The liver has three functional lobes: the right hepatic lobe, the left hepatic lobe, and the caudate lobe. The portal vein (formed from the splenic and superior mesenteric veins) courses from its origin in an oblique upward path toward the patient’s right shoulder. The main portal vein enters the liver through the porta hepatis (Refer to FIGURE 17). It is accompanied by the common bile duct, which courses anteriorly and to the right, and the common hepatic artery which takes an anterior and medial path (FIGURE 19). The three vessels travel throughout the liver bound together by an echogenic membrane, termed Glisson’s capsule. Together, these three vessels are known as the portal triad. Longitudinal images of the vessels within the porta hepatis are usually definitive and should demonstrate what appears as “rungs of a ladder”. If only two parallel channels are seen without the artery between them, spectral Doppler and color flow imaging are mandatory to determine which structure is portal vein, hepatic artery or bile duct. You should also notice that the triad has a “Mickey Mouse” appearance on transverse gray scale images, the two “ears” representing the lateral bile duct and more medial hepatic artery.
Figure 19. Longitudinal gray scale image of the extrahepatic portal triad. The common bile duct is anterior, and the main portal vein is posterior. The common hepatic artery is seen in the transverse image plane lying between them.
Within the liver, the main portal vein divides into two major branches, the right and left portal veins (FIGURE 20). This branching is easily seen in most patients. The right portal vein supplies (and is thus within) the right hepatic lobe; the left portal vein supplies the left lobe. Using this anatomic approach, the right and left lobes of the liver each have two basic segments. The right lobe has an anterior and a posterior segment. These are supplied by anterior and posterior segmental branches of the right portal vein. The left lobe of the liver has a lateral and a medial segment which are supplied by the lateral and medial branches of the left portal vein. The medial segment generally corresponds with what has previously been called the quadrate lobe. On transverse images of the liver, the left lobe segmental branches are almost always seen; they appear as small parallel track-like lines that lie perpendicular to the fissure between the two left lobe segments. The right lobe branches are frequently seen as they arborize from the right main portal vein. Spectral Doppler and color flow imaging can be helpful in differentiating some of these portal branches from bile ducts.
Figure 20. Longitudinal gray scale image of the main portal vein and its two major branches, the right and left portal veins. Note the thrombus within the veins.
The course of the left portal vein needs a little more explanation. Beyond its origin from the main portal vein, it begins in a transverse plane (the “pars transversa”) and then turns anteriorly in the plane of the fissure between the lateral and medial segments of the left lobe (FIGURE 21). The latter portion is called the ascending or “umbilical” portion of the left portal vein; it is that part of the portal system that received blood from the umbilical vein in the fetus. Why is this ascending portion so important? One reason is that the ascending left portal vein is an exception to the other portal vein branches since it is a boundary former – between the lateral and medial segments. Another reason is that a major collateral vein (the recanalized paraumbilical vein) found in many patients with portal hypertension exits from this vein 11. On transverse images of the left lobe, this collateral appears as a round “target” lesion, with the hypoechoic recanalized vein being central, surrounded by an echogenic rim of fat. Other serpiginous collateral vessels can be seen in the region of the main portal vein; these can be confusing unless the normal anatomy is understood and delineated. The vascular response to hepatic disease is discussed in the article “Sonographic Evaluation of the Hepato-Portal System: Mastering the Maze”.
Figure 21. Gray scale image of the left portal vein and its ascending (umbilical) segment. The umbilical segment may serve as an important collateral in patients with portal hypertension.
The Hepatic Veins
The hepatic venous drainage consists primarily of three major hepatic veins –the right, middle, and left–which empty into the IVC (FIGURE 22). There are other smaller veins, but these three are the most important to identify on the ultrasound image. The left and middle hepatic veins often drain into the IVC through a common trunk; the right hepatic vein drains independently. Quite often all three veins can be visualized in a transverse plane by placing the transducer under the xiphoid process and angling cephalad. When only two of the three veins are imaged, sonographers refer to it as the “Playboy Bunny sign”. The veins are seen high in the liver from this view, coursing cephalad to the plane of the main portal vein and its two branches. If a hepatic vein draining the posterior segment of the right lobe is seen at the same level as the major portal vein branches, this most likely represents the most common hepatic variant, the inferior right hepatic vein. This occurs in approximately 10% of patients 12. The right hepatic vein forms the boundary between the anterior and posterior segments of the right lobe; the left hepatic vein forms the boundary between the lateral and medial segments. The middle hepatic vein divides the liver into functional right and left lobes.
