Poal venous gas is seen in association with pneumatosis intestinalis, most commonly with necrotizing enterocolitis. The physiology of this is that the poal vein, the mesenteric veins, drains nutrient-rich blood from the gut to the liver. In the case of necrosis with air in the bowel wall, air migrates into the poal venous system and to the liver. On CT, this has the characteristic appearance of peripheral lucencies following the poal venous system intrahepatically. In cases of more severe pneumatosis, the bowel may rupture and lead to pneumoperitoneum. Also Know: Pneumobilia, or air in the biliary tract, would be seen after instrumentation of the biliary system, such as after an endoscopic retrograde cholangiopancreatogram (ERCP). Other causes include a gas-forming infection within the biliary tree or previous sphincterotomy (endoscopic opening of the sphincter of Oddi). Pneumobilia has a distinct appearance on CT: there is gas located centrally in the liver within the ducts. Hemobilia, or blood in the biliary tract, would be seen after instrumentation of the biliary system, such as after an endoscopic retrograde cholangiopancreatogram (ERCP), from a biliary or hepatic tumor, or secondary to a hypocoagulable state. Hemobilia is found at endoscopy, and is generally not visible on plain radiographs. High attenuation material may be seen within the bile ducts on a CT scan, suggesting hemobilia. Hepatic venous gas would not be seen with pneumatosis because the hepatic veins drain the liver into the inferior vena cava (IVC). Gas from the bowel wall gets trapped in the poal veins and does not traverse the liver to get into the hepatic veins. Ref: Thilo E.H., Rosenberg A.A. (2012). Chapter 2. The Newborn Infant. In W.W. Hay, Jr., M.J. Levin, R.R. Deterding, J.J. Ross, J.M. Sondheimer (Eds), CURRENT Diagnosis & Treatment: Pediatrics, 21e.