Abdominal bowel pathology

Small Bowel Obstruction

Causes of small bowel obstruction include adhesions, herniae and gallstone ileus. The ‘transition point’ is the point at which proximal bowel is dilated, and distal bowel is collapsed. This usually indicates site of obstruction.

Small bowel ileus occurs due to metabolic derangements or post-operatively, with dilatation but no transition point.

Appearance: Dilated small bowel loops proximal to a transition point; Multiple air-fluid levels; Possible ischaemic bowel (a complication).

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Large Bowel Obstruction

Common causes of large bowel obstruction are cancers, diverticulosis and volvulus. An obstruction of the large bowel causes dilatation of proximal large bowel. However, the small bowel may or may not be dilated as well, depending on whether the ileocaecal valve is competent. It is competent in ~70% of people.  A competent valve traps contents in the large bowel, so the small bowel is not dilated. However, this causes the large bowel to dilate much more and much quicker, putting it at a higher risk of perforation.

Appearance: Dilated large bowel loops proximal to a transition point; Multiple air-fluid levels; Dilated small bowel loops (if valve incompetent).

The 3-6-9 rule:

Dilated Small bowel: > 3 cm

Dilated Large bowel: > 6 cm

Dilated Caecum: > 9 cm


Large bowel obstruction on CT

Small Bowel Inflammation

Most common cause is Crohn's disease, which affects patchy areas along the entire gut, most commonly the terminal ileum. Common complications are perforation (free air outside the bowel), abscesses, fistulae to other structures and obstructing strictures causing bowel dilatation.

Appearance: “Target sign” involving the small bowel (on CT).

  • Enhancement (bright) of serosa and mucosa due to more blood flow
  • Low attenuation (dark) wall: oedema

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Large Bowel Inflammation

Ulcerative colitis affects a continuous section of large bowel, most commonly the rectum. Infective causes, such as Clostridium difficile infection, are also common. If the entire large bowel is affected, it is called a pancolitis. Both may cause toxic megacolon (seen as marked colonic dilatation).

Appearance: “Target sign” involving the large bowel (on CT)

  • Enhancement (bright) of serosa and mucosa due to more blood flow
  • Low attenuation (dark), thickened wall due to wall oedema
  • Ascites also seen with C Difficile colitis

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Appendicitis can present at any age. In children, mesenteric adenitis is a common and harmless differential that should be considered. In older people, an underlying caecal carcinoma needs to be considered. The diagnosis can be made on US although the appendix is frequently not visible due to being obscured by overlying bowel gas.


  • Dilated, fluid-filled appendix
  • ‘Fat stranding’ around appendix
  • Possible associated soft tissue mass (lymphoid tissue, cancer); or hyperdense faecolith at base of appendix

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Diverticulitis typically occurs in those over 40, on a background of diverticulosis. It may be referred to as 'right-sided appendicitis' as it can present very similarly (classically periumbilical pain initially, later localising to the left iliac fossa).


  • Diverticulum: Single small outpouching of the bowel
  • Diverticula: Plural of diverticulum (i.e. more than one diverticulum)
  • Diverticulosis: Multiple diverticula, however no evidence of diverticulitis
  • Diverticulitis: Inflammation or infection of a diverticulum
  • Diverticular disease: When diverticula are associated with symptoms (e.g. abdominal pain)


Appearance: Diverticula are multiple small outpouchings, particularly affecting the sigmoid colon. If these are inflamed (diverticulitis), there will be fat stranding, with possible localised perforation or collections.

Diverticulosis on CT

Diverticulosis on CT


Diverticulitis on CT

Diverticulosis on CT

Ischaemic Bowel

Bowel ischaemia may be caused by arterial insufficiency (thrombosis or embolism), venous obstruction, or as a result of bowel obstruction.

Appearance: Variety of appearances including hypo- or hyper-density of the bowel wall, bowel distension and congestion of the mesentery. Arterial filling defect.

Ischaemic bowel on CT

Infarcted Bowel

Infarcted and gangrenous tissues will start to produce gas. Therefore, severely infarcted or necrotic bowel may have gas within its walls. The portal venous system drains the bowel walls. As a result, the portal vein fills with gas that drains into the liver. The liver shows a widespread branching pattern of portal venous gas, which is a premorbid sign (this is different from pneumobilia, which looks similar but the air is actually within the intrahepatic biliary ducts, seen when there is a fistula between bowel and gallbladder)

Appearance: Air within bowel walls; Air in hepatic portal venous system.

Bowel infarction on CT

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