THE THORAX
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 with 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
Fig. 4.41
Cross-sectional a n a t o m y : level T3.
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 with body habitus, and may be 1 cm on the left in obese people with 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, with 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.
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