Cross-sectional anatomy of the upper abdomen

Gas in the stomach may obscure the lesser sac. Bowel gas generally obscures the remainder of the peritoneal cavity, although if free f l u id 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 w i th 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.

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 I II are distinguished from segments IVa and IVb by the fissure for the ligamentum teres on the 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.

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 w i th 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 w i th an adrenal abnormality.

Fig. 5.56 (a) Axial anatomy through upper liver and spleen (T10/T11 approx) (b) CT abdomen: axial image through upper liver and spleen.

THE ABDOMEN 209

Fig. 5.57 (a) Axial anatomy at level of portahepatic (T11 approx); (b) CT section through porta hepatis.

Spleen This section may pass through the splenic hilum. Vessels are seen dividing into branches just medial to the spleen. The splenic vein passes from the spleen posterior to the pancreas and is often visible throughout its length on one or two sections. The splenic artery has a tortuous course and only short segments are visible on axial sections.

Other viscera

The aorta lies in the retrocrural space and its coeliac branch may be seen at this level. The IVC lies close to the liver as above.

The stomach and colon are seen as on the higher section.

Axial section at the level of the body of the

pancreas (Figs 5.58a and b)

Body wall The ribs and intercostal muscles are seen as before. The latissimus dorsi is more lateral in position than before and the serratus anterior muscle is seen interdigitating w i th the external oblique muscle. The internal oblique muscle is seen arising from the deep aspect of the ribs.

Liver

The left lobe of the liver is smaller or not seen at this level. The gallbladder is visible below the porta hepatis.

Pancreas The tail and body lie higher than the head of the pancreas. The tail of the gland is related to the splenic hilum. The body of the pancreas is seen anterior to the splenic vein and the aorta.

Other viscera

The adrenal glands are still visible on this section. The upper pole of the left kidney (higher than that of the right) is visible.

The first part of the duodenum lies superior to the pancreatic head. The lower pole of the spleen lies behind the colon and lateral to the left kidney.

Axial section at the level of the pancreatic head

and renal hila (Figs 5.59a and b)

Body wall The abdominal wall proper is now visible w i th the recti on each side of the linea alba and the three muscle layers of the abdominal wall - the internal and external oblique and transversus abdominis muscles - between the recti and the lower ribs.

Liver The lower part of the right lobe is seen. A smaller left lobe, which may be completely separate from the right lobe, may be seen. The neck of the gallbladder (its highest part) may be seen. The spleen is not usually seen at this level.

Pancreas

The head of pancreas is seen at this level. The pancreatic duct may be identified as a thin, hypodense linear structure. The common bile duct and gastroduodenal artery are seen in cross-section. The bile duct lies in the lateral aspect of the head, close to the duodenal surface. The gastroduodenal artery (a branch of the hepatic artery) lies lateral and more anterior. In slightly lower sections the head of the pancreas is seen to be separated from the uncinate process by the superior mesenteric vessels (the vein lies left of the artery).

Kidneys and renal vessels This section includes one or both renal hila (the left hilum is slightly higher than the right). The left renal vein is seen passing to the IVC from the left hilum anterior to the aorta. The renal arteries have frequently divided already into anterior and posterior branches and may lie both anterior and posterior to the vein and pelvis.

Renal medullary pyramids can be demonstrated in their full length at the level of the hilum, whereas above and below this they are intersected at various planes to their long axis because of their orientation towards the hilum.

The aorta and the inferior vena cava

The aorta remains between the crura. Its superior mesenteric branch is visible at this level. The IVC is separated from the aorta by the crura and, at this level, is free from the liver.

Other viscera

The second part of the duodenum lies lateral to the pancreatic head. The loops of small intestine lie to the left of the head of the pancreas. Vessels and fat are seen in its mesentery.

Axial section below the kidneys (Figs 5.60a and b)

Body wall The crura of the diaphragm may be seen as muscular columns that are not connected to one another, depending on level (not seen below L3). The right crus is usually larger than the left and extends further caudally.

THE ABDOMEN 211

Fig. 5.58 (a) Axial anatomy at level of body of pancreas (T12/L1); (b) CT section through body of pancreas.

Fig. 5.59 (a) Axial anatomy at level of pancreatic head (L1 approx) (b) CT section through pancreatic head and renal hila.

THE ABDOMEN 2 13

Fig. 5.60 (a) Axial anatomy at mid-abdomen (L2/L3); (b) CT section through mid-abdomen.

The psoas muscle lies along the lateral aspect of the vertebral bodies and, although thin here, it increases in bulk in lower cuts. Other muscles are as described in previous sections.

The duodenum The third part of the duodenum is seen passing between the aorta and the superior mesenteric artery. The colon and jejunum The transverse colon is seen close to the anterior abdominal wall, w i th the ascending and descending colon laterally. Loops of jejunum and their mesentery occupy most of the anterior part of this section.