Upper limits of normal bowel diameters – the 3-6-9 rule
3cm = small bowel
6cm = large bowel
9cm = caecum and sigmoid
Interpretation (BOB)
Briefly mention obvious abnormalities first.
Bowel
Small bowel
Identify by: central position; plicae circulares/valvulae conniventes (mucosal folds that cross the whole width of the bowel)
Should be <3cm in diameter (enlarged in small bowel obstruction)
Large bowel
Identify by: peripheral position; faecal contents; haustra (pouches that protrude into the lumen)
Should be <6cm in diameter(enlarged in large bowel obstruction)
Faeces (mottled appearance)
Gas (normal in fundus and large bowel only): extra-luminal gas indicates perforation; check for gas in rectum if bowel obstruction suspected (presence makes complete obstruction less likely)
Fluid levels seen in perforation/infection
Other organs
Soft tissue shadows (may be seen)
Liver
Spleen
Kidneys
Gallbladder
Psoas shadow (lost in retroperitoneal inflammation or ascites)
Calcification of pancreas (chronic pancreatitis), abdominal aorta (atherosclerosis) or renal stones/gallstones