psoas margin

pso·as mar·gin

in abdominal radiography, the appearance of the fat stripe delineating the lateral margin of the psoas muscle shadow; shows a normal retroperitoneum when visible.
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A plain film of the abdomen revealed extensive retroperitoneal air outlining both psoas margins and tracking superiorly below the hemidiaphragms and up into the chest.
Perforation can be identified on plain films as gas under the hemidiaphragms, gas tracking along the psoas margins, or the classic presentation of gas outlining the falciform ligament or both sides of the bowel wall (Rigler sign).