Cross-sectional anatomy

separates the lungs and the line disappears. It may reform below the arch. The line represents the four layers of pleura between the posterior parts of the lungs seen from the front.

Anterior junction line This line is formed by the apposition of the lungs anteriorly. It begins below the clavicles and runs inferiorly and to the left. Its oblique course is because of the differing anterior extent of the two lungs. It always ends at the right ventricular outflow tract.

Pleuro-oesophageal line This is formed by the right lung outlining the right wall of the oesophagus above the level of the azygos vein. If the oesophagus happens to be distended w i th air, the pleurooesophageal stripe is seen. The thickness of this represents the thickness of the oesophageal wall and pleura and should not exceed 2 mm.

Azygo-oesophageal line (Fig. 4.39) The azygos vein is closely related to the right posterolateral aspect of the oesophagus in the posterior mediastinum. The right lung abuts the two structures forming the azygooesophageal line, tucking in behind them in the azygooesophageal recess. On the frontal chest film the line curves to the right as the azygos vein crosses to gain the right tracheobronchial angle before draining into the SVC.

Paraspinal lines These are outlined by lung apposing the spinal column. The right line is sharper than the left as the descending aorta tends to reflect the pleura off the thoracic spine on the left, making the line more indistinct. The distance from the lateral border of the spine to the line may vary w i th body habitus, and may be 1 cm on the left in obese people w i th paraspinal fat deposits. Widening of the paravertebral soft tissue on the right is always abnormal. Widening of the paravertebral soft tissue beyond the transverse process is always abnormal.

Aortopulmonary mediastinal stripe This is the reflected pleura from the aortic arch to the pulmonary trunk and left pulmonary artery. It defines the lateral boundary of the aortopulmonary window.

Posterior tracheal stripe This is seen on the lateral view and represents the posterior wall of the trachea outlined on either side by air. It measures 2-3 mm. If the collapsed oesophagus is apposed to the posterior trachea the stripe may measure 1 cm. If there is air in the oesophagus the stripe represents posterior tracheal and anterior oesophageal walls.

CROSS-SECTIONAL ANATOMY

(Refer to Figs 4.41 and 4.45-4.49 for this section)

Level T3 (Fig. 4.41) This is the superior mediastinal level. The trachea is seen in the midline, w i th the great vessels anteriorly and the oesophagus behind. The brachiocephalic veins are anterior and lateral to the arteries and unite to form the superior vena cava at about this level. The left brachiocephalic vein is seen passing anteriorly to the branches of the aortic arch.

Fig. 4.42 CT scan of thorax: level T3

1. Body of T3 2. Body of manubrium sterni 3. Right brachiocephalic vein 4. Left brachiocephalic vein 5. Right subclavian artery 6. Right common carotid artery 7. Left common carotid artery 8. Left subclavian artery 9. Trachea 10. Oesophagus 11. Right scapula 12. Spine of scapula 13. Erector spinae muscle 14. Trapezius muscle 15. Supraspinatus muscle 16. Infraspinatus muscle 17. Subscapularis muscle 18. Deltoid muscle 19. Fat in axilla 20. Pectoralis minor muscle 2 1. Pectoralis major muscle 22. Subcutaneous fat 1. Body of T3 2. Body of manubrium sterni 3. Right brachiocephalic vein 4. Left brachiocephalic vein 5. Right subclavian artery 6. Right common carotid artery 7. Left common carotid artery 8. Left subclavian artery 9. Trachea 10. Oesophagus 11. Right scapula 12. Spine of scapula 13. Erector spinae muscle 14. Trapezius muscle 15. Supraspinatus muscle 16. Infraspinatus muscle 17. Subscapularis muscle 18. Deltoid muscle 19. Fat in axilla 20. Pectoralis minor muscle 21. Pectoralis major muscle 22. Subcutaneous fat

Fig. 4.43 CT scan of thorax: level T4.

1. Body of T4 2. Thecal sac and spinal cord 3. Arch of aorta 4. Superior vena cava 5. Azygos vein 6. Trachea 7. Oesophagus 8. Erector spinae muscle 9. Trapezius muscle 10. Infraspinatus muscle 11. Subscapularis muscle 12. Pectoralis minor muscle 13. Pectoralis major muscle

THE THORAX 147

Fig. 4 . 44 CT scan of thorax: lower T5 level.

1. Body of T5 2. Sternum 3. Spinous process of T5 4. Ascending aorta 5. Descending aorta 6. Superior vena cava 7. Main pulmonary trunk 8. Right pulmonary artery 9. Left pulmonary artery 10. Carina 11. Left main bronchus 12. Oesophagus 13. Azygos vein 14. Azygo-oesophageal recess 15. Mediastinal fat

Level T4 (see Fig. 4.45) This plane passes through the lower border of T4 and the sternal angle. Above this plane is the superior mediastinum. At this level is the lower part of the aortic arch to the left of the trachea, and the arch of the azygos vein to the right, crossing the right hilum to drain into the posterior aspect of the superior vena cava. The oblique fissure, although demonstrated on these figures, is not always seen on CT as a line.

Fig. 4 . 45 Cross-sectional anatomy: level T4

Fig. 4.46 Cross-sectional anatomy: level T5

Level T5 (Fig. 4.46) This plane is at, or just below, the tracheal bifurcation. The main pulmonary artery is seen dividing into right pulmonary artery, which runs anteriorly to its bronchus, and left pulmonary artery, which arches superiorly out of the plane to pass over the left bronchus. The descending lowerlobe artery is seen lying posterolateral to the bronchus.

The superior pulmonary veins are seen anteriorly. On the right this is anterior to the interlobar artery. On the left it is separated from the lower-lobe artery by the left bronchus.

The ascending aorta is seen in cross-section, lying in a more posterior plane than the origin of the main pulmonary artery.

Anterior to this is the junction of the anterior pleurae of the lungs, which forms the anterior junction line on the frontal chest radiograph.

The oesophagus, azygos veins and descending aorta are seen in the posterior mediastinum. The azygo-oesophageal recess may be seen.

Level T6 (Fig. 4.47) This section passes through the upper part of the heart. The right atrial appendage may be seen overlapping the origin of the aorta. The coronary arteries come off the aorta at this level. The right ventricular outflow tract is seen anterior to the origin of the aorta. The relationship of the oesophagus to the left atrium is seen.

Fig. 4 . 48 Cross-sectional anatomy: level T8

Fig. 4 . 49 Cross-sectional anatomy: level T10

Level T8 (Fig. 4.48) This section passes through the heart and shows the relationship of the chambers to each other. The left ventricular inflow and outflow tracts are separated from each other by the anterior leaflet of the mitral valve. The aortic sinuses are seen as localized bulges. Note that the right heart border is formed by the right atrium, the anterior border is formed by the right ventricle, the left border by the left ventricle and the posterior border by the left atrium. Level T10 (Fig. 4.49) At this level, part of the abdominal cavity is also seen as the dome of the diaphragm reaches the level of approximately T9. The oesophagus traverses the diaphragm at this level, in a relatively anterior position, through the oesophageal hiatus, and joins the stomach at the oesophagogastric junction. The fundus of the stomach and part of the spleen and liver are seen. The inferior vena cava is situated slightly anteriorly; on higher cuts it enters the right atrium. The descending aorta w i th azygos and hemiazygos veins on its right and left are posterior.