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1. Chest radiograph PA projection

In a PA chest film of diagnostic quality the medial ends of the clavicles are equidistant from the spinous process of the adjacent thoracic vertebra. This indicates that it was taken with a truly sagittal X-ray beam. The hemidiaphragm should project at the level of the posterior portion of the tenth rib, or lower. This indicates that the exposure was made during deep inspiration.

In adults the heart and major vessels attached to it cast almost all of the mediastinal soft-tissue density shadow between the two radiolucent lung fields. (The vertebral column and sternum also contribute to the mediastinal shadow.) The right border of the mediastinum is composed of the following set of structures (listed from superior to inferior):

  1. brachiocephalic artery and R. brachiocephalic vein superimposed
  2. superior vena cava and aorta superimposed
  3. R. atrium
  4. inferior vena cava

The left border of the mediastinum is composed of the following set of structures (also listed from superior to inferior):

  1. L. subclavian artery and L. brachiocephalic vein superimposed
  2. posterior part of the aortic arch (the aortic knob)
  3. pulmonary trunk
  4. auricle of the L. atrium (atrial appendage)
  5. L. ventricle

(See Image #4, an MRI in the coronal plane, for better visualization of the structures comprising the mediastinal silhouette)

Even though the diaphragm is one continuous sheet of muscle it radiographs as two distinct hemidiaphragm silhouettes.