The Abdominal Aorta
Sections of the abdominal aorta can be imaged in almost every patient, and in many patients, the entire length of the aorta can be visualized. The aorta is located to the left of the spine and tapers gradually as it courses to its bifurcation (near the level of the umbilicus) where it divides into the right and left common iliac arteries (FIGURE 1). The length of the abdominal aorta approximates 13 centimeters in an average middle-aged male. The anteroposterior (AP) diameter of the proximal segment normally measures up to 27 millimeters and tapers to approximately 13 millimeters at the bifurcation 1, 2, 3. In older patients, the aorta is often tortuous and ectatic, occasionally even lying to the right of the spine. The aorta should be imaged in both the longitudinal (sagittal) and transverse planes, with images from the proximal, mid, and distal portions and any aneurysmal or anatomically aberrant segments retained and presented for interpretation. True transverse, or anterior to posterior, measurements (outer wall- to-outer wall) greater than 3 cm are generally considered aneurysmal. Attention must be given to any lack of normal tapering that might also indicate disease.
Figure 1. Computed tomographic angiogram of the abdominal aorta and iliac arteries.
The image of the proximal abdominal aorta is occasionally obscured by stomach and/or bowel gas but can often be seen well through the acoustic window afforded by the left lobe of the liver. A near- coronal view is often helpful for evaluating the length of the aorta. The patient is placed in the left lateral decubitus position (that is, left side down) with the right shoulder and body rolled a few degrees posteriorly toward the examination table. While scanning head to foot in a longitudinal direction using this coronal plane, the inferior vena cava (IVC) and the aorta can be viewed side-by-side. The IVC will be toward the top of the image since the ultrasound beam entering the right side of the patient encounters the IVC first; the aorta is seen deeper in the image (FIGURE 2). In addition to displaying the aortic bifurcation well, this view is useful for visualizing the proximal renal arteries (especially the right renal artery) particularly in patients with juxta-renal abdominal aortic aneurysms.
Figure 2. Longitudinal color flow image of the IVC and abdominal aorta.
Note duplicated right renal arteries.
The Celiac and Superior Mesenteric Arteries
The celiac artery (a.k.a., trunk, axis) is the first major branch arising from the abdominal aorta (FIGURE 3), originating anteriorly at about the level of the first lumbar vertebra 3, 4. The celiac has three major branches that can be visualized with ultrasound –the splenic, the common hepatic, and the left gastric arteries. The celiac artery supplies blood to the liver, pancreas, spleen, stomach, and small bowel. If you rotate the transducer 90 degrees to image the aorta in a transverse plane at this level, you will obtain a longitudinal view of the celiac trunk arising from the anterior aortic wall. If you follow the celiac trunk to its bifurcation, you will note that the trunk divides into two branches which appear as seagull wings or, if you view the trunk and branches together, you will note that they resemble the letter “T” (FIGURE 4). The left branch, the splenic artery, is directed toward the hilum of the spleen. The right branch, the common hepatic artery (CHA), courses toward the liver. At the point where the first major branch of the CHA (the gastroduodenal artery) descends to supply the pancreas, the CHA becomes the proper hepatic artery (FIGURE 5). The proper hepatic artery eventually branches into the right, middle, and left hepatic arteries within the liver 2. The third, and smallest, branch of the celiac artery, the left gastric artery, is not seen on transverse images. In optimal longitudinal images of the aorta and celiac trunk, the left gastric artery can be seen coursing cephalad for a short distance. It supplies blood to the anterior and posterior portions of the stomach and esophagus. Other abdominal structures, such as the gastroesophageal junction, also can be seen in this same longitudinal view of the proximal aorta. You will encounter the next major branch of the abdominal aorta, the superior mesenteric artery (SMA), approximately one to two centimeters caudal to the origin of the celiac artery 3, 4 (refer to FIGURE 3). Normally, it lies to the left of the superior mesenteric vein, and posterior to the splenic vein and the pancreas (a portion of the pancreas drapes over the SMA) (Refer to FIGURE 15C). If you image the SMA from a transverse aortic view, the vessel will appear disk-like lying immediately inferior to the celiac artery and superior to the left renal vein as it crosses over the anterior aortic wall (FIGURE 6). The superior mesenteric artery supplies much of the blood flow to the small intestines, cecum, ascending colon, a portion of the transverse colon and also sends branches to the pancreas and duodenum. Rarely, the celiac artery and SMA share a common origin; however, there is often the appearance of a common trunk when in fact the two orifices of the celiac and SMA are very near each other, but separate. The SMA has a collar of fairly bright echoes around it. The echogenicity, which is especially appreciated on transverse images, is attributed to peritoneal fat that surrounds the artery and separates it from the pancreas. Just beyond its origin, the SMA proceeds anteriorly for a short distance and then curves until it courses parallel to the aorta as it continues distally, giving rise to multiple branches along its course (Refer to FIGURE 3. The length of the artery that can be visualized depends on patient body habitus and the amount of overlying bowel gas.
