What causes Half moon sign? (Two distinct radiographic contexts: subphrenic free air on abdominal/chest radiograph, and avascular necrosis of the femoral head on hip imaging)
Let me know in the comments.
Click to reveal the answer
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
References
Access all radiology signs posted so far: https://radiogyan.com/radiology-signs/
Improve Content And Help Your Colleagues!
Found an error in the post? Please let us know using our contact page and we will update it with due credits!
If you wish to contribute radiological images for this case (or any other article on the website), you can submit them here and you will be duly credited: Submit radiology case