in the appropriate plane, the image may resemble the spread wings of a seagull (see Fig. 6-3A). The hepatic artery branches off of the right side of the CA and can be followed transversely and superiorly as it travels to enter the liver hilum. The splenic artery branches off of the left side of the CA and courses to the left side of the abdomen to enter the splenic hilum (Fig. 6-5A). The splenic artery can be quite tortuous and is difficult to image in its entirety, especially in elderly patients.

The left gastric artery can occasionally be seen in its proximal aspect. This vessel, however, is usually smaller in caliber than the neighboring hepatic and splenic arteries and is more difficult to image consistently.

After moving the transducer inferiorly, the SMA can be seen. It appears rounded and is surrounded by an echodense collar consisting of mesentery and fat (see Fig. 6-3B).

Immediately inferior to the level of origin of the SMA are the origins of the renal arteries. They are best appreciated in the transverse plane because of their relationship to the acoustic beam. The renal arteries arise from the lateral aspect of the aorta and continue their course, respectively, to the right and left to enter the kidneys. From the right flank, a coronal plane shows the aorta and bilateral renal arteries and gives the appearance of a peeled banana (Fig. 6-5B, C). A transverse plane at this level can demonstrate the nutcracker phenomenon, with the left renal vein being compressed between the aorta and SMA (Fig. 6-5D).

The IMA can be seen approximately midway between the renal arteries and the aortic bifurcation arising anterolaterally from the aorta, sometimes having the appearance of pumpkin stem (Fig. 6-5E). Soon after its origin, the IMA makes an abrupt turn in the posteroinferior direction. High-frequency probes and compression of overlying bowel loops greatly facilitate visualization of this artery, as does color-flow Doppler.

At the level of the umbilicus, the right and left common iliac arteries can be seen as they arise from the aortic terminus as rounded and anechoic vessels emerging from a common source (distal aorta). In the longitudinal plane, the common iliac artery can be seen bifurcating into the external and internal (hypogastric) iliac arteries. Further,

more comprehensive imaging of the iliac arteries is accomplished by placing the transducer in the iliac fossa and angling medially with the scan plane oriented approximately 45 degrees from midline. Demonstration of the length of the iliac arteries is thus achieved. At times, successful imaging of the iliac arteries requires a distended urinary bladder. In this case, the transducer is placed in the midline of the pelvis and oriented 45 degrees from midline. Lateral angulation will result in visualization of the iliac vessels.







Abdominal Veins


Anatomy

The IVC is the large vessel that returns blood to the right atrium from the lower limbs, pelvis, and abdomen. It is formed by the junction of the paired common iliac veins slightly anterior and to the right of the fifth lumbar (L5) vertebral body. The IVC travels superiorly in the abdomen, enters the thoracic cavity and then the right atrium at the level of the eighth thoracic (T8) vertebral body. As the vena cava nears the heart, it courses somewhat anteriorly to form a hockey stick-like configuration before it terminates in the right atrium (Fig. 6-6A, B).

There are many tributaries to the IVC, but most cannot be seen because of their small size. The veins most consistently seen entering the IVC are the common iliac veins at its formation, the renal veins, and the hepatic veins. The right renal vein is generally shorter than the left renal vein because of the right kidney’s proximity to the IVC. The left renal vein traverses the abdomen, coursing anterior to the aorta and posterior to the SMA to finally enter the lateral aspect of the IVC.

The hepatic veins also drain directly into the IVC or right atrium. Normally, there are three hepatic veins: (1) the left, (2) the right, and (3) the middle (Fig. 6-7).


Sonographic Appearance

Sonographically, the IVC is an anechoic structure slightly to the right of midline. Unlike the aorta, which has a relatively consistent diameter and a rounded appearance in the transverse plane, the IVC tends to have a more oval shape. It also responds to respiratory variations. During inspiration, the IVC collapses, owing to the decreased pressure within the thoracic cavity, allowing prompt blood flow from the IVC into the right atrium. The opposite is true for expiration
in which the IVC expands during this maneuver. With suspended inspiration, the IVC expands because of increased intrathoracic pressure and decreased blood flow into the heart. During the Valsalva maneuver, the IVC collapses because of the increased abdominal pressure associated with this technique.8,9











The hepatic veins are best demonstrated in a transverse plane with the transducer just inferior to the xiphoid process and angled cephalic. Identification of the right, middle, and left hepatic veins is relatively easy because they converge to empty into the IVC (Fig. 6-8A). Partial imaging of the right hepatic vein with simultaneous imaging of the middle and left hepatic veins will result in a rabbit ear appearance (Playboy Bunny sign).

