Doppler Liver Assessment, Protocols, and Interpretation of Results

Anatomy and Physiology

The liver plays a vital role in the body's metabolic functions and is composed of a relatively complex vascular architecture.[1] One-quarter of the cardiac output goes to the liver.[2] The liver is divided into various hepatic segments, and each hepatic lobule receives a branch from the portal vein, hepatic artery, and biliary tract called the portal triad.[3]

1. The portal vein constitutes 75% of the hepatic blood supply. The blood from the portal vein is deoxygenated, carrying mainly nutrients. It is formed by the confluence of superior mesenteric and splenic veins. The portal vein drains all the upper and middle parts of the gastrointestinal tract, pancreas, gallbladder, and spleen.[4] Within the liver, the portal is divided into two branches: the left and the right portal veins. The left supplies segments II, III, and IV, while the right portal vein supplies segments V, VIII, VI, and VII. Variants of the portal venous system are not uncommon.[5]
2. Hepatic veins: The hepatic veins are three branches (right, left, and middle hepatic veins) draining blood to the IVC.[6] The most common morphology of the hepatic veins includes a right hepatic vein and a common trunk for the middle and left hepatic veins.[7]
3. The hepatic artery supplies 25% of the hepatic blood flow and constitutes the main bulk of oxygenated blood to the liver.[2] Frequent variations of the origin of the hepatic artery exist, with the most common type constituting the common hepatic artery arising from the celiac artery in 70% of patients.[8] 
4. The inferior vena cava (IVC) represents the confluence of the right and left common iliac veins and is the retroperitoneal draining vessel to the hepatic veins. The inferior vena cava subsequently empties deoxygenated blood to the right heart.[9]

Hepatopetal and Hepatofugal Flow

Hepatopetal flow refers to blood flow towards the liver (from the portal hepatis to the liver periphery). It typically is used in describing the normal blood flow direction in the portal vein. [10] This occurs in a normal liver and allows the liver to detoxify the blood that enters it after absorbing nutrients from the intestine through the portal vein.

Hepatofugal flow refers to blood flow away from the liver in the portal vein and is sometimes referred to as "retrograde" flow. In other words, the portal venous blood flow pattern is from the periphery of the liver towards the porta hepatis.[10] This occurs when the portal venous pressure is high in the case of portal hypertension. Thus portosystemic shunts are reopened with additional findings that include a more narrowed portal vein and prominence of the hepatic artery.[11] Occasionally, a to-and-fro bidirectional blood flow pattern alternating between hepatopetal and hepatofugal flow can be seen in the portal vein before the onset of frank hepatofugal flow.[12]

While HVPG (hepatic venous pressure gradient) is the gold standard for measuring the portal and hepatic venous pressure, it is an invasive procedure with the insertion of a catheter into the hepatic vessels.[13] On the other hand, Doppler ultrasound is a non-invasive procedure and can evaluate for abnormal physiology of the hepatic vasculature.

Indications

Assessment of the Following Clinical Presentations of Diseases

Follow-Up After Liver Transplantation Operation

Hepatic Doppler is used in the early detection of vascular occlusion or graft rejection, and it is the most preferred method of follow-up due to its low cost and non-invasiveness.[14] Sonographic assessment of a post-transplant liver includes a gray-scale evaluation of the hepatic parenchyma and the hepatic biliary tree. Doppler evaluation of the hepatic vasculature involves both gray-scale and Doppler assessment. Evaluation for strictures can manifest as varying degrees of vessel narrowing.[15] 

Thrombus formation in the portal vein and IVC can be evaluated on both gray-scale and Doppler assessments. A normal hepatic artery will demonstrate a rapid systolic upstroke lasting up to 80 milliseconds. The resistive index of the hepatic artery, calculated as the ratio of peak systemic velocity minus peak diastolic velocity divided by peak systolic velocity, should be between 0.5 to 0.7.[15] In the setting of post-transplant evaluation, the portal vein should demonstrate a continuous hepatopetal flow pattern with only mild velocity variations on inspiration and expiration. Hepatic veins and IVC should demonstrate a phasic flow pattern corresponding to the blood flow of the cardiac cycle.[15]

