CT Abdomen Approach
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Direct answer
Read an abdominal CT from the outside in: first free air and free fluid, then the vessels and mesentery, then each solid organ, then the hollow viscera, retroperitoneum and bones. Fat stranding — streaky increased density in mesenteric fat — is the signpost that localises nearly every inflammatory process, from appendicitis to diverticulitis, while the phase of contrast enhancement decides what can be diagnosed at all (portal venous for most parenchyma, arterial for hypervascular lesions, delayed for the pelvicalyceal system). In trauma, the sentinel clot sign and active extravasation outrank everything else. A rigid order protects you from the classic NEET-PG trap: a striking finding in the pancreas that blinds you to a perforation at the rectosigmoid.
What you must remember
- Search order: free air and fluid, aorta and major vessels, solid organs (liver, spleen, pancreas, kidneys), bowel and mesentery, retroperitoneum, pelvic organs, bones, lung bases.
- Fat stranding localises disease: appendicitis (appendix above 6 mm, periappendiceal stranding), diverticulitis, epiploic appendagitis, pancreatitis.
- Free air on CT: rim of gas tracking along the peritoneal folds; look at the porta hepatis and mesenteric borders where it collects first in a supine patient.
- Sentinel clot: the densest haematoma (over about 60 HU) sits next to the injured organ and predicts its source better than free fluid alone.
- Bowel signs: target sign (oedematous wall) in inflammatory or ischaemic enteritis, mesenteric whirl in volvulus, gas in the bowel wall or portal veins in advanced ischaemia.
- Aneurysm thresholds: abdominal aorta above 3 cm is aneurysmal; above 5.5 cm in men triggers repair discussion; iliac above 1.5 cm.
- Never skip the lung bases and bone windows — a basal pneumonia or lumbar fracture hiding on an "abdominal" study is a beloved examiner insert.
- Phase matching: portal venous phase (60-70 seconds) is the routine workhorse; multiphase only when the question is a lesion characterisation or active bleeding.
Walking through a right iliac fossa case
A 24-year-old with 36 hours of right iliac fossa pain and a borderline white count. On the portal venous phase study, begin at the caecum and trace the appendix retrogradely from the base: it is 9 mm in diameter, fluid-filled, with periappendiceal fat stranding and a single 6 mm appendicolith — acute appendicitis, uncomplicated. But do not close the study. Check for a periappendiceal collection or abscess that would convert management to percutaneous drainage first; inspect the terminal ileum and caecum for a right-sided mass or inflammatory fat that suggests Crohn disease mimicking appendicitis; in a woman, confirm both adnexa — a haemorrhagic cyst or torsed ovary produces an identical presentation. Then run the rest of the checklist: free fluid in the pelvis, no free air, ureters unobstructed, spine intact. The diagnosis was visible in ten seconds; the remaining ninety seconds of disciplined reading is what makes the report safe, and it is exactly the habit NEET-PG clinical-case images probe.
Traps in a busy list
Two errors recur. First, treating fat stranding as a diagnosis rather than a pointer — stranding around a normal appendix, a diverticulum or an epiploic appendage tells three different stories, and epiploic appendagitis (a hyperdense ring of inflamed fat) is specifically inserted as a conservative-management distractor. Second, forgetting that an underdistended bladder or an unenhanced phase hides pathology: a bladder tumour needs the excretory phase, and a renal mass read on a single portal phase is characterisation abandoned halfway. In trauma, always grade the solid organ (AAST liver and spleen grades) rather than writing "laceration" without extent, because the grade drives conservative-versus-intervention decisions.
Frequently asked questions
Which contrast phase is standard for a routine CT abdomen?
The portal venous phase at roughly 60-70 seconds after injection, when hepatic parenchyma is maximally enhanced and most inflammatory disease is well shown.
What is the sentinel clot sign?
The highest-attenuation clot lying immediately adjacent to the injured organ, traditionally the spleen, localising the bleeding source in blunt trauma.
How is fat stranding interpreted?
Streaky soft-tissue density within mesenteric fat marking inflammation or venous congestion; it directs attention to the appendix, diverticulum, pancreas or mesentery but is not itself a diagnosis.
What CT features diagnose acute appendicitis?
An appendix distended beyond 6 mm with wall enhancement, periappendiceal fat stranding, sometimes an appendicolith, with or without a collection.
Why review lung bases on an abdominal CT?
Basal pneumonia, pleural effusion and subdiaphragmatic free air routinely explain "abdominal" pain and appear only on the inferiormost slices of the study.