Optimal imaging of the renal veins is also accomplished using a transverse scanning approach. The renal veins should be visualized at about the same level as the renal arteries (just inferior to the origin of the SMA) (Fig. 6-8B). The right renal vein is best imaged with the transducer placed in the right lateral abdomen over the right kidney and angled medially. The renal vein is identified as an echo-free tube exiting the renal hilum. When attempting to visualize the left renal vein, the transducer is placed (in a transverse orientation) in the midline of the abdomen just inferior to the SMA origin. The left renal vein is seen as an anechoic tubular structure coursing between the SMA and the aorta to enter the lateral aspect of the IVC.


Portal Venous System Anatomy

The portal venous system is composed of the veins that drain blood from the bowel and spleen and is separated from the IVC.

The main portal vein is formed at the junction of the splenic vein and the superior mesenteric vein, which can be identified with sonography in most patients (see Fig. 6-7). To image the portal venous system, it is easiest to begin by placing the transducer in the midline of the abdomen substernally with a transverse orientation. The splenic vein can be used as an initial reference point because it is easily seen in this plane. The splenic vein emerges from the splenic hilum and courses medially and superiorly within the abdomen, bordering the posterior surface of the pancreatic body and tail. It is identified sonographically as a tubular structure with a superomedial course within the abdomen coursing anterior to the SMA as it nears the midline. At its termination, the splenic vein can be seen to increase in diameter. This is the point at which the splenic vein merges with the superior mesenteric vein to form the main portal vein (Fig. 6-9A). The junction of the superior mesenteric vein and the splenic vein is known as the portal confluence, and this confluence is immediately posterior to the neck of the pancreas. Visualization of the superior mesenteric vein is accomplished by placing the transducer over the portal confluence (transverse orientation) and rotating the transducer 90 degrees. The length of the superior mesenteric vein will be displayed having a longitudinal course within the abdomen that parallels that of the IVC posteriorly (Fig. 6-9B).






The main portal vein travels somewhat obliquely and anteriorly within the abdomen before it enters the liver hilum (Fig. 6-9C). Placement of the transducer over the portal confluence and subsequent clockwise rotation eventually demonstrates the portal vein in its long axis. It can then be followed into the liver where it soon divides into left and right branches.

Other vessels contributing to portal venous circulation include the inferior mesenteric vein, coronary vein, pyloric vein, cystic vein, and paraumbilical veins. These generally are not seen on routine abdominal examinations but may be identified in abnormal states and are discussed later in this chapter.








Relational Anatomy of the Arteries and Veins

To perform sonography, it is clinically useful to know the relationship of abdominal arteries and veins to each other as well as to the ducts and organs.10 Figure 6-10 illustrates the information summarized in Table 6-1.


ARTERIAL ABNORMALITIES


Atherosclerosis


Description

Atherosclerosis is a form of arteriosclerosis in which the intimal lining of the arteries is altered by the presence of any combination of focal accumulation of lipids, complex carbohydrates, blood and blood products, fibrous tissue, and/or calcium deposits. The media of the arterial wall is also changed (Fig. 6-11).11


Etiology

The cause is not known, but several factors have been linked to the progression of atherosclerosis and they include hyperlipidemia, hypertension, cigarette smoking, and diabetes mellitus.12


Clinical Signs and Symptoms

Generally, there are no symptoms of atherosclerosis until a significant stenosis develops. Then, symptoms vary and are related to the particular stenotic vessel. These are discussed later. Atherosclerotic disease also disposes to the development of aneurysms. There are generally no symptoms unless complications arise because they are often diagnosed as a result of screening or other imaging studies.


Sonographic Appearance

The sonographic findings of atherosclerosis include luminal irregularities (representative of the various changes of the intimal lining of the artery), tortuosity, and vessel wall calcification.

