The projection demonstrates mediastinal divisions into the superior, anterior, middle and posterior. Note the backwards slant of the trachea from the thoracic inlet to the carina. This slant should be borne in mind when tracheal tomograms are performed. Note the position of the outflow tract of the right ventricle and the high position of the left atrium. Note the position of the lung fissures, the left oblique fissure reaching its inferior limit about 5 cm behind the sternum. The right oblique fissure travels more anteriorly at its lower limit. Note also the apparently translucent anterior mediastinum in the normal.
This is the commonest radiogram taken and thus it is important that the normal anatomy is known thoroughly. As with any x-ray, a system must be devised so that all the film is looked at in turn. However, certain hidden areas on a chest film warrant special attention and these include: behind the first ribs, behind the heart shadow, the posterior costophrenic angles which are obliterated on this view by the diaphragmatic shadows, and the hilar regions. Note the air in the trachea and main extra-pulmonary bronchi. Note that the hilar shadows are composed only of vessels and the normal intra-pulmonary bronchi cannot be visualized. The right heart border is formed from the superior vena cava, right atrium and inferior vena cava. The left heart border is formed from the aortic knuckle, the pulmonary conus, the left atrial appendage and the left ventricle.