Diagnosis of Portal Hypertension and Patency of Collaterals

Cirrhosis and an alteration in portal hemodynamics ultimately result in an increased pressure gradient in the portal vein resulting in abnormal portal vein blood flow away from the liver.[16] Doppler ultrasonography can assess blood flow direction and evaluate the presence of collateral vessels. Doppler indices include portal and splenic venous velocities, flow directions, and resistive/pulsatility indices of the arterial vasculature.[17] 

Additional strengths of Doppler assessment include real-time observation, while limitations include a poor detection of slow blood flow and a reduced frame rate.[18] Other useful parameters in the diagnosis of cirrhosis include liver and spleen size, evaluation of liver echotechture, and the evaluation of the bluntness of the liver edge.[19] The normal portal vein pressure is less than 6 mmHg. Portal hypertension is clinically manifested when this pressure exceeds 12 mmHg. 

Diagnosis of Budd Chiari Syndrome

Budd-Chiari syndrome represents an outflow obstruction of the hepatic veins is most typically secondary to a prothrombotic disorder.[20] Doppler in Budd-Chiari syndrome is diagnosed by the occlusion of the hepatic vein drainage of the liver either by stenosis or thrombosis of the hepatic veins or the higher draining inferior vena cava. This is done by measuring the direction and velocity of flow and the vessel diameter.[21] Often there is a non-visualization of the hepatic vein vasculature on Doppler color ultrasound. A fibrous cord, luminal thrombus, and stenosis can be seen.[22] Collaterals can also be seen in a spiderweb morphology, often extending to the inferior vena cava or other hepatic veins. 

Diagnosis and Follow-Up of Thrombosis

Chronic thrombus formation can occur in the portal veins, hepatic veins, or the inferior vena cava. Thrombosis appears as an absent signal on color Doppler evaluation. The gold standard for diagnosing vessel thrombosis is angiography of the suspected vessel, but it is an invasive procedure.[23] In chronic thrombosis, collaterals start to appear along with recanalization of the obstructed vessel. Portal vein thrombosis could be due to benign obstruction of the portal vein (caused by the over-coagulable state, slow blood flow, or vascular injury) or malignant obstruction (caused by infiltration of the endovascular lining of the vessel).[24]

Evaluation of a TIPS (transjugular intrahepatic portosystemic shunt) Stent

The transjugular intrahepatic portosystemic shunt (TIPS) procedure is one treatment option for patients with cirrhosis or noncirrhotic total hypertension. TIPS shunts function to treat those who suffer from refractory ascites and variceal bleeding.[25] TIPS dysfunction has a high incidence and can be secondary to malpositioning, thrombosis, and pseudointimal hyperplasia.[26] Ultrasound can assess TIPS velocities(normally considered 90 to 190 cm/s), normal portal vein blood velocity before entering the shunt (normal reading is around 30 cm/s), and assess waveforms and the presence of stenosis.[27]

Assessment of Hemodynamics of Hepatic Focal Lesions

Doppler evaluation can be used in the assessment of benign hemangiomas and telangiectasia.[28] Hypoechoic hepatic focal lesions may appear through the screening process of the high-risk populations for hepatocellular carcinoma. The lesions may be associated with enlarged porta hepatic lymph nodes or para-aortic lymph nodes in metastasis. The vascular flow on color Doppler around the focal lesion can evaluate for neovascularization.[29][30]

Hemodynamics in Heart Failure

This will result in hepatic venous congestion and retrograde flow, inferior vena cava congestion with dilated diameters of the vessels, and increased portal vein pulsatility.[31]

Contraindications

There are no contraindications to this non-invasive procedure.

Equipment

Doppler ultrasound for the abdominal examination has two types: pulsed and color Doppler. Continuous Doppler is used in high-frequency flow in the cardiac valves and vessel examination but is not suitable for the portal and hepatic veins and IVC.

Personnel

Usually, Doppler ultrasound is carried out by an experienced sonographer, either a radiologist or gastroenterologist.

