Liver

Portal Vein

The portal vein is formed by the junction of the splenic and superior mesenteric vein. The portal/ splenic confluence is found posterior to the neck of the pancreas. The inferior mesenteric vein drains into the splenic vein to the left of the portal/splenic confluence. The left gastric or coronary vein usually joins the splenic vein superiorly near its junction with the superior mesenteric vein. It courses in a craniocaudad plane. From the confluence, the portal vein courses lateral and cephalad in an oblique plane toward the porta hepatis, where it enters the liver. Within the liver, the portal vein is found posterior to the hepatic artery and common bile duct. These three structures course together throughout the liver and are known as the portal triad. The portal vein divides at the porta hepatis into right and left branches. The right portal vein divides into anterior and posterior branches, and the left portal vein divides into medial and lateral branches. The left portal vein is in contact with the ligamentum teres.

The portal vein normally exhibits a monophasic, low-velocity Doppler signal, with slight respiratory variation. The normal range of flow velocity is wide but is usually between 20 and 40 cm/sec. The flow is continuous and should demonstrate little pulsatility

The branches of the portal vein are intrasegmental, traveling within the segments of the liver, whereas the branches of the hepatic veins are intersegmental, traveling between the lobes and segments of the liver. The portal veins can be differentiated sonographically from the hepatic veins by the bright echogenic walls that surround them. This is due to the thick collagenous tissue in the portal vein walls. The hepatic veins do not exhibit echogenic borders (Fig. 1). The cystic vein, which drains into the gallbladder, is a branch of the portal vein. Although it is not usually visualized sonographically, the cystic vein has important implications in the evaluation of portal hypertension. Impaired drainage of the cystic vein into the portal vein can result in varices within the gallbladder wall. These are recognizable by sonographic imaging. There are usually three main hepatic veins within the liver. They are the right, middle, and left hepatic veins. The hepatic veins increase in size toward the superior aspect of the liver, where they drain into the inferior vena cava.

Flow should not cease or reverse in the normal individual. Prominent pulsatility of the portal vein is abnormal and may be indicative of right heart failure, tricuspid regurgitation, hepatic vein/portal vein fistula, or portal hypertension (Fig. 5). The flow in the splenic and superior mesenteric veins is toward (heptopedal) the liver, and both exhibit a low-velocity, monophasic signal. Hepatic artery flow is in the same direction as the portal vein (hepatopetal). The hepatic artery normally demonstrates a low-resistance waveform with continuous forward flow throughout the cardiac cycle (Fig. 6). The hepatic veins (HVs) drain blood from the liver into the inferior vena cava. The normal Doppler waveform obtained from the HVs is triphasic (Fig. 7). This phasicity is dependent on variations in central venous pressure during the cardiac cycle. A lack of pulsatility or continuous waveform in the hepatic vein may indicate compression or stenosis (Fig. 8).

FLOW DIRECTION Flow direction can be very important in abdominal Doppler. Confusion about the direction of flow is a common pitfall. For this reason, it is usually best to avoid use of the color and spectral Doppler invert control. Flow away from the Doppler beam is shown as blue and below the zero baseline, and flow toward the Doppler beam is shown as red and above the zero baseline. Anytime the flow direction is in question, it is helpful to check a baseline vessel in which flow direction is known. For example, to rule out hepatofugal flow in the portal vein, compare its flow direction to that of the hepatic artery. When flow direction is normal in the portal vein (toward the liver), it is the same direction as the hepatic artery. If they are on opposite sides of the spectral Doppler baseline or show opposite colors, blood flow is reversed in the portal vein (Fig. 18).

Color and spectral Doppler are commonly used to determine the presence or absence of flow in a vessel. Another common pitfall is to mistakenly assumethe absence of flow when in fact it is present. This most commonly occurs as a result of poor Doppler angles or inappropriate settings of the Doppler parameters. The Doppler controls should be sensitized for the detection of slow flow whenever thrombosis or occlusion is suspected. This requires adjustment of the pressure-regulating filter (PRF) and filter controls to very low levels. In addition, if the Doppler angle is too great, the frequency shift from slow flow may be too small to detect. This may lead the sonographer to mistakenly assume that flow is absent. Because most abdominal Doppler is performed with curved linear arrays, steering of the Doppler beam cannot be performed, and it is necessary to heel/toe the probe to achieve a good angle of incidence. Commonly, the view that gives the best B-mode image of a vessel is the poorest Doppler approach because the angle of incidence is too great. A common example of this is in evaluating the portal vein. The portal vein is best seen with B-mode imaging in a subcostal approach, where the angle of incidence is near 90 degrees. Color Doppler evaluation of the portal vein in this view may not demonstrate flow, especially if slow flow states are present. The best view of the portal vein for Doppler evaluation is an intercostal approach. In this view, the Doppler angle is much smaller, allowing demonstration of slower flow states.