26 Anatomy and Normal Doppler Signatures of Abdominal Vessels
Recognition of normal vascular anatomy is essential for any investigation into abdominal vascular problems. Doppler studies of the abdominal vessels demand an understanding of normal and abnormal blood flow patterns. The recognition of changes in abdominal blood flow allows accurate diagnosis of arterial and venous abnormalities, including stenosis, occlusion, and thrombosis. This chapter will review the normal flow patterns seen in abdominal arteries and veins that will be encountered in our Doppler examinations.
It is important to recognize that all abdominal arterial branches identified during our Doppler studies receive blood supply via the abdominal aorta. The abdominal aorta leaves its vascular “imprint” on all its branches. Therefore, changes in aortic blood flow related to stenosis (high-velocity flow) or aneurysmal dilatation (low-velocity flow) will be transmitted to its branches. All of our examinations of the mesenteric and renal arteries always begin with an evaluation of the abdominal aorta for plaque, stenosis, aneurysm, dissection, or occlusion. Waveforms are obtained from the abdominal aorta at the level of the branch vessel of interest and compared to the vessel to assess for changes in flow. It is good practice to look at the aorta for all abdominal studies to detect aneurysm or significant atherosclerotic disease. The presence of significant atherosclerotic plaque in the abdominal aorta certainly increases your suspicion of underlying branch disease, particularly at the origin of the vessels.
Doppler waveforms obtained from the proximal abdominal aorta near the origins of the celiac and renal arteries usually have a low-resistance flow pattern, reflecting the need for continuous forward diastolic flow by the liver, spleen, and kidneys. Remember that, like the brain, the highly metabolic parenchymal organs of the abdomen (liver, spleen, kidneys) demand forward flow in systole and diastole (low-resistance flow). The arteries that feed these organs will generate waveforms that look like the internal carotid artery waveforms.
Waveforms obtained from the distal abdominal aorta near the iliac bifurcation usually have a higher-resistance flow pattern, reflecting the peripheral resistance of the lower extremity arteries. Waveforms obtained from the peripheral arteries demonstrate a triphasic character in the resting state with reversal of flow during diastole. This is due to branching into smaller arteries and capillary beds. The average velocity range for the abdominal aorta is 60 to 100 cm/sec.
The celiac artery, also called the celiac trunk or celiac axis, is the first major visceral branch of the abdominal aorta. It arises from the anterior aortic surface, between the diaphragmatic crura (Figure 26-1). It then bifurcates about 1 to 3 cm from its origin into the common hepatic and splenic arteries, which are readily visualized with ultrasound. The celiac artery also gives rise to the left gastric artery, which is generally not visible sonographically. The branching pattern of the celiac artery is quite constant, occurring in approximately 93% of individuals. In the most common variations, one or more of the celiac branches arises separately from the aorta or from the superior mesenteric artery (SMA). In less than 1% of individuals, the celiac artery and the SMA arise from the aorta as a common trunk. In such cases, the common trunk splits into the celiac artery and the SMA within 1 or 2 cm from the aorta.1,2
Ultrasound examination of the celiac artery usually begins with a transverse sweep of the proximal abdominal aorta. The transverse approach allows visualization of the bifurcation of the hepatic and splenic branches, usually resembling a T or “seagull” (Figure 26-2, A). The longitudinal approach is preferred for evaluation of the celiac artery origin. We also utilize the longitudinal view for Doppler interrogation of the celiac artery. Consistent angle correction of greater than or equal to 60 degrees is obtained in this projection. This view also permits evaluation of the SMA, which is found just inferior to the celiac artery.
FIGURE 26-2 Celiac artery. A, Transverse sonogram of the celiac axis (C) as it divides into the common hepatic artery (CHA) and splenic artery (SA). Ao, aorta; IVC, inferior vena cava; SV, a segment of the splenic vein. B, Normal, low-resistance Doppler signal in the celiac artery. Peak systolic velocity is 113 cm/sec, and end-diastolic velocity is 30 cm/sec.
