Anatomy and Normal Doppler Signatures of Abdominal Vessels

Introduction

In this chapter, we will review the normal anatomy and sonographic appearances of the abdominal arteries and veins. We will include an analysis of the normal Doppler waveforms of the abdominal vessels. Of course, recognition of the normal vascular anatomy is essential for the investigation of any abdominal vascular problem. Doppler studies of the abdominal vessels demand an understanding of normal and abnormal blood flow patterns. The detection of changes in abdominal blood flow allows accurate diagnosis of arterial and venous abnormalities, including stenosis, occlusion, and thrombosis. As is mentioned in several chapters in this book, identification of changes in blood flow patterns offers tremendous insight into vascular pathologies that may occur distant from the site of interrogation.

Abdominal Aorta

It is important to remember 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) may be transmitted to its branches. That is why we always begin our examinations of the mesenteric and renal arteries with an evaluation of the abdominal aorta for plaque, stenosis, aneurysm, dissection, or occlusion. It is good practice to include an examination of the aorta during all abdominal arterial studies to detect aneurysm or significant atherosclerotic disease. The presence of significant atherosclerotic plaque in the abdominal aorta certainly increases suspicion of underlying branch disease, particularly at the origin of the vessels. We obtain velocity measurements and waveforms from the abdominal aorta and compare them to the waveforms from the branch artery to assess for changes in velocity and waveform shape.

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 continuous forward flow in systole and diastole (low-resistance flow). The arteries that feed these organs 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. Examples of these flow patterns are seen in Chapter 24 , Fig. 24.1 . Waveforms obtained from the peripheral arteries demonstrate a triphasic pattern 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/s. A similar velocity range is expected for the major branches of the abdominal aorta.

Celiac Artery

The celiac artery, also called the celiac trunk or celiac axis, is the first major visceral branch of the abdominal aorta ( Fig. 23.1 ). It arises from the anterior aortic surface, between the diaphragmatic crura. 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.

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” appearance ( Fig. 23.2A ). 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. The examiner can follow the course of the vessel and optimize the angle correction of less than or equal to 60 degrees in this projection (see Fig. 23.2B ). This view also permits evaluation of the SMA, which is found just inferior to the celiac artery (see Fig. 23.2C ).

The characteristic Doppler waveform of the celiac artery demonstrates a low resistance flow pattern (see Fig. 23.2B ). As mentioned earlier, continuous forward flow throughout diastole is required for adequate perfusion of the liver and spleen. The hepatic and splenic Doppler waveforms also have this low-resistance pattern. The range of normal blood flow velocity in the celiac artery is 98 to 105 cm/s. See Table 23.1 . However, AbuRahma and colleagues reviewed 153 patients and found that the mean velocity for the celiac artery was 148 cm/s with a standard deviation of 28.42.

The mesenteric circulation is notable for extensive arterial anastomoses and a rich collateral network. This network allows for continuous circulation to the splanchnic organs in the event of stenosis or occlusion of mesenteric branches, avoiding end-organ ischemia. 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 proximal celiac artery 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.