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normal variants

Occasionally, air within the most medial and inferior portions of the lower lobes, in apparent isolation from the remainder of the lung, can be seen on CT. This normal variant has been described by Silverman et al. as "retrocrural air" (Fig 1). This finding is visible in about 1 % of normal subjects. The shortened and thickened diaphragmatic muscle fibers near the costal insertion may indent the liver in a nodular or linear fashion (Fig 2). Nodularity of the diaphragm is accentuated on CT scan obtained in deep inspiration. Nodular infoldings of the diaphragm separate from the liver or spleen may mimick intrahepatic lesions or peritoneal tumor implants. The crura may simulate an enlarged lymph node on a single transaxial section on CT scan (Fig 3). Caskey et al. have suggested that many of the diaphragmatic defects identified, especially posteriorly in older patients, represent acquired defects occurring in areas of structural weakness, perhaps themselves embryologic in origin (Fig 4).

Fig 1: Retrocrural air (arrow).

 

Fig 2: Ultrasound image: linear indentation of the diaphragm in the liver.

 

Fig 3: Nodular appearance of the crura: it should not be mistaken for adenopathy (arrow).

 

Fig 4: Indentation of the surface of the liver by the costal slip (arrowheads), diaphragmatic defect (curved arrow) and nodular folds (arrows).