Ultrasound of the portal vein system

Ultrasound anatomy

The portal vein is formed by the confluence of the superior mesenteric vein and the splenic vein. The latter originates from the splenic hilum and runs along the posterior margin of the pancreas, accompanying the artery of the same name. The pattern of intrahepatic branching and hepatic veins is determined by the segmental structure of the liver. The anatomical diagram shows the frontal view of the liver. Coronal MR angiography is an alternative method for visualizing the portal vein system.

Research methodology

The extrahepatic segments are visualized on the extended intercostal image. If this technique fails due to colonic gas or an unacceptable Doppler angle, the extrahepatic portal vein branches can be scanned from the right anterior intercostal approach with the right arm elevated to enlarge the intercostal spaces. Often, the main periportal trunk is visualized only in this plane because the acoustic window provided by the liver is the best. The course of the intrahepatic branches is such that they are best visualized on the subcostal oblique scan. After scanning in B-mode and color, Doppler spectra are recorded to quantify periportal portal vein blood flow.

Normal picture

Ultrasound Dopplerography of the portal vein shows a constant blood flow to the liver, giving a single-phase Doppler spectrum in the form of a strip. By changing the body position and breathing mode, it is possible to control the blood flow. The blood flow velocity in the portal vein, for example, decreases significantly in a sitting position and on full inspiration.

Ultrasound Dopplerography in the Diagnosis of Pathological Changes in the Portal Vein in Various Diseases

Portal hypertension

Color mode in portal hypertension demonstrates decreased blood flow or even significant changes such as flow from the liver through the portal vein or splenic vein and helps to visualize collaterals.

Portal vein thrombosis results in increased resistance in the portal venous circulation. It may result from cirrhosis, tumor invasion, increased blood coagulability, or inflammation. Blood flow in the main hepatic artery increases to compensate for the oxygen deficiency caused by impaired portal vein perfusion. Cavernous transformation may occur along the thrombosed portal vein, resulting in hepatopetal blood flow.

Indirect signs of portal hypertension by ultrasound Dopplerography

• Decrease in blood flow velocity to less than 10 cm/s
• Thrombosis
• Cavernous transformation of the portal vein

Direct signs of portal hypertension by ultrasound Dopplerography

• Portocaval anastomoses
• Blood flow from the liver

Transjugular intrahepatic portosystemic shunt

The placement of a transjugular intrahepatic portosystemic shunt has become the primary method of decompressing the portal vein system. A catheter is inserted through the internal jugular vein into the right hepatic vein and then through the liver tissue into the periportal segment of the portal vein. This communication is held open by a metal stent. One of the results of this procedure is a compensatory increase in blood flow in the common hepatic artery. Recurrent stent stenosis or occlusion are common complications and require repeated intervention.

Ultrasound Doppler, especially in power mode, plays an important role in monitoring after an interventional procedure.

Intrahepatic tumors

Ultrasound Dopplerography helps in the differential diagnosis of undefined vascular and solid liver lesions. Adenomas, focal nodular hyperplasia, and hemangiomas can be distinguished from malignant tumors by characteristic features. The absence of blood flow in a hyperechoic homogeneous formation allows one to suspect a hemangioma. This diagnosis can be clarified by determining additional blood flow characteristics using contrast agents.

Use of contrast agents

In recent years, the use of Doppler and power Doppler modes has improved the differential diagnosis of intrahepatic lesions compared with traditional B-mode, but even experienced specialists may still encounter problems.

First, some deep liver lesions, as well as lesions in very obese individuals, can only be visualized with an unacceptable Doppler angle, which limits the accuracy of the examination. Second, the very slow blood flow that is often observed, especially in small tumors, produces inadequate frequency shifts. Third, in some areas of the liver it is very difficult to avoid motion artifacts due to the transmission of cardiac contractions to the liver parenchyma.

Ultrasound contrast agents in combination with modified scanning techniques can help solve these problems. They significantly enhance the intravascular signal, improving the detection of even slow blood flow in small tumor vessels.

When contrast agents are administered bolus-wise, several phases are distinguished in the enhancement pattern. They may vary to some extent depending on the individual characteristics of the patient's blood circulation.