The characteristic Doppler waveform is a low-resistance arterial waveform (Figure 26-2, B). As mentioned above, continuous forward flow throughout diastole is required for adequate perfusion of the liver and spleen. The hepatic and splenic branches will also demonstrate the low-resistance pattern.3
The mesenteric circulation is notable for extensive arterial anastomoses and rich collateral network. This network allows for continuous circulation to the splanchnic vessels in the event of stenosis or occlusion of mesenteric branches, avoiding significant ischemic insult. With occlusion of the celiac artery, there is collateralization through the pancreaticoduodenal arterial arcade, a network of small vessels surrounding the pancreas and duodenum. These vessels enlarge and feed into the gastroduodenal artery. When the celiac is occluded, there is retrograde flow through the gastroduodenal artery to supply blood to the common hepatic artery. Thus, blood supply to the liver and spleen can be maintained.4
The splenic artery (limb of the celiac T toward patient’s left) follows a tortuous course along the posterosuperior margin of the pancreatic body and tail (Figure 26-3, A) and terminates by splitting into a number of branches in the hilum of the spleen. Along the way, the splenic artery gives rise to several pancreatic branches, short gastric branches, and the left gastroepiploic artery. These vessels are usually not seen with ultrasound. Transverse scans from a midline approach usually reveal the proximal portion of the splenic artery (see Figure 26-2, A). The distal portion of the splenic artery is best seen through the splenic hilum from a left lateral approach. Because of the tortuous course of the splenic artery, flow in this vessel is typically turbulent, (Figure 26-3, B).
The common hepatic artery (Figure 26-4) is the limb of the celiac T that heads toward the patient’s right. After running a short distance along the superior border of the pancreatic head, the common hepatic artery gives rise to the gastroduodenal artery, which can often be seen with ultrasound at the anterosuperior border of the pancreatic head. Beyond the gastroduodenal artery origin, the common hepatic artery becomes the proper hepatic artery, which follows the portal vein to the porta hepatis. At this point, it divides into the left and right hepatic arteries, which penetrate into the hepatic substance. The anatomic relationships among the hepatic artery, the portal vein, and the extrahepatic bile ducts are shown in Figure 26-4, B.
FIGURE 26-4 A, The hepatic artery and its branches. B, Anatomic relationships among the hepatic artery, the portal vein, and the extrahepatic bile ducts. a., artery; IVC, inferior vena cava; Lt., left; Rt., right; v., vein.
The classic hepatic artery configuration just described is seen in 72% of individuals.5 A number of alternative patterns may occur, the most noteworthy of which are the following: (1) the common (4%) or right (11%) hepatic artery may arise from the SMA, and (2) the left hepatic artery may arise from a shared trunk with the left gastric artery (10%).2
The hepatic arteries are usually well visualized sonographically from an anterior abdominal approach. The common hepatic artery is most easily identified at its origin from the celiac artery (see Figure 26-2, A). The proper hepatic artery is seen on ultrasound images near the porta hepatis, anterior to the portal vein, in short- or long-axis views, as can be seen in Figure 26-5. The right and left hepatic artery branches can be followed into the substance of the liver to a variable distance from the porta hepatis. As noted previously, the hepatic arterial system has low-resistance flow characteristics, with continuous forward flow throughout diastole. It is interesting to note that blood flow in the proper hepatic artery and portal vein are both hepatopetal, delivering blood flow toward the liver.
The SMA arises from the anterior surface of the aorta, immediately distal to the origin of the celiac artery (Figure 26-6). The SMA generally consists of a short, anteriorly directed segment and a much longer inferiorly directed segment that ends in the vicinity of the ileocecal valve. SMA branches supply the jejunum, ileum, cecum, and ascending colon, as well as the proximal two-thirds of the transverse colon and portions of the duodenum and pancreatic head. As noted previously, the SMA may also give rise to an aberrant right hepatic artery (11%) or common hepatic artery (4%).1 The SMA communicates with the celiac artery via the pancreaticoduodenal arcade.