The wall irregularities detected by sonography can be seen as low-level echoes along the internal walls of the aorta with a propensity for development at the areas of bifurcation or the branch vessels.1,13 In and of themselves, these areas of plaque formation are not terribly important unless they produce hemodynamically significant stenosis of a particular
branch artery. Detection of a hemodynamically significant stenosis is discussed in detail later in this chapter.













In elderly persons, tortuosity of the aorta is often seen leftward in the patient but occasionally can be right sided.6 Imaging of the vessel is best carried out in the transverse plane because this affords a clearer picture of the course of a tortuous aorta.

Aortic wall calcification is easily detected as an echogenic focus in the arterial wall, which at times may produce acoustic shadows.








Aneurysms of the Abdominal Aorta


Description

An aneurysm is a focal abnormal dilatation of a blood vessel caused by a structural weakness in its wall. True aneurysms involve all three layers of the arterial wall. A false aneurysm (also called a pseudoaneurysm) is an extravascular hematoma communicating with the intravascular space. A saccular aneurysm is a saclike protrusion of the aorta toward one side or the other, is usually larger than a circumferential, and is connected to the aorta by a channel or an opening that varies in size. Most true aneurysms are fusiform and circumferential. A fusiform aneurysm is a gradual and progressive dilatation of the complete circumference of the vessel, which varies in diameter and length. A dissecting aneurysm is when a longitudinal tear in the arterial wall allows bleeding to occur into the wall. Saccular and circumferential aneurysms occur more often in the abdominal aorta, whereas dissecting aneurysms are less common and occur more often in the thoracic aorta (Fig. 6-12). Most abdominal aortic aneurysms occur below the level of the renal arteries.12


Etiology

Smoking is a risk factor for aneurysm as well as atherosclerosis. Syphilis and other diseases can cause aneurysms, although these are not very common causes.2,12


Clinical Signs and Symptoms

Generally, patients with aneurysms are asymptomatic, and the presence of an aneurysm is suspected during palpation of a pulsating mass in the region of the umbilicus, or by calcification seen on an abdominal radiograph. Patients with an expanding aneurysm may have vague lower back or abdominal pain.2


Sonographic Appearance

At the diaphragm, normal aortic diameters have been cited at approximately 2.5 cm.6 During its course, inferiorly the aorta tapers, reaching a diameter of about 1.5 to 2.0 cm at the level of the iliac arteries.6 Ectasia of the aorta, as seen with atherosclerosis, is manifested by a slight widening of the normal aortic diameter up to 3.0 cm. There are also aortic wall irregularities, owing to the atherosclerotic changes that take place in this disease process. A true aneurysm is identified sonographically as a dilatation of the aorta ≥3.0 cm near its bifurcation point, a focal dilatation along the course of the aorta, or lack of normal tapering of the aorta.14, 15, 16 and 17






Aneurysms vary in size and can range from 3 to 20 cm as a result of the abnormal blood flow patterns within an aneurysm. If thrombus is formed, it can usually be detected by sonographic techniques and is a common finding. Sonographically, thrombus typically produces a low-level echo pattern and tends to accumulate along the anterior and lateral walls of the aortic lumen (Fig. 6-13A-H).14,15 Adequate demonstration of the thrombus may require that gain settings be increased from initial settings to display the low-level echoes associated with thrombus. It may also be necessary to scan coronally or obliquely through the aorta to demonstrate thrombus. These maneuvers may help reduce confusion between reverberation artifacts and actual thrombus. Occasionally, there may be calcification within the thrombus. An interesting phenomenon that has been reported in association with aortic aneurysm thrombus is that of an anechoic crescent sign (Fig. 6-13C). In these instances, the anechoic area within the lumen of the aneurysm was found at surgery to be serosanguineous fluid or liquefying clot. When evaluating the aorta for aneurysm formation, it is important to distinguish this finding from aortic dissection because the surgical treatments are different.

Thrombus within the aorta may be difficult at times to visualize, especially in an obese or a gassy patient. Anterior reverberation artifacts from a calcific anterior aortic wall may obscure the clot as well. Instances have been reported in which an obstruction clot of the aorta was not detected sonographically.18 If an obstructing clot is suspected on clinical grounds, Doppler examination of the aorta can confirm the presence or absence of flow within it, thereby solving the problem (Fig. 6-13D).

