5 The abdomen

Anterior abdominal wall

Deep to the skin and subcutaneous fat of the lower anterior abdominal wall is a fibrous layer called Scarpa’s fascia. This extends to the penis and scrotum (or labia majora) and fuses with the deep fascia of the perineum (Colles’ fascia).

Muscle layers

The external oblique muscle arises on the anterior surface of the lower ribs and is inserted into the linea alba, pubic crest, inguinal ligament and iliac crest.

The internal oblique muscle arises from the lumbar fascia, the iliac crest and the lateral two-thirds of the inguinal ligament and is inserted into the linea alba and the costal margin.

The transversus abdominis muscle arises from the lower ribs, interdigitating with the diaphragm, the lumbar fascia, the iliac crest and the lateral half of the inguinal ligament, and is inserted into the linea alba.

The rectus abdominis muscle is a thick ribbon of muscle on either side of the midline from the costal cartilages to the pubic crest. It has three to four tendinous intersections at intervals along its length, which adhere to the anterior rectus sheath (Fig. 6.13B). The rectus sheath is formed by the aponeuroses of the other abdominal wall muscles as they surround the rectus muscle to attach to the linea alba.

Umbilical ligaments (see Fig. 6.13)

The median umbilical ligament is the fibrous remnant of the urachus which runs from the fundus of the bladder on the deep surface of the anterior abdominal wall to the umbilicus as a cord of variable thickness. It is covered by a fold of peritoneum.

The medial umbilical ligaments are the fibrous remnants of the obliterated umbilical arteries and run from the internal iliac artery to the umbilicus on the deep surface of the anterior abdominal wall. These are also covered by a fold of peritoneum.

The term lateral umbilical ligaments is sometimes used to describe the inferior epigastric artery and its covering fold of peritoneum as it passes superomedially from the external iliac artery to enter the rectus sheath posteriorly.

Properitoneal fat

A layer of fat separates the muscles of the anterior abdominal wall from the peritoneum and the abdominal contents. This is referred to as extraperitoneal fat or the properitoneal fat.

Radiological features of the anterior abdominal wall

Plain films of the abdomen (Fig. 5.1)

The muscle layers of the anterior abdominal wall are outlined, especially in obese individuals, between the subcutaneous fat line and the properitoneal fat line. Clearly seen fat lines indicate the lack of oedema in these areas, although in 18% of normal radiographs the properitoneal line is absent.

Radiology pearl

Free intraperitoneal gas may outline the umbilical ligaments and falciform ligament making them visible, thus making a diagnosis of pneumoperitoneum possible on a supine radiograph.

CT of the abdomen (Figs 5.2–5.4, 5.46, 6.13B)

The muscle layers of the anterior abdominal wall can be seen in cross-section. Three muscles can be seen anterolaterally: the external oblique is outermost, then the internal oblique, with transversus abdominis deepest. The rectus muscle and its rectus sheath can be seen in the anterior paramedian position superficial to the other muscles.

Radiology pearl

Skin punctures lateral to the rectus muscles, or in the midline, will avoid the epigastric vessels. Catheter placement that avoids the recti is also better tolerated.

Radiology pearl

In the ruptured anterior urethra contrast can be seen to extravasate into the scrotum, perineum and penis, and then on to the lower abdominal wall because of the attachments of Scarpa’s and Colles’ fascia.

The stomach (Figs 5.5–5.9)

The stomach is J-shaped but varies in size and shape with the volume of its contents, with erect or supine position, and even with inspiration and expiration. The size and shape of the stomach also vary considerably from person to person, differing especially with the build of the subject.

Figure 5.5 • The stomach.

Figure 5.6 • Posterior relations of the stomach (viewed from anterior through the stomach).

Figure 5.7 • Arterial supply of the stomach.

Figure 5.8 • Venous drainage of stomach.

The stomach has two orifices – the cardia and the pylorus. The cardiac orifice or cardia is so-named because of its proximity to the heart and is the anatomic term for the junction of the oesophagus and stomach. The radiological names are gastro-oesophageal junction, oesophagogastric junction and lower oesophageal sphincter. The stomach has two curvatures – the greater and lesser curves. The incisura is an angulation of the lesser curve.

The part of the stomach above the cardia is called the fundus. Between the cardia and the incisura is the body of the stomach, and distal to the incisura is the gastric antrum. The lumen of the pylorus is referred to as the pyloric canal.

The stomach is lined by mucosa, which has tiny nodular elevations called the areae gastricae and is thrown into folds called rugae. Longitudinal folds paralleling the lesser curve are called the ‘magenstrasse’ meaning ‘stomach street’. Rugae elsewhere in the stomach are random and patternless.

There are three muscle layers in the wall of the stomach: (i) an outer longitudinal, (ii) an inner circular and (iii) an incomplete, innermost oblique layer. The circular layer is thickened at the pylorus as a sphincter, but not at the oesophagogastric junction. Fibres of the oblique layer loop around the notch between the oesophagus and the fundus and help to prevent reflux here.

Radiology pearl

The oblique fibres are responsible for the ‘magenstrasse’ (German for ‘stomach street’) and can pinch off the remainder of the stomach allowing fluids and food to pass directly and rapidly from oesophagus along the lesser curve to the duodenum without filling the main lumen of the stomach. This can be seen when liquid contrast is swallowed.

Peritoneum covers the anterior and posterior surfaces of the stomach and is continued between the lesser curve and the liver as the lesser omentum, and beyond the greater curve as the greater omentum.

Anterior relations of the stomach

The upper part of the stomach is covered by the left lobe of the liver on its right and by the left diaphragm on the left (Fig. 5.46A). The fundus occupies the concavity of the left dome of the diaphragm. The remainder of the anterior of the stomach is covered by the anterior abdominal wall.

Posterior relations of the stomach (Fig. 5.6)

Posterior to the stomach lies the lesser sac (see section on peritoneal spaces of the abdomen later in the chapter, and Figs 5.52 and 5.53). The structures of the posterior abdominal wall that are posterior to this are referred to as the stomach bed. The pancreas lies across the midportion of the stomach bed with the splenic artery partly above and partly behind it, and the spleen at its tail. Above the pancreas are the aorta and its coeliac trunk and surrounding plexus and nodes, the diaphragm, the left kidney and the adrenal gland. Attached to the anterior surface of the pancreas is the transverse mesocolon, which forms the inferior part of the stomach bed.