The ribs: anatomic and radiologic considerations.

The ribs are essential structures of the osseous thorax and provide information that aids in the interpretation of radiologic images. Techniques for making precise identification of the ribs are useful in detection of rib lesions and localization of lung lesions. The big rib sign and the vertical displacement sign can be used to differentiate the right and left ribs on lateral chest radiographs. The clavicle, the xiphoid process, and the sternal angle may be used as anatomic landmarks for rib counting on computed tomographic scans. For rib counting on lateral chest radiographs, the sternal angle or the 12th rib may be used. Anatomic rib variants include developmental deformities, cervical rib, and short rib and may mimic true rib diseases. Detection of thoracic deformities such as funnel chest (pectus excavatum) and barrel-shaped thorax requires an awareness of the strong correlation between the transverse appearance of the thorax and costal shape. Shadows around the rib cage (eg, rib companion shadows, sharp lines along the lower margin of the ribs, rib overlying shadows) may mimic pleural and extrapleural disease on frontal chest radiographs. It is imperative that the radiologist be familiar with normal rib anatomy, normal rib variants, and the radiologic appearance of the ribs to prevent misdiagnosis.