Preparation

Fasting is preferred (4 to 6 hours before examination) to decrease gaseous distension and fluids in the abdomen and increase the visibility of the vessels.[32] Before starting the examination, device adjustments of the gain, frequency, and depth are crucial.[33][34]

Technique or Treatment

Using hepatic Doppler ultrasound as part of the abdominal ultrasound will increase the operator's experience in refining the probe movement and better visualizing the hepatic vessels. The time consumed for both examinations (abdominal ultrasound and hepatic doppler) will not increase much.

• The patient could be asked to hold his breath to improve visualization.[35]
• The red color on the screen is usually set up in most devices as the direction of flow towards the probe, while the blue indicates the direction of blood flow away from the probe.
• The portal vein has a thickened fibrous tissue wall that will be reflected as drawing an echogenic line around the vessel, while hepatic vein walls are thin and non-visualized.
• Portal vein diameter can range from 7 to 15 mm.[36] Hepatic veins can have a diameter of 5 to 7 mm. The IVC normal diameter ranges from 13 to 22 mm with a collapsible wall on pressure.[37]
• The patient is best positioned in the left lateral or supine position. Scanning starts in the right subcostal area, which allows for visualization of the portal vein. The confluence of the splenic and superior mesenteric veins is visualized when moving left towards the midline.
• Then scanning in the substernal position, the operator will view the IVC (which should be compressible) and aorta at the midline while the probe faces posteriorly (either longitudinally or transversely).
• Hepatic veins can be best viewed from the right intercostal position where the probe is facing medially, and this position is best to view the drainage of the three hepatic veins into the IVC. The hepatic veins could also be viewed from the subcostal position directing the probe posteriorly. Left and middle hepatic veins are best visualized from the substernal position.

Limitations

• Obesity can cause limitations in the visualization of vessel flow and velocity.
• Eating may cause an increase in the portal pressure and widen the diameter of the vessel. Ingested contents and resultant gaseous distension may hinder the visualization of vessels. Therefore, fasting for at least 4 to 6 hours before the Doppler examination is recommended.
• Changes in the liver hemodynamics may be falsely diagnosed if the gain and frequency are not well adjusted in the device.
• The low frequency may falsely diagnose portal vein thrombosis.
• Patients who cannot hold their breath will create some difficulty for the operator to visualize the vessels, but this could be overcome by the timing of inhalation and moving the probe synchronously. 

Clinical Significance

Hepatic Doppler ultrasound is a non-invasive investigative tool that can help clinicians, especially if added to an abdominal ultrasound exam. The procedure is cost-effective and has no complications.

Doppler ultrasound is the first imaging technique used in assessing the vascular condition in the following diseases: 

• Portal hypertension associated with cirrhosis 
• Portal or hepatic vein thrombosis[38]
• Vascularity of the hepatic focal lesions[39]
• Follow-up after TIPS or liver transplantation[40]
• Measuring the hepatic vein and artery patency, blood velocity, and direction

Referral to other invasive investigative or treatment procedures such as hepatic venous pressure gradient HVPG, angiography, transjugular intrahepatic portosystemic shunt TIPS, and other procedures depends on initial assessment on Doppler ultrasound.

Enhancing Healthcare Team Outcomes

Evaluation of the liver starts with clinical assessment and laboratory workup. Radiological evaluation in the form of abdominal ultrasound accompanied by hepatic Doppler could give the physician a complete picture of the patient's liver condition with a non-invasive procedure. Doppler can detect early hepatic focal lesions and be part of the liver ultrasound screening process in high-risk individuals or post-operative hepatic procedures such as TIPS and liver transplantation.[27]

Hepatic Doppler also can detect portal hypertension and guide further management to include prophylactic band ligation or early medical therapy before the condition deteriorates.[41] It can also be used in the follow-up of patients post-ligation.

Although sonographers and radiologists mainly conduct hepatic Doppler exams, it offers a real-time evaluation of the liver. Doppler can also be an essential part of the hepatic assessment by gastroenterologists and surgeons. As part of their training and practice, such subspecialists are familiar with abdominal ultrasound and hepatic Doppler.