The etiology of portal vein thrombosis falls into the Virchow trias, comprising reduced blood flow within the vessel, changes in the consistency of blood which affects flow properties, and pathologies of the vessel wall. Thus, etiological factors of portal vein thrombosis are slow flow secondary to cirrhosis, obstruction of the vessel by porto-hepatic lymphadenopathy, direct invasion by cancer, inflammatory changes secondary to pancreatitis, sclerosing cholangitis, abdominal infections, poly-cytemia vera and benign masses [1,45,51,75].
While portal vein thrombosis often occurs in cases of HCC, it may also be the result of other primary or secondary neoplasms of the liver.
In cases of portal vein occlusion, portal perfusion is maintained due to the periportal collateral veins. With progression, the draining collaterals dilate while the thrombosed portal vein retracts to form a "cavernous transformation" which on US may be misinterpreted as patency of the portal vein [44,76].
MRI is an accurate method to non-invasively depict portal-venous blood flow, intraluminal thrombus and collateral circulation and can be generally carried out without the administration of contrast medium. Moreover, as it is not restricted by body habitus, ascites or abdominal gas, it is superior to duplex sonography [19,34,67,81,82].
MRI not only aids in the diagnosis of portal vein thrombosis, but also in the planning of shunt surgery and hepatic transplantation, and in the monitoring of shunt patency following surgery [5, 17,60].
Patency of the portal vein can be interpreted on spin-echo images by the demonstration of a flow void within the vessel. However, increased SI is frequently seen at the confluence of the splenic and the mesenteric veins. In cases of portal vein occlusion, the thrombosis is usually isointense compared to the liver parenchyma on T1-weighted images, and hyperintense on T2-weighted images .
A diagnosis of portal vein thrombosis is likely when the lesion is present on all sequences with comparable size and shape. The suspicion of portal vein thrombosis may be confirmed or excluded following acquisition of flow sensitive gradient echo images .
Chronic occlusion of a branch of the portal vein may be accompanied by segmental atrophy and compensatory hypertrophy of other segments .
Tumoral obstruction of lobal or segmental portal branches may present on T2-weighted images as wedge-shaped regions of increased SI. In such cases the apex usually points to the obstructing tumor, and therefore, MR images should be examined carefully .
In livers with preexisting fatty infiltration, the area affected by segmental portal vein obstruction shows a decreased accumulation of fat, as fat deliverance correlates with portal flow  (Fig. 14).
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