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AbdomenDemonstration data

Abdominal X-Ray (KUB)

The plain abdominal radiograph (KUB - kidneys, ureters, bladder) is a low-cost bedside test that surveys the bowel gas pattern, calcifications, soft tissues and lower thoracic and pelvic bones. Although CT has largely replaced it for most acute abdominal pathology, the KUB retains a role in suspected bowel obstruction, assessment of foreign bodies and enteric/urinary tube position, and evaluation of constipation. Its diagnostic yield is limited, so appropriate patient selection is essential.

Indications

  • Suspected small or large bowel obstruction
  • Suspected bowel perforation (usually combined with an erect chest film for free air)
  • Assessment of severe constipation or faecal loading
  • Localisation and follow-up of radiopaque foreign bodies and ingested objects
  • Suspected renal or ureteric calculi (limited sensitivity; CT KUB preferred)
  • Confirmation of nasogastric, gastrostomy or ureteric stent position
  • Suspected toxic megacolon in inflammatory bowel disease
  • Suspected sigmoid or caecal volvulus

Contraindications & Cautions

  • No absolute contraindications, but low diagnostic yield means requests should be justified
  • Pregnancy: avoid where possible; use ultrasound or MRI alternatives and justify if performed
  • Effective dose is moderate (~0.7 mSv, roughly 3-4 months of background) - higher than a chest film
  • Avoid as a routine screen for non-specific abdominal pain where CT or ultrasound is more appropriate
  • Apply ALARA; do not repeat unnecessarily

Patient Preparation

  • Generally no specific preparation for the acute setting
  • Remove metallic objects, belts and clothing with radiopaque fasteners from the field
  • Confirm pregnancy status in women of reproductive age
  • Ensure the bladder is not deliberately emptied when assessing lower urinary calculi
  • Explain the breath-hold (expiration) to reduce motion and diaphragmatic obscuration
  • For suspected obstruction, plan for an additional erect or decubitus view

Technique & Parameters

  • Standard projection: supine AP with the patient breath-holding in expiration
  • Coverage from the diaphragms superiorly to the symphysis pubis inferiorly, including both flanks
  • Centre at the level of the iliac crests (L4)
  • Moderate kVp (70-80 kVp) with a grid for soft-tissue and calcification contrast
  • Erect abdominal or left lateral decubitus view when obstruction or perforation is suspected to demonstrate air-fluid levels and free air
  • Erect chest radiograph is the most sensitive plain film for subdiaphragmatic free gas
  • Ensure both properitoneal fat lines and the symphysis are included

Systematic Review

  • Assess adequacy: coverage from diaphragm to symphysis, both flanks, penetration
  • Bowel gas pattern: small bowel (central, valvulae conniventes spanning the lumen) versus large bowel (peripheral, haustra)
  • Calibre: small bowel >3 cm, large bowel >6 cm, caecum >9 cm suggest dilatation
  • Look for gas where it should not be: pneumoperitoneum, pneumobilia, portal venous gas, intramural gas
  • Calcifications: renal tract, gallstones, pancreas, vascular, phleboliths, appendicoliths
  • Solid organs and soft tissues: liver, spleen, renal outlines, psoas margins, bladder
  • Bones: lower ribs, spine, pelvis, sacroiliac joints and hips
  • Foreign bodies, tubes, lines and stents

Key Findings & Significance

  • Dilated central loops with valvulae conniventes and multiple air-fluid levels indicate small bowel obstruction
  • Dilated peripheral colon with haustra and a collapsed distal segment indicates large bowel obstruction
  • A 'coffee bean' or inverted-U loop suggests sigmoid volvulus; a distended caecum displaced to the left upper quadrant suggests caecal volvulus
  • Rigler sign (gas on both sides of the bowel wall) and free subdiaphragmatic gas indicate perforation
  • Continuous diaphragm sign and the falciform ligament outlined by gas confirm pneumoperitoneum
  • Ureteric calculi appear as calcific densities along the expected ureteric course
  • Mural thickening with 'thumbprinting' suggests colitis or ischaemia; loss of haustra with dilatation suggests toxic megacolon
  • Faecal loading throughout the colon supports constipation

Differential Considerations

  • Dilated bowel: mechanical obstruction, adynamic ileus, pseudo-obstruction (Ogilvie), volvulus
  • Abnormal gas: pneumoperitoneum, pneumatosis intestinalis, emphysematous cholecystitis/pyelonephritis, abscess
  • Right upper quadrant calcification: gallstones, renal calculus, costal cartilage, hepatic lesion
  • Pelvic calcification: phleboliths, bladder calculus, uterine fibroid, dermoid, appendicolith
  • Curvilinear calcification: aortic aneurysm, splenic artery, vas deferens
  • Absent psoas outline: retroperitoneal fluid, mass or haemorrhage (non-specific)
  • Multiple air-fluid levels: obstruction, ileus, gastroenteritis, or normal in small numbers

Pearls & Pitfalls

  • The erect chest radiograph detects as little as 1-2 mL of free intraperitoneal gas - more sensitive than the abdominal film
  • Only about 10-15% of gallstones and a variable proportion of renal stones are radiopaque; a normal KUB does not exclude them
  • Distinguish small from large bowel by mucosal folds and distribution, not just position
  • Phleboliths (rounded with lucent centres) are commonly mistaken for distal ureteric stones
  • A left lateral decubitus film is useful when the patient cannot stand for free-air assessment
  • Beware overcalling ileus versus mechanical obstruction; a transition point favours mechanical obstruction
  • Retained surgical items, patches and external artefacts can mimic pathology - correlate clinically

Structured Report

  • State projection(s) and adequacy of coverage
  • Describe the bowel gas pattern, calibre and any transition point or dilatation
  • Report presence or absence of free intraperitoneal or abnormal gas
  • Describe calcifications, soft tissues, solid organ outlines and bones
  • Note tubes, stents, lines and foreign bodies with their positions
  • Impression: state whether findings support obstruction, perforation or other pathology and recommend CT where appropriate

References

  • ACR-SPR Practice Parameter for the Performance of Abdominal Radiography
  • Royal College of Radiologists iRefer guidelines
  • ACR Appropriateness Criteria: Suspected Small-Bowel Obstruction
  • Begg JD. Abdominal X-Rays Made Easy, Elsevier
  • Gore RM, Levine MS. Textbook of Gastrointestinal Radiology, Elsevier

Educational clinical decision support only. Protocols vary by institution and equipment; always confirm with a qualified radiologist and local guidelines before clinical use.