If an aneurysm is detected during sonographic examination, it is prudent to attempt to identify the origins of the renal arteries as well as to extend the examination to the iliac arteries to look for aneurysmal involvement in these areas.

Associated renal artery aneurysm in conjunction with abdominal aortic aneurysm has been reported to be 1% or less.19 Nonetheless, it is important for the surgeon to know of this coexistence because of the difference in treatment procedures. When the renal arteries are involved in an aneurysm, renal artery enlargement generally coexists with aortic dilatation. Demonstration of this complication, however, can be quite difficult because large aneurysms tend to displace surrounding bowel superiorly and subsequently cover the renal artery origins.19 Consequently, diligent scanning techniques involving multiple patient positions and numerous transducer angulations may be necessary before adequate visualization of the renal artery origins is achieved. Color-flow Doppler may also aid visualization of the renal arteries in such patients. If efforts to identify the renal arteries are unsuccessful, an attempt should be made to visualize the SMA. Because of the proximity of the renal arteries to the SMA, any aneurysm shown to involve the SMA also involves the renal arteries.

Abdominal aneurysms may also extend into the iliac arteries. When this occurs, the iliac arteries will be abnormally dilated, and the thrombus may or may not be present in the dilated areas. Isolated iliac artery aneurysms are an occasional finding.2 On occasion, aneurysm thrombosis may lead to peripheral thromboembolism or acute limb ischemia.

The accuracy rate for the sonographic detection of aortic aneurysms approaches 100% in most reports.5,20, 21 and 22 Because of this, sonography is a very good screening tool as the first step

in the evaluation of suspected aortic aneurysm and can be used to monitor the growth of aneurysms over time.23 However, there are some important considerations to keep in mind to avoid misdiagnosis of an aneurysm. Tortuosity may make the aortic diameter appear larger than it is. This occurs when the plane of imaging is not truly perpendicular to the aortic walls. Therefore, careful observations should be made of the aortic curvature in these instances to avoid misrepresentation of a tortuous aortic segment as an aortic aneurysm. Excessive air in the abdomen or obesity may obscure the distal aorta and iliac vessels and render some aneurysms invisible. Lymphadenopathy may also confound the picture.21,22






Normally, the abundant lymph nodes that are linked together chainlike along the anterior and lateral aspects of the aorta are invisible sonographically. When enlarged, however, their appearance can be dramatic—and initially confusing. Sonographically, enlarged lymph nodes are echo poor, but with increased gain settings, fine internal echoes may be appreciated. Several patterns of lymph node enlargement have been described: isolated large masses, which tend to develop along the aortic chain; mantle-like distributions of enlarged nodes draped atop the aorta and IVC; symmetric nodal enlargement along the aortic chain bilaterally; multiple spindle-shaped nodes dispersed in the mesentery; and large, confluent masses surrounding the aorta and IVC.14 It is conceivable that the mantle-like configurations and the confluent mass effects may be confused with aortic aneurysm with thrombus. Close inspection of the area should reveal linear separations between lymph node masses. In addition, the general appearance of extensive lymph node enlargement seems to be slightly more irregular, or “lumpy,” than an aortic aneurysm. The most reliable and accurate tool to use to determine artery versus vein versus lymph node is to Doppler sample for the presence or absence of arterial or venous flow patterns.

The rates with which sonography can accurately detect renal artery involvement and other abnormalities (ruptured aneurysm) are unfortunately not as high as those for aneurysm detection. Therefore, other diagnostic imaging tests may be necessary to further evaluate these complications if they are suspected.24

Although other imaging procedures may be the primary tool used to evaluate the extent of various aortic pathologies, sonographic evaluation is a sensitive and specific imaging technique of choice for screening patients suspected of having aortic aneurysms. The sonographic measurements are accurate, repeatable, and noninvasive and do not involve ionizing radiation.20,25


