Portal Vein Thrombosis Imaging 

Portal vein thrombosis (PVT) is being recognized with increasing frequency with the use of ultrasonography. Reduced portal blood flow caused by hepatic parenchymal disease and abdominal sepsis (ie, infectious or ascending thrombophlebitis) are the major causes. Transient PVT is also being recognized with increasing frequency, partly because of the great increase in the use of ultrasonography in the evaluation of patients with abdominal inflammation such as appendicitis. Hypercoagulable syndromes can lead to portomesenteric and splenic vein thrombosis. Patients with these conditions may present with acute or subacute intestinal angina. In late stages, patients may have variceal bleeding.^{[1, 2, 3, 4, 5, 6, 7]}

See the images of portal vein thrombosis provided below. Click any single image to see all the images in a slideshow format.

Generally, the portal vein enters the porta hepatis and divides into the right and left main branches. The right main branch divides into anterior and posterior branches that supply the anterior and posterior segments of the right lobe. The left main branch courses horizontally to the left before turning vertically to form the medial and lateral segmental branches. Several variations of the portal venous anatomy have been described by using ultrasonography, CT, and cadaveric dissection.

Preferred examination

Preferred examinations include duplex Doppler ultrasonography and/or color Doppler ultrasonography, CT, magnetic resonance angiography (MRA) (see the image below), and arterial portography or splenoportography.^[8]

Tumor in the portal vein may have an appearance identical to that of thrombosis, but this appearance is far less common than others. Tumor in the portal vein is most frequently related to hepatocellular carcinoma. The thrombus may be partial or complete. It may be mixed with bland thrombus as well.

Adults who have acute PVT secondary to abdominal sepsis may completely recover, and the vessel may recanalize with successful treatment of the underlying sepsis.^[9] In children, the portal vein may recanalize with the development of multiple, small, collateral channels. These channels are seen as a partly echogenic band of small vessels extending to the porta hepatis (cavernous transformation). These have a reduced flow velocity of 2-7 cm/s. Nonvisualization of the portal vein is strongly suggestive of occlusion. The portal vein may then be seen as a band of high-level echoes at the porta hepatis.^[10]

Intervention

The development of PVT can precipitate the need for emergency endoscopy for sclerotherapy of varices, transjugular intrahepatic portosystemic shunt (TIPS) creation, surgical portocaval shunt creation, transjugular or transhepatic portomesenteric thrombolysis and thrombectomy, or even resection of ischemic bowel or liver transplantation.^[11] However, PVT may complicate sclerotherapy. Fine-needle aspiration biopsy of PVT can be performed with color Doppler sonographic guidance to assess therapeutic effectiveness.

Early complications of TIPS creation that are detectable with ultrasonography include the following: intraperitoneal hemorrhage, shunt thrombosis, neck hematoma, compromise of hepatic blood supply, PVT, hepatic artery occlusion, hepatic infarction, failed stent deployment, inadequate stent expansion, stent retraction, stent fracture, and biliary obstruction.^{[12, 13, 14]}

Delayed complications of TIPS creation that are detectable with ultrasonography include shunt stenosis caused by pseudointimal hyperplasia and hepatic vein stenosis.^{[15, 16]}

Plain radiographs may reveal hepatosplenomegaly, an enlarged azygos vein, and paraspinal varices. Esophageal varices that consist of dilated submucosal veins in the lower esophagus occur chiefly as a consequence of portal hypertension, mostly in cirrhosis. The varices appear as beaded or serpiginous translucent filling defects. Large esophageal varices are obvious and appear as nodular or vermiform, changeable filling defects within the esophagus. Smaller varices appear as scalloped esophageal folds, which are better seen on recumbent radiographs, because they tend to disappear on upright images.

The portal vein supplies 75% of the blood flow to the liver. Therefore, peak liver contrast enhancement occurs during the portal venous phase, about 60 seconds after the start of a bolus injection of contrast material.^[17] With helical CT, a liver examination requires about 20 seconds to complete; images can usually be acquired in one breath hold.^[18]

This technique can be extended to acquire a dual-phase contrast-enhanced CT scan in which the liver is imaged twice with a single contrast agent bolus, first during the arterial phase and then through portal venous phase. Dual-phase CT is indicated in some cases involving benign or malignant lesions in which vascular characteristics suggest the correct diagnosis (see the images below).

