CT proves valuable in portal vein thrombosis

Multislice CT has certain advantages over ultrasound in the diagnosis of portal vein thrombosis, and clinicians are beginning to notice, according to an educational poster at the 2005 European Society of Gastrointestinal and Abdominal Radiology meeting in Florence.

"Contrast-enhanced CT has gained an increasingly important role in patients with acute mesenteric venous thrombosis. It is considered the diagnostic test of choice at many centers," said lead author Dr. Euan Armstrong, specialist registrar in the department of radiology at Derriford Hospital in Plymouth, U.K.

The Plymouth team wrote that differentiation of tumor from bland thrombus can be difficult but important. The most common cause for portal vein tumor invasion is hepatocellular carcinoma.

Ultrasound is the modality of choice for initial investigation of portal vein thrombosis. The features of PVT demonstrated on B-mode ultrasound include increased portal vein diameter, visualization of the thrombus, collateralization, and cavernous transformation. Sensitivity is improved by color Doppler and the use of contrast agents, according to the poster.

Doppler sonography can differentiate bland from tumor thrombus, but Doppler parameters must be fully optimized. Pulsatile flow in portal vein thrombi occurring in patients with cirrhosis as a sign for malignant portal vein thrombus has a sensitivity of 62% and specificity of 95%, the poster said. Continuous or absent flow can be detected in both benign and malignant portal vein thrombus and is not useful in differentiating between the two.

The limitations of ultrasound are its operator dependence and reduced sensitivity with increased patient body habitus, gas, and ascites. Low flow can also be mistaken for thrombosis. Portal vein visualization is difficult in cirrhotic patients, due to sound attenuation, and may be improved with contrast agents. Overall detection of PVT by ultrasound has a reported sensitivity of 89% and specificity of 92%. Endoscopic ultrasound may be more sensitive than transabdominal ultrasound, but it is invasive.

Multislice CT scanners offer the advantages of significantly shorter acquisition times, retrospective thin- or thick-section reconstruction from the same raw data, improved 3D rendering with decreased helical artifact, and increased z-axis coverage without compromise of image quality.

Overall CT detection of bland PVT has a reported sensitivity of 90% and specificity of 99%. CT may also detect the underlying cause of PVT. CT characterization of malignant PVT has a reported sensitivity of 86% and specificity of 100%.

Timing parameters can be optimized to allow imaging of both arterial and portal venous phases. CT can evaluate vascular structures, the bowel wall, and adjacent mesentery. Findings of PVT include persistent, well-defined intraluminal filling defects with central low attenuation, which may be surrounded by well-defined, rim-enhancing venous walls.

Features suggesting malignancy include identification of a main PVT diameter greater than or equal to 23 mm, PVT enhancement, and neovascularity.

"A pitfall of portal vein imaging by CT is the appearance of pseudothrombus. This appearance occurs during the hepatic arterial phase in the main portal vein lumen and is due to mixed flow from the enhanced splenic vein return and the nonenhanced superior mesenteric vein return," the authors said. "However, a homogeneously enhanced portal vein is seen during the late portal venous phase."

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