Aortic Dissection


Description

In aortic dissection, there is a separation of the layers of the arterial wall by blood or hemorrhage, which generally begins in the proximal portion of the aorta. According to the DeBakey model, there are three types of dissections (Fig. 6-14). Type I and type II involve the ascending aorta and the aortic arch, and type III involves the descending aorta at a level below the left subclavian artery. There is a high incidence of mortality with type I and type II dissections because of the propensity of the dissection to extend into the pericardium. Once dissection has begun, it may extend for varying distances along the length of the aorta.26 The Stanford classification is used to separate aortic dissections into those that need surgical repair and those that usually require only medical management. The Stanford classification divides dissections by the most proximal involvement. Type A “affects ascending aorta” and requires surgical management. Type B “begins beyond the brachiocephalic vessels” and is treated with medical management with blood pressure control. Dissections that involve the aortic arch but not the ascending aorta have been addressed in the American surgical consensus 2020.27







Etiology

The etiology of aortic dissection is not clear. Presumably, the dissection results from a tear of the intimal lining of the aorta. It has been demonstrated, however, that this is not always the case, and postulation has been made that rupture of the vasa vasorum can initiate a dissection.12,27 Hypertension is strongly associated with dissections, and cystic medial necrosis of the vessel is also well recognized as an underlying cause. Other entities that contribute to aortic dissection include Marfan syndrome, pregnancy, aortic valve disease, congenital cardiac anomalies (coarctation, aortic hypoplasia, bicuspid aortic valve, persistent patent ductus arteriosus, atrial septal defect, and tricuspid valve abnormalities), Cushing syndrome, pheochromocytoma, and catheter-induced needle wounds.12,27, 28 and 29


Clinical Signs and Symptoms

Intense chest pain is the most common symptom of aortic dissection. Abdominal, as well as lower back, arm, or leg, pain may occur, depending on the extent of the dissection. There may also be vomiting, paralysis, transient blindness, coma, confusion, syncope, headache, and dyspnea, and extremity pulses may be absent.26,27,29



Sonographic Appearance

Sonographically, aortic dissection appears as a thin, linear echo flap within the arterial lumen (Fig. 6-15).30 Because of the presence of blood flow along both sides of the dissection, there is usually motion of the flap with each cardiac cycle. Doppler interrogation is an additional diagnostic aid, providing demonstration of arterial blood flow on both sides of the flap. When evaluating a patient for aortic dissection, it is important to utilize both longitudinal and transverse imaging planes to carefully examine the aorta because an intimal flap can be overlooked if it is located laterally in the artery.26


Aortic Rupture


Description

Abdominal aortic aneurysms of any size may rupture, but the risk increases with aneurysms larger than 7 cm in diameter.21,31,32 Most aneurysms rupture into the peritoneal space, with no predilection for a specific site. They may also rupture into the duodenum, left renal vein, IVC, or urinary tract. An aortic rupture is a medical emergency because the mortality rate for untreated aortic rupture is virtually 100%; with surgery, the mortality rate ranges between 40% and 60%.33







Clinical Signs and Symptoms

Typically, aortic rupture presents clinically as central back pain and hypotension.6,12


Sonographic Appearance

Because of the leakage of blood outside the vessel, aortic rupture may be diagnosed by identification of a hematoma in the abdomen in association with aneurysmal dilatation of the aorta. These hematomas may be located close to the aorta or may extend to varying degrees through the retroperitoneum. Aortic rupture may appear in a variety of stages, from a completely cystic mass to a complex mass. If large enough, the hematomas may also displace surrounding
organs and structures.12 Other findings suggestive of aortic aneurysm rupture include irregular intra-abdominal fluid collections in association with aortic aneurysm and diffuse irregular hypoechoic areas near an aortic aneurysm.

It is difficult to identify the actual rupture site by sonographic examination, although they may be inferred by hematoma “geography.” Computed tomography (CT), on the other hand, is well suited for the detection of aortic rupture and is the diagnostic test of choice because it allows for clear depiction of the extent and density of the hematoma as well as the site of rupture.34 For high-risk patients, contrast-enhanced three-dimensional magnetic resonance angiography (MRA) is a preferred and accurate examination with iodinated contrast material or carbon dioxide contrast agents.35


Inflammatory Aneurysms


Description

Inflammatory aneurysms are enveloped by a dense, fibrotic reaction, generally including many inflammatory cell infiltrates and fatty tissue. This fibrotic reaction is also vascular in nature and involves the retroperitoneum to different degrees. The inflammatory reaction around the aneurysm may become adherent to the duodenum, sigmoid colon, small bowel, ureter, iliac vein, and IVC.36, 37, 38 and 39

Inflammatory aneurysms are an uncommon entity, reportedly between 5% and 20% of all aortic aneurysms.37 They tend to occur in relatively younger persons than arteriosclerotic aneurysms. Even though the risk of rupture is less than that of a “normal” aneurysm, rupture is still a possible scenario.