Angiographically assisted CT, or CT arterial portography, can provide better delineation of the portal venous system and portal venous enhancement of the liver. An angiographic catheter is placed in the common celiac axis, hepatic artery, or superior mesenteric artery by using a modified Seldinger technique via the femoral artery. Image acquisition begins 3-5 seconds after injection of the contrast agent is initiated. The examination should be completed as soon as possible, before the contrast material recirculates. To prevent significant artifacts related to the density of the contrast material, 70 mL of dilute (1-30%) iodinated contrast agent is used with an infusion rate of 2 mL/s. Although angiographically assisted CT can produce elegant images, it is invasive, expensive, and not widely accepted.

On contrast-enhanced CT scans, PVT may be depicted as a low-attenuating center in the portal vein surrounded by peripheral enhancement. Portal vein attenuation is 20-30 HU less than that of the aorta.

CT angiography (CTA) is an application of helical CT. The rapidity of helical CT allows the maintenance of a higher concentration of intravenous contrast medium, particularly through the arterial enhancement phase, and it has the capability of 3-dimensional (3D) reconstruction. Both peripheral intravenous injections of contrast agent and CT arterial portography have been used as with CTA. CTA has shown great promise in the evaluation of hepatic vessels before liver resection. It provides preoperative surgical information about the segmental location of liver tumors, the segmental venous anatomy, and the presence of significant arterial anomalies. The value of CTA in the evaluation of portal hypertension is unclear, but CTA is likely to be useful because it may delineate the collateral vessels, varices, and other findings in patients with portal hypertension.

The vascular anatomy of the liver may be outlined by using spin-echo and gradient-recalled-echo (GRE) techniques, but these techniques cannot demonstrate the direction of portal flow. Time-of-flight MRI with bolus tracking has been successful in the assessment of portal hypertension and its sequelae. Phase-contrast sequences can also be used to evaluate the portal vein, and phase-contrast cine MRA can show the direction of portal venous flow and the presence of portal vein thrombus. Magnetic resonance evaluation of the portal venous system accurately demonstrates thrombosis and the collateral circulation. Gadolinium enhancement is useful in this application (see the images below).^{[19, 20, 21]}

Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.

NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.

Degree of confidence

MRCP coupled with dynamic 3D gradient-echo imaging can not only detect portal vein occlusion, cavernous transformation, and gallbladder varices but also depict bile duct abnormalities associated with portal biliopathy.^{[22, 23, 24]}

False positives/negatives

Lin and associates compared surgery and X-ray portography with 3D CE MRA in patients with portal vein involvement from hepatocellular carcinoma.^[25] The overall sensitivity with 3D CE MRA was 99%; the specificity was 96%; the positive predictive value was 91%; and the negative predictive value was 99%. The accuracy in diagnosis of the main portal vein was 100%. 3D CE MRA resulted in 7 false-positive interpretations involving 6 left portal veins and 1 right portal vein. One false-negative diagnosis was made with right portal vein involvement.

Shah and associates compared MRI with intraoperative findings in the diagnosis of portal vein thrombosis in transplantation candidates. In this study, the sensitivity and specificity of MRI for detecting main PVT were 100% and 98%, respectively. The cause of discordance between findings on MRI and at transplantation in 2 cases was a diminutive caliber of the main portal vein that was interpreted as recanalized chronic thrombosis on MRI. The major reason for a false-positive MRI is a diminutive but patent portal vein.^[21]

An extremely slow portal venous flow is considered to be the cause of false-positive findings with unenhanced MRI and sonography.^[26]

PVT is being recognized with increasing frequency at ultrasonography. Abdominal sepsis and reduced portal blood flow resulting from hepatic parenchymal disease are the major causes. Transient PVT is also being recognized with increasing frequency, partly because of the great increase in the use of ultrasonography in the evaluation of patients with abdominal inflammation, such as appendicitis. Tumor in the portal vein may have an appearance identical to that of thrombosis,^{[27, 28]} but this appearance is far less common than others (see the images below).

