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Textbook/Clinical imaging and interpretation/CT in the acute abdomen
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CT in the acute abdomen

Select and protocol CT for acute abdominal presentations, interpret the study as a search for cause and complications, and preserve time-critical imaging while managing radiation and contrast risk proportionately.

25 min full readUK scopeSources checked 13 Sept 2026Clinical review pending
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Unstable acute abdomen

Shock, peritonism, active haemorrhage or suspected bowel ischaemia requires immediate resuscitation and senior decision-making.

Action: Contact surgery and radiology early, choose the fastest diagnostic protocol that changes source control, and do not delay life-saving enhanced CT for routine renal testing.

Open the sections you need. The overview is shown first.
01Core principlesThe concepts and mechanisms needed to understand the subject.

CT provides rapid cross-sectional assessment of bowel, viscera, retroperitoneum, vessels and abdominal wall. Its value comes from matching acquisition to the suspected process. A generic abdomen and pelvis label leaves unanswered whether unenhanced images, arterial timing, portal-venous enhancement or delayed excretory imaging is needed.

Many inflammatory presentations are assessed in a portal-venous phase after intravenous contrast. Suspected active bleeding or mesenteric ischaemia needs arterial information, often with additional phases. Renal colic is usually evaluated with low-dose non-contrast CT in adults, while CT urography addresses urothelial anatomy. Additional phases should have a defined purpose because each adds radiation.

Interpretation should begin with immediate threats: free gas, active bleeding, vascular occlusion, ischaemic bowel, obstruction, solid-organ injury and sepsis source. Then examine each organ and compare prior studies. A technically good negative examination does not override evolving physiology or a disease that was too early or outside the protocol.

Key points

  • Define the management question before CT: inflammation, obstruction, perforation, bleeding, ischaemia, trauma, stone or another process needs a different protocol.
  • Portal-venous contrast CT is versatile for many acute abdominal diagnoses; use unenhanced, arterial, delayed or urographic phases only when they answer a stated question.
  • Give radiology the pain site and onset, physiology, examination, relevant laboratory results, surgery, pregnancy possibility and contrast history.
  • A negative CT is limited by timing, protocol, image quality and pre-test probability; reassess persistent or evolving clinical concern.
  • Do not delay enhanced emergency CT for renal results when postponement threatens life, limb or access to immediate definitive therapy.
  • The requester and clinical team must receive, acknowledge and act on urgent findings rather than relying on passive report release.
02Mechanisms and patternsImportant relationships and how to distinguish them.
Inflammatory pattern

Focal wall thickening, fat stranding, fluid and enhancement changes can localise appendicitis, diverticulitis, cholecystitis or another inflammatory source.

Obstruction pattern

Dilated bowel, a transition point and collapsed downstream segments indicate mechanical obstruction; assess cause, closed loop and perfusion.

Active haemorrhageRed flag

Arterial-phase contrast extravasation that enlarges or changes on later phases supports active bleeding and guides intervention.

Mesenteric ischaemiaRed flag

Arterial occlusion, venous thrombosis, reduced enhancement, pneumatosis or portal venous gas with the clinical syndrome needs urgent escalation.

PerforationRed flag

Extraluminal gas, focal wall discontinuity, leaked contents and regional inflammation may identify a perforated viscus.

Protocol mismatch

A study can be technically complete yet unable to exclude the target because enhancement phase, coverage or timing was inappropriate.

Red flags requiring action

  • Pain out of proportion, severe metabolic acidosis or abrupt atrial-fibrillation-associated pain raises mesenteric ischaemia and may require CT angiography.
  • Active bleeding requires an arterial-capable multiphase protocol; a routine portal-venous scan may miss the source or timing.
  • Peritonism, free gas, closed-loop obstruction, reduced bowel enhancement or organ injury requires direct urgent communication.
  • Pregnancy, acute kidney injury or prior severe contrast reaction changes planning but does not create an automatic ban when maternal life is threatened.
03Interpreting evidenceInformation, measurements and their limitations.
Reasoning sequence

Consider the information, its meaning and its limitations before deciding what follows.

