The mediastinal divisions

the thoracic aorta if they arise from it. Owing to the variable origin of the bronchial arteries it may be necessary to catheterize the subclavian, internal thoracic or intercostal arteries.

As the spinal arteries also arise from these vessels (cf. Chapter 3) there is a risk of spinal ischaemia using this procedure.

Computed tomography (see Figs 4. 40 and 4. 42-4. 44) Fissures On conventional CT fissures are less visible than on plain radiographs. They are seen as regions of relative avascularity on the outer cortex of the lobe, where tapering vessels are less visible. Discrete lines are only seen if the vertical axis of the fissure is perpendicular to plane of the CT slice, which sometimes occurs in parts of the oblique fissure but not in the transverse fissure. On high-resolution CT fissures are seen as sharp lines. Bronchi The bronchi may be seen depending upon their size and orientation. Narrow slices improve visualization.

The horizontally orientated bronchi, such as the anterior segment bronchus of the upper lobes, the superior segmental bronchi of the lower lobes and the proximal part of the middle-lobe bronchus, may be seen as tubular structures.

The vertically orientated bronchi, such as the main bronchi, bronchus intermedius, lower-lobe bronchi and apical segmental bronchi, may be seen as circular air-filled structures.

The posterior wall of the right main bronchus and its divisions into upper-lobe bronchus and bronchus intermedius should be outlined by lung as it invaginates into the azygo-oesophageal recess.

Occasionally, a pulmonary vein may pass behind the bronchus intermedius on its way to the left atrium, simulating a small mass, which is usually less than 1 cm in diameter.

The posterior walls of left main and upper-lobe bronchi are usually outlined by lung. Below the hilum, lung tissue may also be seen in contact w i th the posterior wall of the lower-lobe bronchus.

Vasculature The vessels account for most of the lung markings seen on CT. The relationships of the pulmonary arteries and veins to the bronchi are best seen at hilar level. The right pulmonary artery is anterior to the right bronchus, and the right superior pulmonary vein may be seen anterior to this. The left pulmonary artery is seen anterior to the left main bronchus, and above it on a higher section. The lowerlobe artery is seen posterolateral to the lower-lobe bronchus. The left superior pulmonary vein is separated from the lower-lobe artery at hilar level by the left bronchus.

Magnetic resonance imaging Because the lungs are of very low proton density and move w i th respiration they are poorly seen by this method. Third-order pulmonary arteries are visible as are pulmonary veins close to the hilum. Segmental bronchi and fissures are not seen. The axial anatomy as seen is the same as that of CT. MR angiography is increasingly being used, however, to image the pulmonary arteries non-invasively.

Isotope ventilation-perfusion scanning A ventilation scan outlines the trachea and main bronchi in addition to the lungs. A gap is seen owing to the mediastinum and a cardiac notch is seen in the anterior border of the left lung.

Perfusion scanning may show differential isotope distribution from apex to diaphragm, owing to variations in blood flow associated w i th this posture.

THE MEDIASTINAL DIVISIONS (Fig. 4. 19) The mediastinum is the space between the lungs and their pleura. It is arbitrarily divided into superior, middle, anterior and posterior sections. These divisions are not anatomical. They are used to describe the location of pathological processes. The superior mediastinum is above a line drawn from the lower border of T4 to the sternal angle. Below this line are anterior, middle and posterior compartments. The middle mediastinum is occupied by the heart and its vessels. The anterior mediastinum is between the anterior part of the heart and the sternum. The posterior mediastinum is between the posterior part of the heart and the thoracic spine, extending down behind the posterior part of the diaphragm as it slopes inferiorly.

The superior mediastinum contains the: • Aortic arch and branches; • Brachiocephalic veins and superior vena cava;