What causes Half moon sign in abdomen on chest radiograph?
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Answer:
Half moon sign: Semilunar radiolucency under the diaphragm due to subphrenic free gas accumulation, seen in perforated viscus such as peptic ulcer perforation or bowel perforation; also describes crescentic subchondral radiolucency in avascular necrosis of the femoral head.
Why is it called so?
Named for its characteristic semilunar or half-moon shaped radiolucency, resembling a crescent moon.
Pathophysiology
Free intraperitoneal gas from gastrointestinal perforation rises to the non-dependent subphrenic space, outlining the diaphragm with lucency; in avascular necrosis, bone infarction leads to subchondral collapse and fracture, producing a crescentic lucency parallel to the femoral head surface.
Alternative names: Crescent sign
Other associated named signs: Rigler sign, football sign (in subphrenic gas); for AVN: snake sign
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