Continuing Education Activity
Hepatic doppler ultrasound is a part of the abdominal ultrasound examination. It is used to assess the patency of the hepatic vasculature and the direction and velocity of blood flow. It is a non-invasive procedure that has no contraindications or hazards and could benefit the patients immensely.
- Review the indications for hepatic Doppler ultrasound and its significance in clinical practice.
- Review the limitations of hepatic doppler ultrasound
- Describe the anatomy of the hepatic vessels as shown in doppler ultrasound.
- Explain the types of flow in hepatic doppler ultrasound.
Doppler ultrasound is a non-invasive procedure; hepatic Doppler could enrich abdominal ultrasound examination. The benefits are significant; first, if the examiner uses liver Doppler will be able to know the abnormal vascular structures even if the lesion is not apparent (iso-echoic). Second, it will give clinicians and sonographers tremendous experience to catch the hemodynamic abnormalities easily.
Anatomy and Physiology
The liver is one of the most vascular organs in the human body. One-quarter of the cardiac output goes to the liver. The liver is divided into hepatic segments, and each hepatic lobule receives a branch from the portal vein, and hepatic artery and biliary tract called the portal triad.
The Livers Dual Blood Supply Accounting for the High Vascularity From the Portal Vein and Hepatic Artery
- Portal vein constitutes 75% of the hepatic blood supply, but the blood is deoxygenated, carrying mainly nutrients. It is formed by the confluence of superior mesenteric and splenic veins. Portal vein drains all the upper and middle parts of the gastrointestinal tract, pancreas, gall bladder, and spleen.
- Hepatic veins: three branches (right-left and middle) draining the blood to the IVC.
- The hepatic artery supplies 25% of the hepatic blood flow and constitutes the main bulk of oxygenated blood to the liver.
- Inferior vena cava IVC: is the draining vessel to the hepatic veins before reaching the heart and the systemic circulation.
Hepatopetal and Hepatofugal Flow
Hepatopetal flow means blood flow towards the liver; it is also called sluggish or slow portal flow. This occurs in normal liver provided that the patient is fasting. This allows the liver to detoxify the blood that enters it after the absorption of the nutrients from the intestine through the portal vein.
Hepatofugal flow means blood flow away from the liver; this occurs when the portal venous pressure is high in the case of portal hypertension. Thus portosystemic shunts are reopened, and the pressure is reflecting the arterial side.
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. On the other hand, Doppler ultrasound is a non-invasive procedure that could be a part of the abdominal ultrasound examination provided the right device is available.
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.
Diagnosis of portal hypertension and patency of collaterals:
High pressure in the portal vein will cause shunting from the portal vein to the systemic venous system. It will result in variceal bleeding through the patency of the collaterals (mainly the short and left gastric and recanalized umbilical veins). A direct correlation between the degree of rising portal pressure and hepatic fibrosis and survival is present. Response to medical therapy could be monitored by the decline in the portal venous pressure.
Destruction of the liver architecture will lead to portal vein congestion.
Congestive index: Portal vein diameter/portal vein velocity
The normal portal vein pressure is less than 6 mmHg pressure; portal hypertension is clinically manifested when it exceeds 12 mmHg pressure. It will result in increasing the pressure of the sinusoids and reopening of the collaterals.
Diagnosis of Budd Chiari syndrome:
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.
Diagnosis and follow up of thrombosis:
This occurs in the portal or hepatic veins or IVC inferior vena cava. Thrombosis appears as an absent signal in color Doppler. The gold standard for diagnosing vessel thrombosis is angiography of the suspected vessel, but it is an invasive procedure with contrast ingestion hazards. 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.
Insertion of the TIPS (transjugular intrahepatic portosystemic shunt) stent.
Doppler ultrasound to guide TIPS insertion and follow-up of vascular patency is the commonest non-invasive method, measuring the velocity and patency in the haptic and portal veins, but the doppler accuracy is still controversial.
Assessment of hemodynamics of hepatic focal lesions:
Benign Haemangioma and telangiectasia: vascular anastomosis or shunts (Hepatic artery and vein, porto-hepatic, peripheral venous) occur along with; hyper-echoic or normo-echoic focal lesion. Assessing the vascular diameters and velocity across the hepatic vessels is diagnostic.
