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CT of the acute abdomen: what to look for

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CT is the main scan for a sudden, severe belly pain in adults. Reading it in the same order every time finds the cause and the emergencies.

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What it is

CT of the abdomen and pelvis is the main test for an adult with an acute abdomen. The standard study is taken with IV iodinated contrast in the portal venous phase, which shows bowel wall, solid organs and veins well; oral contrast is often unnecessary. A non-contrast scan is used for kidney stones, and CT angiography (an arterial phase) for suspected mesenteric ischaemia, active bleeding or aortic disease. Before contrast, check for previous serious reactions and, in patients at risk, kidney function, but do not delay an emergency scan for a blood result. Read in a fixed order: free air and free fluid first, then each organ, then the bowel from stomach to rectum, the vessels, the retroperitoneum, bones and lung bases. Fat stranding (hazy, streaky fat) is a key clue: it points to the inflamed organ. In children and pregnancy, ultrasound or MRI usually come first because of radiation.

What to check

  • Protocol: IV contrast portal venous phase for most; non-contrast for stones; arterial phase (CT angiography) for ischaemia, bleeding or the aorta.
  • Free air on lung window, free fluid in the pelvis and around the liver and spleen.
  • Fat stranding: hazy fat around an inflamed appendix, diverticulum, gallbladder or pancreas.
  • Bowel: calibre, transition point, wall thickening, wall enhancement, pneumatosis.
  • Appendix: diameter over about 6 mm, thickened enhancing wall, surrounding stranding, appendicolith.
  • Vessels: aortic diameter and leak, mesenteric artery and vein filling, portal venous gas.

Reading it step by step (and why)

  1. Check the protocol and phase against the question — a non-contrast scan cannot show ischaemia or a leak.
  2. Scroll the whole study on lung window first — small bubbles of free air are easy to miss on soft-tissue window.
  3. Look for free fluid in the pelvis, paracolic gutters and around liver and spleen — dense fluid suggests blood.
  4. Review the liver, gallbladder, spleen, pancreas, adrenals and kidneys one by one — each can be the source.
  5. Follow the bowel from stomach to rectum — find dilatation, a transition point, thick or poorly enhancing wall.
  6. Find the appendix from the caecum — a normal appendix seen clearly argues against appendicitis.
  7. Look for fat stranding and follow it to its centre — it points to the inflamed organ.
  8. Check the aorta, its branches and the mesenteric and portal veins — ischaemia and aneurysms are time-critical.
  9. Review the bladder, uterus and ovaries or prostate, groin hernias, bones and lung bases — pain can arise there.

Common mistakes

  • Reading only on soft-tissue window and missing small pockets of free air.
  • Ordering the wrong phase: a portal venous scan can miss early arterial ischaemia; a non-contrast scan misses bowel and organ ischaemia.
  • Delaying an unstable patient for CT; an unstable patient goes to resuscitation and surgery or bedside ultrasound first.
  • Stopping at the first finding: a stone does not exclude appendicitis or an aneurysm.
  • Scanning early in typical pancreatitis: CT in the first days is not needed for diagnosis and underestimates necrosis.
  • Forgetting radiation in the young and in pregnancy; ultrasound or MRI first when they can answer the question.

Next step

  • Free air, closed loop, strangulation, mesenteric ischaemia or a leaking aneurysm: call the surgical or vascular team now.
  • Suspected mesenteric ischaemia or active bleeding: ask for CT angiography (arterial and venous phases), not a routine scan.
  • Contrast risk: check previous reactions and kidney function in those at risk; do not delay a life-threatening diagnosis for it.
  • Pancreatitis not improving after about 72 hours: contrast CT to look for necrosis and collections.
  • Findings you cannot explain, or results that do not fit the patient: discuss with the radiologist before acting.

Test yourself

Think of the answer, then tap the question to see it.

Which CT phase is standard for an adult acute abdomen?

The portal venous phase after IV iodinated contrast. Non-contrast CT is for stones; CT angiography is added for ischaemia, bleeding or the aorta.

Why scroll the abdominal CT on lung window?

Small bubbles of free air look almost like fat on soft-tissue window; on lung window they stand out black against grey.

What does fat stranding mean?

Hazy, streaky density in the fat from oedema and inflammation; following it to its centre points to the inflamed organ, such as the appendix or a diverticulum.

When is CT useful in acute pancreatitis?

Not for typical early cases; it helps when the diagnosis is uncertain or when the patient is not improving after about 72 hours, to look for necrosis and collections.

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Educational content; it does not replace a radiologist's report and is not a clinical instruction. Each study is ordered on the patient's assessment and your institution's protocols.

Sources: Intravenous Contrast Media in Medical Imaging (StatPearls (NCBI Bookshelf), 2023); Bowel Perforation (StatPearls (NCBI Bookshelf), 2025); Appendicitis (StatPearls (NCBI Bookshelf), 2024); Acute Pancreatitis (StatPearls (NCBI Bookshelf), 2025).

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