On sonograms, echogenic lesions may be present in the portal vein. Clot with variable echogenicity may be depicted. The clot usually has moderate echogenicity, but if it is recently formed, it may be hypoechoic. Patent vessels may have increased intraluminal echogenicity because of erythrocyte rouleaux formation, which makes slow-flowing blood slightly echogenic. Increased or decreased echogenicity may be observed in the lumen of the portal vein. In isolation, this finding is not sufficient to diagnose or exclude PVT.

PVT eliminates the usual venous flow signal from the lumen of the portal vein during either pulsed or color flow Doppler imaging. Color flow Doppler images can show flow around a thrombus that partially blocks the vein. However, if flow is sluggish, the Doppler signal may not be detected. Color flow may be present in other small collateral vessels.^[29]

Incomplete occlusion may occur. This is common with neoplastic invasion. Alternatively, thrombolytic recanalization may occur. The 2 cannot be differentiated on sonograms. Cavernous malformation, spontaneous shunts, and splenorenal and portosystemic collaterals may be seen. The underlying cause (eg, hepatocellular carcinoma, metastases, cirrhosis, pancreatic neoplasms) may be evident. The incidence of PVT is reported to be low in portal hypertension.

The string sign—that is, thickening of the portal vein with narrowing of its lumen—is assumed to be caused by portal phlebitis. This is considered a precursor of PVT in patients with acute pancreatitis. The portal vein thrombus may be calcified. The diameter of the portal vein is larger than 15 mm in 38% of the cases of PVT.^[30]

Among catheter-directed techniques, arterial portography is now the preferred method for evaluating the portal venous system because it is less invasive and has a lower complication rate than the other methods (eg, splenoportography). The method involves the indirect opacification of the portal venous system with an injection of contrast material into the splenic vein to outline the splenic and portal veins or superior artery to outline the superior mesenteric and portal veins.

The 3 major indications for arterial portography are the following: (1) to examine patients with portal hypertension and its sequelae, particularly when surgical treatment is planned; (2) to determine the resectability of hepatic and pancreatic tumors when both the arterial- and venous-phase angiographic findings make a significant contribution; and (3) to perform transcatheter embolization, in cases of metastases of islet cell tumors or carcinoid metastases, or chemoembolization, in cases of hepatocellular carcinoma. (A widely or partly patent portal vein is a prerequisite for such treatment.)^[31]

Patient preparation and contraindications are the same as those of conventional angiography. Selective catheters are used to cannulate the appropriate vessel. In a superior mesenteric artery injection, the tip of the catheter is placed to enable opacification of all the branches with contrast material. Before the delivery of the contrast agent, a vasodilator (tolazoline [Priscoline], papaverine, or prostaglandin E) is administered to improve opacification of the portal vein.

Manual or digital subtraction is necessary. If digital subtraction is used, the administration of an anticholinergic drug (eg, glucagon) before imaging reduces bowel motion. Selective injection of the superior mesenteric artery can be used to outline the superior mesenteric and portal veins. For splenic and/or portal vein delineation, splenic artery catheterization is chosen. Injection of the left gastric artery consistently demonstrates esophageal varices.

Another technique for opacifying the portal system is wedged hepatic venous injections of carbon dioxide.^[32] The low viscosity of the gas allows it to readily float across the hepatic sinusoids, successfully opacifying the portal vein in most cases.

Degree of confidence

Diagnostic modalities such as ultrasonography, CT, and MRI have reduced the diagnostic importance of arteriography in the diagnosis of liver tumors. The role of liver angiography seems to be limited to the occasional mapping of the vascular anatomy before surgery (now largely performed with MRA or CTA) and the transcatheter treatment of liver tumors. Venous-phase imaging in the celiac axis and superior angiography provide sufficient detail to make direct portography unnecessary in most cases.