What is the cause of this chest x-ray opacity?

click picture to enlarge

CT of opacity.  Click picture to enlarge.










Eventration of the diaphragm.  Caused by a thinned and weakened section of diaphragm - often congenital - permitting abdominal contents to bulge towards the thoracic cavity.  While eventration may cause respiratory distress in the newborn, adults are often asymptomatic.  The distinction between a localized eventration or a pathological mass is best made using CT or MRI.


Source 

Clinical Imaging: An Atlas of Differential Diagnosis.  5th ed.

Adam: Grainger & Allison's Diagnostic Radiology, 5th ed.