Etiology

The cause of inflammatory aneurysms is uncertain, but because they are always seen in the presence of aneurysm, it has been postulated that the aneurysm itself may be the cause of the inflammatory reaction.36,38


Clinical Signs and Symptoms

Clinically, the symptoms of inflammatory aneurysms are similar to those of aortic aneurysm. Other symptoms may develop in accordance with the extent of inflammatory involvement to the neighboring areas. These may include leg edema, bothersome pulsations in the epigastrium, and constipation. Hydronephrosis with concomitant flank pain may develop in the presence of ureteral obstruction, and there may be anorexia, early satiety, and dyspnea if bowel adheres to the aneurysmal inflammation.38,40


Sonographic Appearance

Typically, the sonographic features of an inflammatory aneurysm include aneurysmal dilatation of the aorta with a hypoechoic mantle, usually seen anterior and lateral to a thickened aortic wall.36, 37 and 38 CT can also demonstrate this phenomenon, and it is actually better able to depict the extension of the inflammatory process to the surrounding structures in the retroperitoneum.37

It is important to distinguish inflammatory aneurysms from a condition known as retroperitoneal fibrosis.36 Whereas inflammatory aneurysms are always associated with an aortic aneurysm, retroperitoneal fibrosis is not. In addition, the makeup of the two fibrotic reactions is somewhat different. The symptoms of retroperitoneal fibrosis generally do not occur until there is vascular or ureteral compromise. Sonographically, it appears as an echo-free area around the anterior and lateral aspects of the aorta, similar to that seen in association with inflammatory aneurysms, although no aneurysm is present.41


AORTIC BRANCH VESSEL ANEURYSMS


Splanchnic Artery Aneurysms


Splenic Artery Aneurysms


Description

Splenic artery aneurysms are the most common type of splanchnic artery aneurysm. These usually occur in the middle to distal aspect of the splenic artery. There is apparently a female preponderance. Splenic artery aneurysms, although not very common, are life threatening (Fig. 6-16A-C).40,42


Etiology

The causes of splenic artery aneurysm encompass fibromuscular disease of the renal arteries, pancreatic inflammation, peptic ulcer disease, primary arterial injury, and mycotic lesions. There is also a greater potential for patients with portal hypertension and multigravidas to develop splenic artery aneurysms.32,40,42,43


Clinical Signs and Symptoms

The symptoms vary and may range from none to nonspecific left side upper quadrant pain, nausea, vomiting, and a palpable mass if the aneurysm is large enough. There is about a 10% risk of rupture of a splenic artery aneurysm into the peritoneal cavity, with a lesser incidence of rupture into the gastrointestinal tract, spleen, or pancreas.40


Hepatic Artery Aneurysms


Description

Hepatic artery aneurysms are the second most common type of splanchnic vessel aneurysms encountered. About 75% of all hepatic aneurysms are extrahepatic in origin. The remaining 25% occur intrahepatically, the right hepatic arterial branch being more often affected than the left.44,45 Hepatic artery aneurysms are rare and tend to male preponderance (Fig. 6-17).40,42


Etiology

The most common causes of reported hepatic arterial aneurysms are systemic infection, arteriosclerosis, and blunt abdominal trauma. Other less common causes include iatrogenic trauma, vasculitis as a result of pancreatitis, chronic cholecystitis, polyarteritis, and congenital abnormalities.45, 46, 47, 48 and 49


Clinical Signs and Symptoms

Generally, hepatic artery aneurysms are silent or asymptomatic until the aneurysm attains a large size or tapers. When symptoms do occur, they are often vague and unclear and may include epigastric pain (two-thirds of patients), gastrointestinal bleeding because of rupture of the aneurysm into the biliary tract and resulting hemobilia, or obstructive jaundice.45,46 Because of the propensity of hepatic artery
aneurysms to rupture, early detection is important so that prompt treatment can be obtained.

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Dec 10, 2022 | Posted by in ULTRASONOGRAPHY | Comments Off on Vascular Structures
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