1. I MARZOUK MOUSSA, A BEN TEKAYA, A FENNIRA, L BEN FARHAT, N DALI, L HENDAOUI MEDICAL IMAGING AND INTERVENTIONNAL DEPARTMENT MONGI SLIM LA MARSA TUNISIA GI 11

2.  The third sector is defined by aberrant vessels, mainly by veins entering directly into the liver independently of the portal system.  This contribution is constant, it leads eventually to localized metabolic changes that create pitfalls in imaging.  The purpose of this work is to explain the pathophysiology of this entity and illustrate aspects of imaging through two new cases.

3.  Two patients, aged 62 and 51, were reffered to our department for an acute intestinal obstruction for the first and acute pancreatitis for the second.  An ultrasonography and an abdominal computed tomography (CT) were performed for one, a CT scan for the other.

5. Observation 1  Mrs M.H 51 years old aged woman, without previous medical history was hospitalized in surgery for acute pancreatitis.  An abdominal ultrasonography was performed (Fig. 1), that showed in addition of a gallbladder lithiasis, a hyperechoic liver steatosis with an ill- defined hypoechoic area of segment IV in relation with an island of "healthy“ liver.

6. fig. 1: Ultrasound of the liver, gallbladder stones. Hypoechoic area of segment IV of the liver (arrow) within an overall hyperechoic liver parenchyma.

7.  Anabdominal CT scan (Fig. 2 and 3) was performed for the pancreatic grader and has shown in addition of an edematous pancreas, an ill-defined area of the segment IV of the liver that enhanced in the arterial phase of injection and became isodense to liver parenchyma in portal time ; realizing a disorder of hepatic perfusion. Observation 1

8. Fig 2aFig 2b fig3 Fig. 2 and 3: Axial abdominal CT after injection of contrast during the arterial phase (Fig. 2) and portal (Fig. 3): ill- defined area of the posterior segment IV of the liver with enhancement in the arterial time and which disappeared at the portal time.

9. Observation 1  There was no hepatic tumor mass and no arterial, venous, or portal vascular hepatic anomaly.  The semiology of the perfusion disorder was compatible with the vascularization of segment IV by an aberrant vessel.

11.  Mr L. H. 62 years old aged man, was hospitalized for a bowel obstruction. He was operated a year ago for acute appendicitis.  The questioning and physical examination were in favor of a high occlusion. Observation 2

12.  Computed tomography was performed,it showed in addition of small bowel distension, a disorder of hepatic perfusion of segment IV of the liver as a parenchymal enhancement during the arterial phase of injection becomes isodense to the liver in portal phase.

13. Fig. 4 and 5: Axial abdominal CT after injection of contrast during the arterial phase (Fig. 4) and portal (Fig. 5): ill-defined area around the gallbladder bed (arrow) with enhancement in the arterial time and which disappeared at the portal time. Fig 4 Fig 5

14. Observation 2  There was no hepatic tumor mass and no arterial, venous, or portal vascular hepatic anomaly.  The patient was operated and the occlusion was secondary to a flange.  The semiology of the perfusion disorder was compatible with the vascularization of segment IV by an aberrant vessel.

16. Discussion  The liver has the particularity to have a dual system of arterial and portal input.  There is in normal state a communication between these two systems at different anatomical locations such as liver sinusoids and parabiliary plexus.  These communication systems are brought into play physiologically in normal state or in case of abnormal delivery systems.

17. Discussion  Some hepatic areas have a third systemic origin of vascular supply that is done through aberrant veins, and which enter directly into the liver independently of the portal system.  These veins anastomose with the intrahepatic portal system at various sites and are responsible for a focal decrease of portal perfusion and a small increase in arterial supply.

18. Discussion  These haemodynamic changes are also responsible for metabolic changes in the hepatocytes, including lipid metabolism which may be the cause of healthy liver area in steatosis or fatty accumulation zones.  There are mainly three vascular networks: the Cystic vein to the gallbladder bed, the parabiliary venous system, the para-umbilical and epigastric veins.

19. A single large cystic vein does not exist. Small venous channels on the hepatic side of the gallbladder lead directly into the liver. Other small veins flow toward the cystic duct and merge with channels from the common bile duct before terminating in the portal venous system. Cystic venous system

20. Drawing shows the anatomy of the parabiliary venous system.

21. Diagram shows the anatomy of the paraumbilical vein.

22. Discussion  Imaging this third sector is responsible for visible disorders of hepatic perfusion CT or MRI after injection of contrast by intravenous way.  Indeed, the vascularized hepatic parenchyma by these aberrant veins shows an enhancement more important than the liver during the arterial time and becomes isodense to the liver at the portal time.  Aberrant drainage of the cystic vein in the liver gives the pseudo- lesions around the gallbladder bed.

23. Discussion  Aberrant drainage of the parabiliary venous system causes artifacts typically localized to the posterior part (dorsum) of segment IV.  The epigastric venous system typically provides perfusion trubles in subcapsular liver areas.  In some situations this third sector is revealed by rounded or oval perfusion disorders, making the differential diagnosis of a tumor almost impossible.

24.  The aberrant vessels are rare and responsible for most pitfalls in CT image.  The differential diagnosis with a hypervascular focal liver lesion can be difficult requiring a good knowledge of their symptomatology.

25.  Yong Yeon Jeong et al Hepatocellular Carcinoma in the Cirrhotic Liver with Helical CT and MRI: Imaging Spectrum and Pitfalls of Cirrhosis- Related Nodules.AJR 2005 vol. 185no. 4 1024-1032.  Kengo Yoshimitsu et al. Unusual Hemodynamics and Pseudolesions of the Noncirrhotic Liver at CT. RadioGraphics October 2001,21, S81-S96.  Mathieu D, Luciani A, Achab A, Zegai B, Bouanane M, Kobeiter H. Les pseudo-lésions hépatiques. Gastroenterol Clin Biol 2001;25: B158-B166.  Itai Y, Matsui O. Blood flow and liver imaging. Radiology 1997, 202: 306- 314.  Chen WP, Chen Jeon-Hor, Hwang JI, Tsai JW, Chen JS. Spectrum of transient hepatic attenuation differences in biphasic helical CT. AJR 1999; 172: 419-424