It should be noted that the left hepatic vein and the ascending (“umbilical”) portion of the left portal vein describe the same boundary between lateral and medial segments of the left lobe, only at slightly different levels. The left hepatic vein is more cephalad and the portal vein more caudal.
Figure 22. Longitudinal image of the three major hepatic veins – the right, middle, and left-demonstrating their confluence with the IVC.
The Caudate Circulation
In normal patients, the caudate lobe is the smallest of the three hepatic lobes. It has two processes: the papillary process (which is more anterior and medial) and the caudate process. The papillary process has been confused with lymphadenopathy in some patients. Care must be taken to trace its connection to the caudate lobe if there is any question about a papillary structure in this region. The caudate lobe receives its blood supply from both the right and left portal veins; its venous drainage is into the IVC or a hepatic vein 13. Occasionally, these small veins in the caudate lobe can be seen sonographically. Because of this dual blood supply, the caudate lobe may be the last area involved with such hepatic disorders as cirrhosis.
While the gallbladder does not have an easily visualized vascular supply, it is so readily seen in many patients that you should always note whether there are gallstones (FIGURE 23), masses, wall thickening, etc. These issues can then be pursued for more detailed examination if indicated. The gallbladder lies in the plane which divides the right and left hepatic lobes; this is the same plane as the middle hepatic vein although it is more caudal. There is an incomplete fissure in this area called the interlobar or chief fissure of the liver. This fissure can be seen to extend to the right portal vein on longitudinal views, and can be a means of locating the gallbladder when it is small and contracted and difficult to see. To accomplish this task, locate the right portal vein on a longitudinal image, trace the interlobar fissure caudally and it will lead you to the gallbladder!
Figure 23. Multiple gallstones, which are seen as bright angular echoes within the gallbladder, casting acoustic shadows.
Next, locate the pancreas lying anterior to the SMA. The pancreas generally has a somewhat oblique position in the body, the tail being more cephalad than the head. An oblique-transverse orientation of the transducer may be necessary to visualize the splenic vein in the region of the pancreatic tail. Occasionally, a flank approach can be helpful in further delineating this vein.
In many patients, the gastroduodenal artery (GDA) can be seen along the anterior border of the head of the pancreas (Refer to FIGURE 5). The GDA serves as an important collateral in patients with mesenteric ischemia; I’ll discuss this in more detail in an article on ultrasound evaluation of the mesenteric arterial system. The common bile duct is seen posteriorly. The main pancreatic duct (the duct of Wirsung) can be seen in the normal pancreas; it normally measures 2 millimeters or less. Care must be taken to not confuse it with a blood vessel –again, a good use for spectral Doppler!
Let’s take a closer look at the spleen lying in the left upper quadrant of the abdomen. Because of bowel gas in the region of the splenic flexure, the best approach is usually a left posterior coronal plane. The coronal plane will usually display the splenic hilum well; this can be embellished by a 90 degrees turn of the transducer to create a transverse image originating laterally (FIGURE 24). With these two image planes, the splenic artery and vein can be demonstrated entering and exiting the splenic hilum. In the majority of patients, the kidney and its hilar vessels can be imaged quite well (FIGURE 25). The right kidney can usually be seen from either a parasagittal, oblique parasagittal, or coronal plane. A generous right hepatic lobe helps to provide a good acoustic window in many patients. It is important to keep in mind that normally the lower pole is more lateral and anterior. The entire renal contour (both upper and lower poles) should be delineated. Intercostal scanning through the liver may be required to adequately visualize the upper pole of the right kidney. The left kidney is often a bit more difficult to evaluate, requiring a coronal, or coronal oblique approach. In some patients, scanning with the posterior approach (patient in a decubitus or prone position) may be necessary.
Figure 24. Transverse gray scale image of a spleen from a patient with splenomegaly. Note the marked enlargement of the organ and the splenic vein.
Figure 25. Longitudinal gray scale image of a normal kidney. The central echogenic sinus is primarily composed of fat, renal collecting system, small vessels and lymphatics. The cortex is less echogenic than adjacent liver.
The kidneys should be evaluated for echo texture (which is normally less than the liver), for any evidence of dilation of the collecting system, for masses, calcifications, contour defects, and the presence of any perirenal abnormality such as fluid collections or masses. When evaluating the renal arteries and veins, it is important to be aware of the renal axis and position; this may provide information regarding a horseshoe kidney, ectopic location, or presence of an adjacent extra-renal mass.