Figure 17.  Rib companion shadows. Sagittal reformatted CT scan of the thorax shows the posterior or posterolateral portion of the second rib (2) located at the superoposterior aspect of the lung. Note the slightly protruding soft tissues in the second intercostal space (arrow), which may cause the rib companion shadow below the second rib. 1 = first rib, 3 = third rib, 4 = fourth rib.