Abdominal imaging - CT scan

Principle

CT is the workhorse cross-sectional imaging modality for the abdomen - rapid, widely available, and providing detailed anatomical detail of solid organs, bowel, vasculature, and free fluid/air in a single acquisition
IV contrast timing (arterial, portal venous, delayed phases) is selected based on the clinical question - arterial phase for vascular/active bleeding assessment, portal venous phase for most routine solid organ/general abdominal imaging, delayed phase for specific indications (e.g. characterising a renal or hepatic lesion further)

The default first-line cross-sectional test for acute abdominal pain of unclear cause, and central to staging/surveillance of most intra-abdominal malignancy
Acute abdominal pain workup, bowel obstruction, intra-abdominal malignancy staging, suspected perforation/ischaemia

Oral/rectal contrast can help delineate bowel loops in specific scenarios (e.g. suspected fistula, some obstruction workups) but is not always required for a standard study, and is omitted or replaced by water in some modern protocols
CT is highly sensitive for free air (perforation), significant free fluid, obstruction, and most solid organ pathology, though has some limitations for subtle bowel wall/mucosal detail compared with endoscopy or MR enterography

Requires adequate renal function for IV contrast (or an alternative/reduced-contrast protocol in renal impairment); involves a meaningful radiation dose, relevant when considering serial/repeated imaging, particularly in younger patients or with frequent surveillance needs
Findings directly guide urgent surgical/interventional referral (e.g. perforation, ischaemia, obstruction) or outpatient further workup depending on acuity

CT's speed and sensitivity make it the default modality for undifferentiated acute abdominal presentations, but its radiation burden and contrast requirements mean ultrasound or MRI are preferred first-line in specific scenarios (pregnancy, children, certain hepatobiliary questions) where they offer comparable or superior information without these costs

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