Central and Upper Abdomen
Knowing the location of the central and upper abdominal vessels is a vital key to understanding any abdominal sonogram. The transverse plane images are the most helpful in this regard. The superior mesenteric artery is a major landmark. As you will recall from the initial part of our journey, the SMA is located from eleven to three o’clock in relation to the transverse image of the aorta (ref to FIGURE 6). The general area of the pancreas is anterior to the SMA. The splenic vein and the SMA (coursing posterior to the splenic vein) form the dorsal boundary of the pancreas on the ultrasound image. The splenic artery is usually just cephalad to the pancreas, and occasionally, tortuous loops of the splenic artery may be seen entering in and out of the image of the pancreas. These arterial loops should not be confused with a prominent or dilated main pancreatic duct; when in doubt, confirm their identity with spectral Doppler. The splenic vein should be evaluated in every patient by following it to the right until the portal confluence (PSC) is seen. The portal vein should then be scanned throughout its length including its entry into the liver through the porta hepatis (FIGURE 17).
Figure 16. Longitudinal gray scale image of a prominent superior mesenteric vein. Note the bulbous area posterior to the pancreas where it joins with the splenic vein to form the portal confluence.
Figure 17. Color flow image of the portal vein and the common hepatic artery within the porta hepatis. Note the bifurcation of the main portal vein into the right and left portal veins.
Just a short distance caudal to this central abdominal region, you can see the left renal vein course under the SMA and over the anterior aortic wall. From this image, you can determine the approximate level of the renal hila. Only two major structures normally pass between the SMA and the aorta – the left renal vein and the duodenum.
These two transverse planes, a) the level of the pancreas, SMA, SMV, and splenic vein and b) the aorta, SMA, left renal vein and renal arteries should be evaluated carefully in every patient.
There are several common anatomic variations in this region which call for very careful examination of the superior mesenteric artery and the hepatic artery. The most common aberrancy is the so-called replaced right hepatic artery, which originates from the SMA and then courses posterior, instead of anterior, to the portal vein (FIGURE 18 A, B). This variant is found in up to 17% of the population 10. Also of note, the common hepatic artery can arise from the SMA (2% to 3% of cases) or from the aorta (1% to 2% of cases) rather than the celiac trunk. A very uncommon finding (less than 1% of the population) is a shared origin of the celiac artery and the SMA from the aorta 3,4. This common trunk is termed a celiacomesenteric artery.
Figure 18. Transverse gray scale images demonstrating an aberrant hepatic artery. (A) The common hepatic artery originates from the SMA and courses toward the liver on the patient’s right side. (B) Slightly more cephalad, there is only a splenic artery branching from the celiac artery. No common hepatic artery is seen at this level.
The Liver Anatomy
There are many different descriptions of liver anatomy in the literature depending on the approach of the anatomist. Historically, liver anatomy was described by its gross anatomic appearance. More recent classifications utilize a functional approach, basing anatomic description on vascular supply. The explanation which follows is a simplified approach, one which will help you identify basic liver anatomy based on its major arterial and venous circulation. Once mastered, the more complex divisions can then be pursued if desired.
There are two general, and very helpful, rules to keep in mind: 1) hepatic veins are boundary formers; that is, they describe the boundaries of the hepatic lobes and segments, and 2) portal veins are not boundary formers (with one exception); rather, they are located within the lobes and segments.