Hepatocellular carcinoma and metastatic lesions: hypoechoic hepatic focal lesions may appear through the screening process of the high-risk populations for HCC. In metastasis, the lesions may be associated with enlarged porta hepatic lymph nodes or Para-aortic lymph nodes. The vascular flow in the color Doppler around the focal lesion shows neovascularization and high vascularity, but Doppler will not detect the cancerous tissue in slow blood flow tumors. So the diagnostic picture is hypervascularity with centripetal or hepatofugal flow.
Hemodynamics in heart failure:
This will result in hepatic venous congestion and retrograde flow, inferior vena cava congestion with dilated diameters of the vessels, along with increased pulsatility of the portal vein.
There are no contraindications to this non-invasive procedure.
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 not suitable for the portal and hepatic veins and IVC.
Usually, Doppler ultrasound is carried by an experienced sonographer, either a radiologist or gastroenterologist.
Using hepatic Doppler ultrasound as part of the abdominal ultrasound will increase the operator experience in refining the probe movement and better visualize 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.
- The red color on the screen is usually set up in most devices as the direction of flow towards the probe, while the blue color means the direction of blood flow is away from the probe.
- The portal vein has thickened fibrous tissue wall that will be reflected as drawing a white line around the vessel, while hepatic veins’ walls are thin and non visualized.
- Portal vein diameter is from 7 to 14 mm when the patient is fasting for 8 hours. Hepatic veins each normally has a diameter of 5 to 7 mm. IVC normal diameter ranges from 13 to 28 mm with a collapsible wall on pressure.
- The patient is best to be positioned in the left lateral or supine positions. Scanning starts in the right subcostal area, which is best to view 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, you will view the IVC (compressible) and aorta at the midline; the probe faces posteriorly (either longitudinal or transverse).
- Hepatic veins can be best viewed from the right intercostal position where the probe is facing medially; this position is best to view the drainage of the three hepatic veins into the IVC. But the HVs could also be viewed from the subcostal position directing the probe posteriorly. Left and middle hepatic veins are best viewed from the substernal position.
- Obesity can cause limitation of the visualization of the flow of vessels, and pressure on the abdomen as done in the normal ultrasound may alter the pressure on the vessel, and the velocity detected.
- Eating may cause an increase in the portal pressure and widen the diameter of the vessel; also, gaseous and fluids may hinder visualization of the vessel. So it advised that the patients fast for at least 4 to 6 hours before Doppler examination.
- 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 cause some difficulty to the operator to visualize the vessels, but this could be overcome by the timing of inhalation and moving the probe synchronously. The operator's experience can improve this by refining the movement of the probe.
Hepatic doppler ultrasound is a non-invasive investigative tool that could help clinicians, especially if added to abdominal ultrasound. The cost-effectiveness of the procedure is high, with no hazards or complications.
Doppler ultrasound is the first imaging technique used in assessing the vascular condition in the following diseases; the degree of portal hypertension associated with cirrhosis, portal or hepatic vein thrombosis, and vascularity of the hepatic focal lesions, and follow after TIPS or liver transplantation. measuring the vessel patency, blood velocity, and direction will yield tremendous information helpful to guide the clinician on planing the management of the patients.
Referral to other invasive investigative or treatment procedures as hepatic venous pressure gradient HVPG, angiography, Transjugular intrahepatic portosystemic shunt TIPS, etc., all depends on first assessing the condition by Doppler ultrasound as the easiest and cheapest way to direct the patient to the most effective measure later. It also could be used for follow-up of the patients where repeated invasive procedures are not ethically accepted or plain harmful.
Enhancing Healthcare Team Outcomes
Evaluation of the liver condition of the patient starts with clinical evaluation and laboratory diagnosis. Radiological assessment in the form of abdominal ultrasound and accompanied by hepatic Doppler could give the physician a complete picture of the patient's liver condition in a non-invasive procedure. Doppler could detect early hepatic focal lesions and be part of the liver ultrasound screening process in high-risk individuals or post-operative hepatic procedures as TIPS and liver transplantation.
Hepatic Doppler also could detect the condition of varices and degree of portal hypertension; thus, the patient could be referred for prophylactic band ligation or medical therapy early before the condition deteriorates. Also, it could be used in the follow-up of patients post ligation.
Although hepatic doppler is mainly conducted by radiologists, it offers an instant clinical evaluation of the liver condition. Thus Doppler is also an important part of the hepatic assessment by gastroenterologists and surgeons. Most specialists in that area are familiar with abdominal ultrasound and hepatic Doppler as part of their training and practice.