Figure 3. Longitudinal gray scale image of the abdominal aorta demonstrating the origin of the celiac artery and, 1-2 cm distally, the superior mesenteric artery coursing parallel to the aortic wall.
Figure 4.Longitudinal gray scale image of the celiac artery and its branches, the common hepatic artery and the splenic artery.
Figure 5. Computed tomographic angiogram of the celiac artery bifurcation. The gastroduodenal artery (GDA) can be seen arising from the common hepatic artery and descending caudally.
Figure 6. Transverse color flow image of the abdominal aorta. The superior mesenteric artery (SMA) appears disk-like superior to the left renal vein and anterior wall of the aorta.
The renal arteries are the next major exits on the aortic highway but we’re going to skip over those for just a bit and finish our tour of the mesenteric arteries. You will find the small inferior mesenteric artery (IMA) by scanning further distally along the abdominal aorta. The IMA usually originates at the level of the fourth lumbar vertebra 5 which is about two finger widths above the umbilicus or 4 centimeters above the bifurcation. It is commonly surrounded by small bowel and mesenteric fat. The IMA supplies the descending and rectosigmoid colon. While it can be seen for a short distance beyond its origin (from a longitudinal image of the distal aorta), it is best viewed from a transverse image plane arising anterolaterally from the left aortic wall (FIGURE 7).
Figure 7. Transverse color flow image of the distal abdominal aorta. The inferior mesenteric artery (IMA) can be seen arising from the left anterolateral wall of the aorta.
The Renal Arteries
Now, let’s scan back up the aorta to the level of the SMA to locate the renal arteries which arise from the proximal aorta just caudal to the SMA (FIGURE 8). The right renal artery usually originates anterolaterally from the aortic wall and then courses posterior to the Inferior vena cava (IVC). The left renal artery most often arises from the lateral or posterolateral aortic wall and then takes a gradual path, coursing posterior to the left renal vein, to enter the hilum of the kidney. While the majority of patients will have a single renal artery on each side, up to 30% of the population has multiple renal arteries 6 (FIGURE 9). The most common anatomic variants are duplicated main renal arteries, accessory renal arteries, and polar renal arteries 7, 8. Duplicated renal arteries originate from the aorta and enter the renal hilum. Accessory renal arteries also enter the renal hilum but they may originate from the aorta or the iliac arteries. Like accessory renal arteries, polar renal arteries may arise from the aorta or the iliac arteries but, in contrast, they course to the surface of a pole of the kidney and occur more often on the left side than the right for reasons that are not well understood. The proximal renal arteries can most often be identified from a transverse aortic image or from a coronal view of the aorta (described earlier), particularly when using color flow imaging. From a transverse view, they are found posterior to the renal veins (refer to FIGURE 8). The right renal artery can also be identified on longitudinal scans of the inferior vena cava as a disk-like structure coursing behind the IVC (FIGURE 10). Dependent on patient body habitus and the extent of bowel gas, the length of the arteries from aorta to renal hilum may be imaged (FIGURE 11). In the majority of patients, however, the best views of the distal-to-mid segments of the renal artery are obtained from a transverse image of the kidney where the distal artery can be seen coursing into the renal hilum lying in proximity to the renal vein (FIGURE 12).
Figure 8. Transverse gray scale image of the proximal aorta demonstrating the origins of the right and left renal arteries. Note the Left renal vein crossing over the aorta and the SMA (transverse image) just superior to the vein.
Figure 9. Longitudinal color flow image demonstrating multiple renal arteries arising from the aorta and entering the hilum of the kidney.
Figure 10. Longitudinal gray scale image of the inferior vena cava (IVC). Note the cross-sectional image of the right renal artery coursing behind the IVC.
Figure 11. From a transverse image of the kidney, the length of the renal artery can be viewed from the renal hilum to its origin from the aorta.
