Coffee bean sign (sigmoid colon)
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The coffee bean sign (also known as the kidney bean sign or bent inner tube sign) 4 is a radiological finding on an abdominal plain radiograph of a sigmoid volvulus although some authors have also used the term to refer to closed loop small bowel obstructions.
Cecal volvulus may be confused with sigmoid volvulus using plain radiography, and the number of gas-fluid levels may help in differentiating the two entities. In cecal volvulus only one gas-fluid level is usually seen, compared to sigmoid volvulus with a few gas-fluid levels 4.
This thick 'inner wall' represents the double wall thickness of apposed loops of bowel, with thinner outer walls due to single wall thickness.