THE ABDOMEN
Gas in the stomach may obscure the lesser sac. Bowel gas generally obscures the remainder of the peritoneal cavity, although if free fluid is present the peritoneal cavity is well seen. Computed tomography (Fig. 5.55) The peritoneal spaces and their ligaments are well seen if there is free fluid present. The bare area can be identified as it lies directly on the diaphragm, whereas the remainder of the liver is surrounded with fluid. The ligaments usually contain some fat as well as vessels. The greater omentum can also be seen as a thin fat-filled structure anterior to bowel loops beneath the anterior abdominal wall. CROSS-SECTIONAL ANATOMY OF THE UPPER ABDOMEN Axial section through upper liver and spleen (Fig. 5.56a and b) Body wall The ribs and intercostal muscles are sectioned obliquely around most of the perimeter of the section. Anteriorly the costal cartilages forming the costal margin are separated by the two rectus abdominis muscles and the midline linea alba between. Superficial to the ribs and intercostal muscles lie the serratus anterior muscles laterally and the latissimus dorsi posterolaterally. The erector spinae muscles lie posterior to the vertebra on each side.
anterior surface of the liver. The fissure for the ligamentum venosum on the visceral surface separates the caudate lobe (segment I) posteriorly from segments II and IV anteriorly. Branches of the portal veins and bile ducts may be seen especially on contrast-enhanced scans. This level is above the porta hepatis. Spleen Lying posteriorly on the left, close to the diaphragm and the ribs, the spleen has a smooth diaphragmatic surface and a concave visceral surface in axial section. Other viscera The stomach lies deep to the left lobe of the liver. The transverse colon may be seen anterior to the stomach, although its position is variable. The splenic flexure is found anterior to the spleen. Axial section at the level of the porta of the liver (Fig. 5.57a and b) Body wall This is as for the level above. The gap between the costal margins becomes progressively wider in lower cuts. Diaphragm As for above level. Usually only the crura and adjacent diaphragm are visible.
The diaphragm The domes of the diaphragm can be distinguished from the liver and other intra-abdominal viscera only where these are separated by fat. The lung bases lie posteriorly, separated from the liver and spleen by the diaphragm. The crura are visible anterior to the vertebral bodies. With the adjoining diaphragm these form linear structures extending from the posterior aspect of the liver and spleen to the anterior surface of the abdominal aorta. The retrocrural space so formed is the lowest recess of the mediastinum. In addition to the aorta it contains the azygos vein on the right side and the hemiazygos vein on the left, the thoracic duct on the right posterior to the aorta (not usually visible on CT scans unless dilated), lymph nodes and fat. The IVC remains on the abdominal side, that is, lateral to the crura below T8 level, where it pierces the diaphragm. Liver The liver occupies most of the right half of this section. Segments II and III are distinguished from segments IVa and IVb by the fissure for the ligamentum teres on the
Liver The falciform ligament is intrahepatic at this level. On this section, a line dividing the liver that passes through the gallbladder and the IVC defines the morphological right and left lobes (division of segment IV from segments V and VI). (This line has to be extrapolated back from the gallbladder, which is seen on lower sections). That part of the anatomical right lobe left of this line and anterior to the porta is the quadrate lobe and that part posterior to the porta is the caudate lobe. At the porta hepatis the portal veins lie posteriorly with the branches of the hepatic artery and of the bile ducts anteriorly. Adrenal glands The right adrenal gland is a linear structure directly behind the IVC between the crura and the liver. The left adrenal gland extends deeper in front of the left kidney than does the right. Nearby splenic vessels may be confused with an adrenal abnormality.
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