Enhancement phases after intravenous administration of contrast agent

• Early arterial: 15-25 sec after administration
• Arterial: 20-30 sec after administration
• Portal: 40-100 sec after insertion
• Late venous: 110-180 sec after administration

Benign liver lesions: focal nodular hyperplasia and adenoma

Benign liver tumors, unlike malignant ones, do not contain pathological shunts. As a result, they remain enhanced even in the late venous phase. This is typical for focal nodular hyperplasia and hemangioma. Focal nodular hyperplasia most often affects women who constantly use oral contraceptives. Liver adenomas have an almost identical picture in B-mode, and differentiation often requires histological assessment. When using color and power Doppler modes for focal nodular hyperplasia, a typical blood flow pattern is determined, which allows for differential diagnosis.

The choroid plexus in focal nodular hyperplasia diverges from the central artery, demonstrating centrifugal blood flow with the formation of the "spokes of the wheel" sign. Focal nodular hyperplasia and adenoma may have similar symptoms due to enlargement due to growth or hemorrhage. On CT, focal nodular hyperplasia and adenomas are most clearly defined in the early arterial phase of enhancement. In the parenchymal phase, they are hyper- or isoechoic relative to the surrounding liver tissue.

Hemangiomas of the liver

In contrast to focal nodular hyperplasia, hemangiomas are supplied from the periphery to the center. In the arterial phase, the outer areas of the lesion enhance, while the center remains hypoechoic. The central portion becomes significantly more echogenic in the subsequent portal phase, and the entire lesion becomes hyperechoic in the late venous phase. This pattern of enhancement from the periphery to the center, also called the "iris diaphragm" sign, is typical of hepatic hemangiomas. It is also seen on CT.

Hepatocellular carcinoma

The detection of intra- and peritumoral arterial Doppler signals, vascular ruptures, vascular invasion, spiral configurations, and an increase in the number of arteriovenous shunts by ultrasound Dopplerography are considered as criteria for malignancy. Hepatocellular carcinoma usually has a heterogeneous pattern of signal enhancement in the arterial phase after administration of a contrast agent. It remains hyperechoic in the portal phase and becomes isoechoic relative to normal liver parenchyma in the late venous phase.

Liver metastases

Liver metastases may be hypovascular or hypervascular. Although the exact location of the primary tumor cannot be determined from the vascular pattern of a liver metastasis, some degree of vascularity has been found in some primary tumors. Neuroendocrine tumors such as C-cell thyroid cancer or carcinoid tend to form hypervascular metastases, whereas metastases from primary colorectal tumors are usually hypovascular.

In the arterial phase after contrast administration with standard scanning, metastases are characterized by slight contrast enhancement depending on the degree of vascularization. They usually remain hypoechoic relative to the liver parenchyma in the late venous phase or may become isoechoic. This low echogenicity in the late venous phase after contrast administration is a key criterion for the differential diagnosis of metastases from the above-described benign liver lesions. What follows from this? A distinctive characteristic of metastases is their tendency to form arteriovenous shunts. This may explain why contrast agents are cleared more quickly from liver metastases than from normal liver parenchyma, which is why the picture of metastases is relatively hypoechoic in the late phase of contrast perfusion.

Typical features of liver metastases are an uneven enhancement pattern, a spiral or corkscrew configuration of the vessels, and the presence of a large number of arteriovenous shunts. As a result of the latter aspect, the contrast medium enters the hepatic veins within 20 seconds instead of the normal 40 seconds. The clinical picture can also help in the differential diagnosis between hepatocellular carcinoma and metastases: patients with hepatocellular carcinoma often suffer from cirrhosis of the liver, chronic hepatitis, and/or have elevated levels of alpha-fetoprotein in the blood. This combination is much less common in patients with liver metastases.

Special scanning techniques

When scanning with a low mechanical index (MI ~ 0.1), often combined with phase inversion, small microbubbles are immediately destroyed during the initial passage of the bolus. This prolongs the contrast enhancement. At the same time, using a low mechanical index reduces the sensitivity of the study. For example, when using a low mechanical index, posterior acoustic enhancement is no longer an effective criterion for differentiating cysts from other hypoechoic formations. In some cases, posterior acoustic enhancement reappears only when the mechanical index rises to “normal” values from 1.0 to 2.0.

