Radiodiagnosis
Abdominal Imaging
Abdominal imaging for MBBS and NEET-PG: acute abdomen plain films, bowel obstruction, gallstones, renal colic and appendicitis on ultrasound and CT, mapped to NMC codes RD1.
MedNext Academy | 3 min read
Abdominal Imaging
Abdominal imaging for MBBS and NEET-PG: acute abdomen plain films, bowel obstruction, gallstones, renal colic and appendicitis on ultrasound and CT, mapped to NMC codes RD1.
This chapter covers the imaging of the acute and chronic abdomen across plain films, ultrasound and CT. It teaches the plain-film signs of perforation and obstruction, the ultrasound approach to gallstones, renal disease and jaundice, and the central role of CT in appendicitis, pancreatitis and the wider surgical abdomen.
High-yield: Abdominal Imaging
- The erect and supine abdominal radiograph and an erect chest film are the traditional plain-film series for the acute abdomen.
- Free gas under the diaphragm on an erect chest film indicates a perforated hollow viscus.
- Multiple air-fluid levels with dilated bowel loops on an erect film suggest intestinal obstruction.
- Small bowel obstruction shows central dilated loops with valvulae conniventes crossing the full width, while large bowel obstruction shows peripheral loops with incomplete haustra.
- A coffee-bean or inverted-U loop pointing to the right upper quadrant suggests sigmoid volvulus.
- Ultrasound is the first-line investigation for gallstones, showing an echogenic focus with posterior acoustic shadowing that moves with position.
- Ultrasound is the modality of choice for suspected renal colic in pregnancy and for hydronephrosis, while non-contrast CT is the gold standard for urinary tract stones.
- About 90% of urinary tract calculi are radio-opaque and may be seen on a plain film, whereas most gallstones are radiolucent.
- The target or doughnut sign on ultrasound suggests intussusception, most common in young children at the ileocolic junction.
- CT is the workhorse for the acute abdomen, characterising appendicitis, diverticulitis, pancreatitis, abscess and bowel ischaemia.
- A dilated appendix over 6 millimetres with wall thickening and periappendiceal fat stranding supports appendicitis on CT or ultrasound.
- Contrast studies such as barium swallow and barium enema assess the gut lumen, but water-soluble contrast is used if perforation is suspected.
- Ultrasound is first line for the jaundiced patient to distinguish obstructive from non-obstructive causes by looking for duct dilatation.
- A focused assessment with sonography in trauma looks for free fluid in the hepatorenal, splenorenal and pelvic spaces at the bedside.
First-line imaging by problem
- **Suspected perforation:** Erect chest radiograph for free gas under the diaphragm; CT to confirm and localise.
- **Gallstones and jaundice:** Ultrasound first line: stones, wall thickening and duct dilatation.
- **Renal colic:** Non-contrast CT is the gold standard; ultrasound preferred in pregnancy.
- **Acute abdomen:** CT characterises appendicitis, diverticulitis, pancreatitis and ischaemia.
NMC competencies in this chapter
- **RD1.5:** Radiological investigation and interpretation in common internal medicine conditions
- **RD1.6:** Radiological investigation and interpretation in common surgical conditions
Frequently Asked Questions
What is the first-line imaging for gallstones?
Ultrasound. Gallstones appear as echogenic foci with posterior acoustic shadowing that move with the patient's position, and ultrasound also shows gallbladder wall thickening and bile duct dilatation.
How is small bowel obstruction distinguished from large bowel on a plain film?
Small bowel loops are central with valvulae conniventes crossing the full lumen width, while large bowel loops lie peripherally with haustra that only partly cross. Air-fluid levels are seen on the erect film.
Why is non-contrast CT the gold standard for renal stones?
It detects nearly all calculi regardless of their composition, shows their exact size and location, and reveals secondary signs of obstruction such as hydronephrosis, without needing contrast.
What plain-film sign indicates a perforated viscus?
Free gas under the diaphragm on an erect chest radiograph. The patient should sit upright for several minutes before the film so that free air rises beneath the diaphragm and becomes visible.
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