abdominal X-ray
Summary
Abdominal X-ray (KUB - Kidney, Ureter, Bladder) is a plain radiograph used as a first-line imaging modality to evaluate the abdomen, primarily for detecting bowel obstruction, perforation (free air), foreign bodies, and certain calcifications like renal stones.
Detail
Plain abdominal radiography uses ionizing radiation to produce a 2D image of abdominal structures based on differential tissue density (air, fat, soft tissue, bone/calcification). Key clinical uses include: (1) Suspected bowel obstruction - dilated loops of bowel with air-fluid levels, distinguishing small bowel (valvulae conniventes, central location) from large bowel (haustra, peripheral location) obstruction; (2) Free air under the diaphragm (pneumoperitoneum) on an upright or lateral decubitus film, indicating perforated viscus - a surgical emergency; (3) Toxic megacolon in ulcerative colitis, showing colonic dilation; (4) Radiopaque foreign bodies or ingested objects; (5) Renal/ureteral calculi (only ~90% are radiopaque); (6) 'Double bubble' sign in neonates suggesting duodenal atresia (associated with Down syndrome); (7) Sentinel loop or 'colon cutoff sign' in pancreatitis; (8) Sigmoid or cecal volvulus showing characteristic 'coffee bean' sign. Limitations include poor soft tissue resolution compared to CT/ultrasound, making it insufficient for diagnosing solid organ pathology, appendicitis, or many causes of acute abdomen. It is often obtained as an initial screening test in the emergency setting due to speed and accessibility, but CT abdomen/pelvis with contrast is frequently required for definitive diagnosis. In pregnant patients, ultrasound is preferred to avoid radiation exposure.
Sources
- First Aid for the USMLE Step 1
- Sabiston Textbook of Surgery
- Radiopaedia.org
- UpToDate - Plain abdominal radiography
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