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ANATOMY FOR DIAGNOSTIC IMAGING
Radiological features of the pleura Plain films On a chest radiograph the pleura is visible only if tangential to the beam and if fat or air is on each side of it. Thus the pleura is visible in a normal subject at: • Fissures; • Sites where parietal pleura lies on extrapleural fat: — seen just below the second rib and — extending vertically upwards from the costophrenic recess; and • Junction lines (see Figs 4. 6, 4. 9, 4. 40, 4. 41 and 4. 45) (see also section: Mediastinal lines): — Anterior junction line: anterior to the arch of the aorta the two lungs may come in contact with one another, separated only by four layers of pleura. This pleura is then seen as the anterior junction line on a PA chest radiograph; — Posterior junction line: if the lungs lie close to one another posteriorly, a posterior junction line is seen on a PA chest radiograph, extending vertically downwards from the apices (approximately T1) for a variable distance. It disappears where the lungs envelop the aortic arch and may reform inferiorly. Where the junction lines are seen well, a mass between the lungs in that area can be excluded. Computed tomography On axial CT the pleura cannot usually be distinguished from the thoracic wall or mediastinum unless it is thickened (see section on fissures, p. 119). The pulmonary ligaments can occasionally be seen extending below the inferior
Fig. 4. 10 Trachea and main bronchi: anterior relations.
pulmonary vein caudally and posteriorly to the diaphragm. The right pulmonary ligament lies close to the inferior vena cava (IVC), whereas the left pulmonary ligament lies close to the oesophagus. THE TRACHEA AND BRONCHI (Figs 4. 10-4. 12) The trachea The trachea begins at the lower border of the cricoid cartilage at the level of C6 vertebra. It extends to the carina at the level of the sternal angle (T5 level, T4 on inspiration and T6 on expiration). The trachea is 15 cm long and 2 cm in diameter and is made up of 15-20 incomplete rings of cartilage that are bridged posteriorly by the trachealis muscle. The trachea is lined by ciliated columnar epithelium. The trachea in children is very pliable. It may be deviated to the right at almost 90° in a normal expiratory film. It only deviates to the left if the aortic arch is on the right side. Relations of the trachea Cervical (see Figs 1. 34-1. 36) The anterior relations are as follows: • Anterior: — Isthmus of thyroid anterior to the second, third and fourth rings — Inferior thyroid veins — Strap muscles: sternohyoid and sternothyroid; • Posterior: oesophagus and recurrent laryngeal nerves; and • Lateral: lobes of thyroid gland — Common carotid artery.
