The pleura

1 14 ANATOMY FOR DIAGNOSTIC IMAGING

1. Anterior wall of trachea 2. Posterior tracheal stripe 3. Scapulae 4. Left lower-lobe bronchus 5. Right lower-lobe bronchus

G. Aorta (not well seen) 7. Vertebral body of T4 8. Anterior aspect of right ventricle 9. Pulmonary outflow tract 10. Main pulmonary artery 11. Right pulmonary artery 12. Left pulmonary artery 13. Left atrium 14. Left ventricle 15. Inferior vena cava 16. Horizontal (minor) fissure 17. Oblique (major) fissure 18. Sternum 19. Manubriosternal joint 20. Left hemidiaphragm 2 1. Right hemidiaphragm 22. Stomach bubble 23. Lung projected anterior to sternum in intercostal space 24. Retrosternal airspace Ultrasound The diaphragm is readily imaged by ultrasound, using the liver or spleen as an acoustic window. It is seen as an echogenic line outlining the upper surface of these organs. The diaphragmatic interdigitations may occasionally be pronounced to give the spurious impression of an echogenic mass on the surface of the liver.

Computed tomography The diaphragm is not usually visible as a structure discrete from the liver or other abdominal organs, unless there is a lot of fat on its abdominal aspect.

The costal origins may be prominent w i th deep inspirations. The crura are usually visible on the anterior surface of the upper lumbar vertebrae. In young, muscular subjects the crura may be very thick or even nodular; however, their tubular nature and changes w i th respiration serve to distinguish these from lymph nodes. The right crus extends more inferiorly than the left.

The retrocrural space contains fat, the azygos and hemiazygos veins, the thoracic duct and lymph nodes, and should not be more than 6 mm wide.

Magnetic resonance imaging (see Fig. 4. 5) This technique yields excellent sagittal and coronal images of the diaphragm as a thin muscular septum of intermediate signal intensity. The crura are elegantly displayed on coronal images.

THE PLEURA (Figs 4. 8 and 4. 9) The pleura is a serous membrane that: (i) covers the lung (i. e. the visceral pleura); and (ii) lines the thoracic cavity and mediastinum (i. e. the parietal pleura). Parts of the pleura are named according to site, for example costal, diaphragmatic, mediastinal and apical.

The visceral and parietal layers are continuous w i th each other anterior and posterior to the lung root, but below the hilum the two layers hang down in a loose fold called the pulmonary ligament. This may extend to the diaphragm or have a free inferior border, and allows descent of the lung root in respiration and also distension of the pulmonary veins (note that these lie inferiorly in the lung root).

The visceral pleura extends into interlobar and accessory fissures. At rest the parietal pleura extends deeper into the costophrenic and costomediastinal recesses than do the lungs and visceral pleura (see Table 4. 1 for lower limits of lungs and pleura).

The parietal pleura is supplied by the systemic vessels. The visceral pleura receives arterial supply from both the bronchial and the pulmonary circulation.

THE THORAX 1 15

Table 4. 1 Lower limits of lung and pleura at rest

Anterior

Visceral pleura and lung Parietal pleura

6th costal cartilage 7th costal cartilage

Mid-axillary line 8th rib

Posterior T10 T10 10th rib

T12