Figure 12. Longitudinal color flow image of the right renal artery and right renal vein at the level of the renal hilum. Note the renal artery coursing posterior to the IVC.
The Inferior Vena Cava
The large abdominal and pelvic veins can be visualized quite well in most patients. The inferior vena cava (IVC) is the largest of the highways that return blood flow from the extremities to the heart and is located to the right of the spine. It has a variable diameter depending on the stage of respiration during which it is visualized; it can be quite prominent and still normal. With the patient in a left lateral decubitus position, and using a near coronal view, large segments of the IVC can be seen well in most patients (Refer to FIGURE 10). It is important to keep in mind that venous filling is optimized when the patient is lying on an examination table placed in reverse Trendelenburg and that extrinsic compression of the abdominal and pelvic veins may occur secondary to masses, fluid collections, or during late stages of pregnancy.
The Iliac Veins
In the lower abdomen and pelvis, the IVC receives blood flow from the lower extremities. It is possible to visualize the larger of these veins (especially the common iliac veins) as they converge to form the IVC (FIGURE 13). Generally, a lateral decubitus position works well for visualizing the iliac veins. It is often helpful to begin scanning over the common femoral vein at the level of the inguinal crease and moving the transducer superiorly following the long axis of the vein from the common femoral through the external and common iliac veins to the inferior vena cava. You will note that the external iliac vein dives deep and then curves upward as you enter the common iliac.
Figure 13. Longitudinal color flow image of the confluence of the right and left common iliac veins to form the inferior vena cava.
The Renal Veins
In the upper abdomen, the renal veins provide drainage from the kidneys. The right renal vein courses directly to the IVC. The longer left renal vein, which will serve as a valuable landmark on our journey, passes between the SMA and the anterior aortic wall as it courses toward the IVC (Refer to FIGURE 6). If you don’t see it at this location, look beneath the aorta. It may be retroaortic or it may even encircle the aorta (a.k.a. bifid) as a normal variant in some patients (FIGURE 14). The renal veins should be evaluated for possible thrombus, especially in suspicious clinical settings including, but not limited to, nephrotic syndrome, membranous glomerulonephritis, IVC or ovarian vein thrombus, hypercoagulable states, and renal cell carcinoma.9 Oblique views may be useful in such evaluations, as well as the routine transverse and longitudinal views. Identification of acute renal vein thrombosis may be technically challenging because the most common findings of kidney enlargement and changes in parenchymal echogenicity are nonspecific and spectral Doppler signals may be difficult to obtain. The diagnosis is dependent on visualization of thrombus within the renal vein.
Figure 14. Transverse gray scale image of the abdominal aorta demonstrating a retroaortic left renal vein, a normal variant.
The Superior Mesenteric, Splenic and Inferior Mesenteric Veins
The superior mesenteric vein (SMV) can be seen coursing parallel to the IVC providing drainage from the small bowel, cecum, and colon. It serves as a valuable sonographic landmark for locating the head of the pancreas, a portion of which wraps around its proximal segment (FIGURE 15 A, B, C). The SMV joins the splenic vein which exits the hilum of the spleen and courses across the left abdomen. As you follow the often tortuous course of the splenic vein from the patient’s left toward the right side, you will encounter a bulbous area located posterior to the head of the pancreas (FIGURE 16). This is the site of the convergence of the splenic vein, the superior mesenteric vein, and the inferior mesenteric vein (IMV). This area may be referred to as the portal confluence or the portal splenic confluence (PSC). The portal vein provides the majority of blood flow to the liver. Yes, that’s correct…the portal vein doesn’t drain the liver, it is responsible for supplying at least 70% – 80% of the liver’s vascular needs! The inferior mesenteric vein, which is variable in size, is much more difficult to visualize than the SMV. It can occasionally be seen as it courses to the left of the SMV and empties into the splenic vein.
Figure 15. (A) Longitudinal gray scale image of the superior mesenteric vein (SMV) coursing parallel to the IVC (deeper in the image).
Figure 15. (B) Illustration demonstrating the confluence of the SMV and splenic vein to form the main portal vein. Modified from Gill KA, Abdominal Ultrasound.
Figure 15. (C) Illustration demonstrating the anatomic relationship of the SMV, IMV, splenic vein, and the pancreas. Modified from Pellerito JS and Polak JF, Introduction to Vascular Ultrasonography.