🔬 CT Abdomen and Pelvis

CT Abdomen and Pelvis is a core diagnostic tool for evaluating acute and chronic abdominal pathology. It provide high-resolution cross-sectional images of the viscera, vasculature, and lymphatics. While highly effective, it involves significant radiation and requires IV contrast for optimal detail.

Common indications include acute abdominal pain (e.g., suspected perforation, bowel obstruction, or mesenteric ischaemia), staging of intra-abdominal malignancies, and assessment of multi-system trauma. It is also used to investigate unexplained weight loss, change in bowel habit where colonoscopy is incomplete, or the evaluation of complex inflammatory bowel disease. It remains the gold standard for diagnosing renal colic in many UK trusts.

The patient is positioned supine and scanned from the diaphragm to the pubic symphysis. Intravenous iodinated contrast is usually administered to opacify vessels and organs. Depending on the indication, oral or rectal contrast (water or Gastrografin) may be given to distend and delineate the bowel loops. The timing of the scan (arterial vs. venous phase) is tailored to the clinical question.

A normal scan shows uniform enhancement of solid organs without masses, normal bowel wall thickness without dilatation, and no free fluid or gas within the peritoneal cavity. The aorta should be of normal calibre, and no significant lymphadenopathy should be present. The urinary tract should be unobstructed with normal renal cortical enhancement.

Interpretation requires a systematic review of the 'solid organs' (liver, spleen, pancreas, kidneys, adrenals), the 'hollow viscera' (stomach, small and large bowel), the vasculature (aorta, mesenteric vessels), and the musculoskeletal structures. Radiologists look for abnormal enhancement patterns, fluid collections, and the presence of gas where it shouldn't be. Comparison with previous scans is vital for oncology patients.

Abnormal findings are vast and include bowel wall thickening or 'target signs' in Crohn's disease, free gas (pneumoperitoneum) indicating perforation, or dilated bowel loops in obstruction. In trauma, findings include solid organ lacerations (liver, spleen, kidney) and haemoperitoneum. In oncology, it identifies primary tumours, lymphadenopathy, and metastatic deposits in the liver or peritoneum. Inflammatory processes like appendicitis or diverticulitis show fat stranding and localized fluid.

CT A/P is indispensable for the 'acute abdomen' where the diagnosis is uncertain. It guides surgical intervention by localising perforations or identifying causes of obstruction (e.g., volvulus vs. malignancy). In cancer care, it is the primary tool for staging (TNM) and monitoring response to chemotherapy. Its high diagnostic yield often prevents unnecessary exploratory laparotomies.

Poor bowel opacification can mimic masses or abscesses. 'Partial volume' effects can sometimes make small lesions difficult to characterise. A major pitfall is failing to identify 'closed-loop' obstructions or early mesenteric ischaemia, which can be subtle. Over-reliance on CT can lead to 'incidentalomas'—benign findings that lead to unnecessary follow-up.

The major limitation is the high radiation dose, necessitating a high threshold for use in children and pregnant women. Contrast-induced nephropathy is a risk for those with pre-existing renal disease. It is less sensitive than MRI for certain pelvic pathologies (e.g., rectal cancer staging or prostate assessment) and less sensitive than ultrasound for gallstones.

Understand the 'SBO' (Small Bowel Obstruction) features: dilated loops (>3cm), air-fluid levels, and the transition point. Note the importance of the portal venous phase for liver metastases. Be aware of 'CT KUB' (Kidneys, Ureters, Bladder) as a specific low-dose, non-contrast variant for stones.