In normal patients, the portal veins provide 70–75% of the hepatic blood supply. The main portal vein is formed by the confluence of the splenic vein and the superior mesenteric vein in the mid epigastrium, posterior to the pancreas. It travels into the liver at the porta hepatis, and then branches into the right and left portal veins. The right portal vein branches further into anterior and posterior branches shortly after the main portal vein bifurcation. (Fig. 46.2). Variations of this anatomy are common, seen in about 35% of patients. The most common variant is trifurcation of the main portal vein into the right posterior, right anterior, and left portal veins, with other variants including bifurcation of the main portal vein into the right posterior branch and a common trunk for the right anterior branch and the left portal vein. The portal vein and its branches are considerably larger than the hepatic arteries. As a result, the main portal vein and its branches can be seen farther out into the liver parenchyma with grayscale imaging than the hepatic arteries [8].

Normal Doppler waveform of the portal veins demonstrates continuous antegrade flow toward the liver with a mildly undulating pattern. Flow is affected by respiration and cardiac pulsatility, although the effect is normally mild. The mean velocity in the portal vein is typically 15–30 cm/s, while the peak velocity has a broad range of normal from 15 to 40 cm/s. In general, variations in velocities in the main portal vein are small, with the lowest velocity measuring more than half the highest velocity. That is,
(Fig. 46.3) [7, 9, 10].

The hepatic veins drain all the blood from the liver into the inferior vena cava. That is, all the blood entering the liver through the portal vein and hepatic arteries is drained through the hepatic veins directly into the inferior vena cava. Three main hepatic veins are typically present: the left, middle, and right. The left hepatic vein travels between the medial and lateral segments of the left lobe of the liver. The middle hepatic vein courses between the right and left lobes of the liver, and the right hepatic vein courses between the anterior and posterior segments of the right lobe (Fig. 46.4). Variations of hepatic venous anatomy are common, with an additional right inferior hepatic vein in some patients and merging of the middle and left hepatic veins into a single vessel in others [11].

Normal hepatic veins have a multiphasic pattern during the cardiac cycle, with flow predominantly away from the liver and toward the heart except during short segments of the cycle. Starting with atrial systole, the first part of the hepatic vein waveform is typically retrograde, or back into the liver, as a result of increased pressure in the right atrium as the atrium contracts. This is termed the A-wave. In some patients, the A-wave is not retrograde, but rather has markedly diminished antegrade flow compared to the rest of the waveform. After atrial systole is ventricular systole, during which blood is propelled out of the heart into the great arteries. In addition, the tricuspid valve is drawn into the right ventricle, increasing negative pressure in the atrium, drawing blood back to the heart. During ventricular systole, the hepatic vein waveform demonstrates flow toward the inferior vena cava and is called the S-wave. Flow is maximal toward the heart during this portion of the cardiac cycle. Toward the end of systole, the tricuspid valve shifts away from the right ventricle. During this period, the hepatic waveform usually demonstrates a decrease in flow velocity toward the heart, or even a bit of reversed flow away from the heart, making the V-wave. Lastly, during diastole, the tricuspid valve is open, and the atria and ­ventricles fill with blood. The hepatic waveform during this part of the cycle is called the D-wave and characteristically demonstrates antegrade flow with velocities almost as great as during cardiac systole (Fig. 46.5) [11].

The multiphasic pattern of hepatic venous flow is sometimes considered to have four components, the A-wave, V-wave, S-wave, and D-wave, and is sometimes called triphasic. The latter term is used by those who consider the V-wave to be a transitional period and not its own wave. No matter which term is used to characterize the waveform, it is important to recognize the normal appearance of the hepatic venous waveform in order to identify abnormalities of flow in these vessels (Fig. 46.6) [11].

The hepatic venous waveform is affected by respiration. In particular, during inspiration and Valsalva, the multiphasic waveform may become monophasic, or flattened, with diminished flow toward the heart. Thus, it is best to assess the hepatic veins at end expiration or during quiet respiration. Because of this, imaging and Doppler acquisition may be difficult because the scan should not be performed with the patient in suspended respiration after taking a deep breath. Intercostal views may be necessary, as the subcostal approach may not be adequate [11].

Color and spectral Doppler are used in conjunction with grayscale imaging of the liver to identify blood vessels in the liver and assess the vessels for presence and direction of flow. In addition, Doppler is used to evaluate the dynamics of flow within the vessel [11]. Diseases of the liver alter the parenchyma, which in turn, alters blood flow in the arteries and veins leading into the liver, within the liver, and exiting the liver [7]. Thus, Doppler studies should be performed to help determine if the liver parenchyma is abnormal. Once the presence of liver disease has been established, Doppler is used to help determine the severity of disease and to monitor progression [12].

Common indications for liver Doppler are listed in Table 46.1. The most common indication is suspected or known portal hypertension. These patients are monitored closely to detect changes in portal venous flow and to look for the development of collateral vessels. Patients with known or suspected liver disease, such as cirrhosis or hepatitis B or C, may benefit from Doppler studies, because the Doppler assessment may provide information about the severity of disease. Doppler is also an important tool for monitoring the progression of these diseases to determine when intervention is necessary. In addition to the indications above, patients with systemic diseases, such as hypercoagulability and atherosclerosis, may develop abnormalities of the vessels entering or exiting the liver, leading to damage to the liver.

Altered blood flow in the hepatic artery typically results from one of two pathologic processes, arterial stenosis, or hepatic parenchymal disease. The most common site of atherosclerotic disease affecting blood flow in the hepatic artery is in the celiac artery near its origin from the aorta. A stenosis of 70% or greater in the celiac axis is considered hemodynamically significant and can lead to diminished blood flow to the liver through the hepatic artery. The diagnosis of celiac stenosis is made with spectral Doppler when the peak systolic velocity measures greater than 200 cm/s in the celiac artery (Fig. 46.7). Additional findings with celiac stenosis may include an abnormal tardus-pardus waveform in the main hepatic artery [13, 14].

Liver parenchymal disease and liver congestion may cause increased vascular resistance in the liver. The increased resistance is reflected in the hepatic artery waveform as diminished diastolic flow and an elevated Doppler resistive index measuring >0.70 (Fig. 46.8). Common pathologies associated with an elevated resistive index in the hepatic artery include cirrhosis, acute hepatitis, metastatic disease, congestive heart failure, and hepatic vein obstruction.

A few rare entities may lead to an abnormally low resistive index in the hepatic artery that measures <0.55. Such entities include vascular malformations with arteriovenous shunting, arteriovenous or arterioportal shunt, and significant stenosis of the celiac artery or the hepatic artery.