Right Left paramedian paramedian segment segment

Right liver Left liver

Figure 11-3 The three anatomic segmentations with respect to hemiliver (right and left liver), Goldsmith and Woodburne (lateral, paramedian, and dorsal right or left segments), and Couinaud (numbers). (Source-. Used with permission from Soler L, Delingette H, Malandain G, et al. Fully automatic anatomical, pathological, and functional segmentation from CT scans for hepatic surgery. Comput Aided Surg 6.131-142, 2001.)

As the middle hepatic vein is within the liver parenchyma, it cannot be visualized without the aid of modern imaging studies such as computed tomography (CT) scans, magnetic resonance imaging (MRI), ultrasound (US), and angiography. The right liver is defined as segments 5, 6, 7, and 8 and is made up of the right lateral segment and right paramedian segment. The left liver is defined as segments 2, 3, and 4 and consists of the left lateral segment and the left paramedian segment. Segment 1 is the caudate lobe and stands alone anatomically as its venous drainage is not dependent on the right, middle, and left hepatic veins. It drains directly into the vena cava by small venous branches.3 The unique venous drainage of the caudate lobe becomes very important when discussing disorders of the hepatic venous system such as Budd-Chiari syndrome. Imaging of the liver and bile ducts and the identification of the segments are essential in planning operations on the liver (Fig. 11-4).

One important aspect of liver and bile duct anatomy is its highly variable composition. Both intra- and extrahepatic biliary ducts, as well as the arterial blood supply to liver and gallbladder are highly variant. Even simple procedures such as laparoscopic cholecystectomies can become treacherously difficult because of aberrant anatomy.4 One of the more common anatomic variants is the right hepatic artery either being completely replaced or with an accessory branch, arising from the superior mesenteric artery. Likewise, the left hepatic artery can be replaced completely or with an accessory branch, arising from the left gastric artery. A replaced or accessory right hepatic artery will be found in the porta hepatis posterior to the common bile duct and the replaced or accessory left hepatic artery will be found coursing through the gas-trohepatic ligament. Presence of these arterial variations may affect the surgeon's ability to perform specific procedures (i.e., pancreaticoduodenectomy) or facilitate planned resections (i.e., a right hepatectomy).

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