The mediastinum on the chest radiograph

to the thoracic viscera and forms the splanchnic nerves to the abdominal viscera.

THE MEDIASTINUM ON THE CHEST RADIOGRAPH

Mediastinal contour on the frontal chest

radiograph (see Fig. 4.6) The heart and great vessels form a characteristic contour on the frontal chest radiograph. The right side of the mediastinal contour is formed from above downward by the brachiocephalic vein and the SVC. The SVC forms a shallow angle w i th the right atrium, which forms the right heart border. The terminal part of the IVC may be seen just medial to the cardiophrenic angle, which is usually sharp. Occasionally a fibrofatty pad displaces the right pleura laterally, obscuring the cardiophrenic angle.

The left side of the mediastinal contour is formed by the composite shadow of the subclavian vessels superiorly. The artery is lower and actually forms the contour. This fades out laterally and is usually indistinct. Below this the aortic prominence is termed the 'aortic knuckle' or 'knob'. This is formed by the posterior part of the arch. It may be indistinct in young people and very prominent in older people, especially if there is aortic unfolding. Sometimes a small 'nipple' may be seen projecting from the aortic knuckle. This is caused by the left superior intercostal vein as it crosses the aorta to drain into the left brachiocephalic vein (Fig. 4.38). In older people the left side of the descending aorta may be visible descending from the aortic knuckle. Below the aortic knuckle is an air space called the aortopulmonary window. Failure to identify this clear space indicates pathology.

Fig. 4.38 PA chest radiograph showing an 'aortic nipple' (arrows) caused by the left superior intercostal vein as it passes anterior to the arch of the aorta. Below the aortopulmonary window is the main pulmonary artery, which has a straight upper border, and below this is the left ventricle. The left atrial appendage lies embedded in fat below the pulmonary artery but is not contour forming unless enlarged. The right cardiophrenic angle is not as sharp as the left. In deep inspiration air-filled lung may be seen under the apex of the left ventricle. Occasionally, a fat pad is present in the left cardiophrenic angle.

The pulmonary arteries and veins form the densities of the hila on the frontal chest radiograph (see radiological features of the lung, p. 122).

Mediastinal contours on the lateral chest

radiograph (see Fig. 4.7) The heart shadow lies behind the lower third of the sternum. The anterior border is formed by the right ventricle and outflow tract. Higher up the lungs are in contact w i th each other behind the sternum and in front of the ascending aorta, forming the retrosternal air space. The posterior contour of the heart shadow is formed by the left atrium above and the left ventricle below. The IVC may be identified as a triangular structure crossing the diaphragm to enter the right atrium, which lies in a more anterior plane than the left atrium.

The aorta The aorta may be invisible in young people but is usually seen, at least in part, in middle-aged subjects. The ascending aorta is indistinct. The arch curves evenly from front to back and the descending aorta is seen anterior to the vertebral column. Its walls should be parallel. In older people, unfolding may cause it to overlie the vertebral bodies.

Mediastinal lines (Figs 4.6, 4.7, 4.9, 4.39 and 4.40) Wherever air-filled lung outlines a linear soft-tissue structure, the difference in density is detected by the plain radiograph as a line. If air outlines two sides of a thin structure the soft-tissue density is seen as a stripe. This relationship may be appreciated on CT scans.

Right paratraeheal stripe The lung is in contact w i th the trachea from the level of the clavicles to the azygos vein in the right tracheobronchial angle. As the trachea is air-filled the right tracheal wall is seen as a stripe, which should not measure more than 3 mm (the left tracheal wall is separated from lung by the aorta and great vessels and is not seen).

Posterior junction line (Fig. 4.9) This is formed by the apposition of the two lungs posteriorly. It extends from well above the clavicles vertically downwards to the arch of the aorta. The aortic arch

1 44 ANATOMY FOR DIAGNOSTIC IMAGING

Fig. 4.39 PA chest radiograph showing the azygo-oesophageal line (arrows).

Fig. 4.40 CT scan of thorax: upper T4 level showing mediastinal lines.

1. Anterior junction line 2. Air-filled trachea 3. Oesophagus 4. Right paratracheal stripe 5. Azygo-oesophageal stripe 6. Superior vena cava 7. Aortic arch 8. Right paraspinal line 9. Left paraspinal line 10. Azygos vein