  1. 01
    Portal-venous contrast CT
    Why
    Evaluate many inflammatory, infective, obstructive and solid-organ causes in a stable acute abdomen.
    Interpretation and limitations
    A broad study still needs a focused question and may not optimise arterial bleeding, stones or urothelium.
  2. 02
    CT angiography
    Why
    Assess mesenteric arterial occlusion, active haemorrhage, aneurysm or other vascular emergency.
    Interpretation and limitations
    Timing and coverage are critical; involve radiology early when the vascular hypothesis is plausible.
  3. 03
    Low-dose non-contrast CT
    Why
    Detect urinary calculi in non-pregnant adults with suspected renal colic.
    Interpretation and limitations
    It shows stones and obstruction but does not provide the same visceral enhancement or urothelial evaluation as contrast studies.
  4. 04
    Ultrasound
    Why
    Assess biliary, pelvic, obstetric, urinary and vascular questions without ionising radiation.
    Interpretation and limitations
    Operator dependence and bowel gas can make a negative or incomplete study non-exclusionary.
  5. 05
    Serial clinical assessment
    Why
    Detect evolving peritonitis, shock or diagnostic discordance before and after imaging.
    Interpretation and limitations
    Clinical deterioration after a negative CT triggers re-review, alternative protocol or surgery rather than automatic reassurance.
04Applied reasoningWorked examples connecting principles to decisions.
01Worked caseUndifferentiated pain and feverA stable adult has severe diffuse pain, fever and guarding without a confident source.
  1. 1Context: assess haemodynamics, peritonism, onset, site, operations, pregnancy possibility, renal function and previous contrast reaction.
  2. 2Reasoning: state the leading dangerous alternatives and ask radiology for an urgent protocol that can distinguish inflammation, obstruction and perforation.
  3. 3Outcome: perform contrast CT, prioritise immediate threats and involve the specialty responsible for drainage or surgery.
  4. 4Verification: receive the report actively, correlate it with the patient and reopen the diagnosis if symptoms or physiology remain discordant.
02Emergency CTChoose the useful phaseCT will determine immediate source control or disposition in an acute abdomen.
  1. 1Continue resuscitation; if delaying necessary enhanced CT would threaten life, limb or access to immediate definitive treatment, proceed without waiting for a recent eGFR, document the emergency justification and review renal function after imaging.
  2. 2Use portal-venous imaging for many inflammatory processes, angiographic phases for bleeding or ischaemia and unenhanced imaging for stones.
  3. 3Limit coverage and phases to the decision while ensuring the study is diagnostically adequate.
  4. 4Communicate critical findings directly and document recipient, acknowledgment and agreed action.
03Negative resultResolve clinical discordanceCT reports no acute cause but pain, examination or physiology remains concerning.
  1. 1Check whether the correct anatomy and enhancement phase were acquired and whether motion or timing limits sensitivity.
  2. 2Discuss focused re-review with radiology and compare all prior imaging.
  3. 3Reconsider vascular, early inflammatory, intermittent obstructive, metabolic and extra-abdominal causes.
  4. 4Observe, repeat examination and select repeat or alternative testing according to the evolving risk and decision.
05Checking understandingVerify the reasoning, revisit uncertainties and apply feedback.
  • Track haemodynamics, urine output, abdominal findings, lactate and organ function before and after transfer to CT.
  • Document contrast product and any immediate reaction, including phenotype and treatment.
  • Review the full report for limitations, incidental findings and recommendations in addition to the main diagnosis.
  • Ensure every urgent or unexpected finding has a named owner and time-bound action.
  • Reassess persistent pain after negative imaging and record why observation, re-imaging or discharge is safe.
06Special situationsVariants, exceptions and circumstances that change the usual approach.

One phase rarely fits all

Portal-venous imaging is versatile, but active bleeding, ischaemia, stones and urothelial lesions require different timing or contrast use.

Oral contrast is selective

Routine positive oral contrast can delay emergency imaging and obscure extravasated iodine; its use depends on the specific question.

Dose follows purpose

Eliminate phases that do not affect the clinical decision, while avoiding an underpowered examination that must be repeated.

Contrast risk is contextual

Acute kidney injury and previous hypersensitivity need review, but the harm of delaying source control can dominate in a genuine emergency.

Negative CT has timing

Early appendicitis, intermittent torsion, evolving ischaemia and small contained leaks may remain subtle, especially with protocol mismatch.

Extra-abdominal sources exist

Basal pneumonia, myocardial ischaemia, ketoacidosis and abdominal-wall disease can mimic an acute intra-abdominal process.

07Common pitfallsFrequent interpretation and management errors.
  1. 01

    Requesting CT without stating the decision or dangerous differential.

  2. 02

    Using one generic phase for suspected bleeding, ischaemia and stones.

  3. 03

    Adding phases reflexively and increasing radiation without diagnostic value.

  4. 04

    Treating a negative CT as final despite deterioration or a mismatched protocol.

  5. 05

    Waiting for renal tests when enhanced CT is required to preserve immediate life-saving treatment.

Practice

Two practice questions

Question 1 of 20 correct
Clinical imaging and interpretationOriginal SBA

Tailor the acute CT protocol

A haemodynamically stable adult has severe non-localised abdominal pain, fever and guarding. The diagnosis remains uncertain after examination and blood tests. Which imaging request is most appropriate?

Sources and review status3 sources · checked 13 Sept 2026 · clinical review pending
Sources

Sources and review status

National guidance is shown before implementation-dependent detail. Apply principles in context and verify current guidance when a decision affects care. Source check completed 13 Sept 2026; clinical approval remains outstanding.

Authoring stateComplete draftClinical stateAwaiting reviewJurisdictionUnited Kingdom