Variable transmission of two ultrasound pulses per second instead of 15 (variable harmonic imaging) allows visualization of even the smallest capillaries, since a longer interpulse delay leads to less microbubble destruction. As a result, their higher concentration leads to capillary signal enhancement when the delayed pulse passes through the tissue.

When using the variable pulse transmission technique at a low mechanical index, even hypovascular metastases become hyperechoic in the early arterial phase (within the first 5-10 seconds of the passage of the contrast agent), thereby creating a visible difference between the early arterial and arterial phases of contrast enhancement.

An important rule for differential diagnosis of liver lesions

The use of contrast agents allows the use of the following differential diagnostic rule: lesions with a longer duration of signal enhancement are most likely benign, whereas metastases in hepatocellular carcinoma are often hypoechoic compared to the surrounding liver parenchyma even in the late venous phase.

Inflammatory bowel disease

Despite the difficult conditions of scanning the gastrointestinal tract, some pathological conditions can be detected and assessed using ultrasound. B-mode allows one to suspect an inflammatory process by the presence of exudate and thickening of the intestinal walls. Detection of hypervascularization allows one to assume chronic or acute inflammatory bowel disease. Fluoroscopic enterography (contrast examination of the small intestine using the Sellink technique) determines the segment of the residual lumen. Acute enteritis and radiation enteritis are also characterized by nonspecific hypervascularization, which leads to an increase in the blood flow velocity and its volume in the superior mesenteric artery. In appendicitis, nonspecific hypervascularization of the thickened and inflamed intestinal wall is also determined.

Critical assessment

Ultrasound Dopplerography is a non-invasive examination technique with various possibilities for assessing abdominal organs and vascular systems. The liver is easily accessible for ultrasound examination even in difficult clinical conditions. Specific indications have been defined for assessing focal and diffuse changes in the liver parenchyma and vessels. Ultrasound Dopplerography has become the technique of choice in the diagnosis and assessment of portal hypertension, as well as in planning and monitoring the placement of transjugular intrahepatic portosystemic shunt. Ultrasound Dopplerography allows for non-invasive measurement of blood flow velocity and volume, and the detection of complications such as stenosis and occlusion.

Doppler ultrasound is used for postoperative monitoring of liver transplants to determine organ perfusion. However, there are no standard criteria for diagnosing liver transplant rejection.

The characteristics of focal liver lesions are based on the degree of vascularization. There are some criteria of malignancy that help to more accurately diagnose a volumetric liver lesion. The use of ultrasound contrast agents allows for improved vascularization display and assessment of changes in the perfusion pattern in different contrast phases.

In the study of abdominal vessels, Doppler ultrasound is used for screening and evaluation of aneurysms. Additional methods such as CT, MRI, and DSA may be required for planning medical and surgical treatment. Doppler ultrasound is also a screening method for chronic intestinal ischemia.

The ability of Doppler ultrasound to detect increased vascularity in inflammatory diseases such as appendicitis and cholecystitis has expanded the capabilities of ultrasound diagnostics.

An experienced sonographer can identify specialized, non-standard indications for Doppler ultrasound using a high spatial resolution transducer. However, there are limitations to this method. For example, it can take a considerable amount of time to perform a complete examination. Moreover, Doppler ultrasound is quite operator-dependent when examining the abdominal cavity. Advances in electronic data processing will continue to improve examination results, becoming more detailed and easier to interpret, for example, using the panoramic SieScape technique and 3D reconstructions.

Tissue harmonic imaging is a new technique used in diagnostically challenging cases, allowing for improved imaging under poor abdominal scanning conditions. The use of various contrast agents has significantly improved the capabilities of ultrasound diagnostics, especially in patients with large liver lesions. Thus, ultrasound Dopplerography is a noninvasive diagnostic technique with high development potential, which must be used much more widely in abdominal examination than it is currently.