Anterior and Posterior Junction Lines on Chest Radiograph

Key points

• Anterior junction line: four pleural layers (two visceral, two parietal) where the upper lobes appose anterior to the pericardium; visible on about 40% of upright frontal radiographs as an oblique line running downward from right to left at the level of the aortic arch.
• Posterior junction line: pleural apposition behind the trachea and esophagus, in front of the spine; seen more often than the anterior line, projected as a vertical line over the tracheal air column and extending above the clavicles down to the aortic arch.
• Both are normal potential spaces containing a little mediastinal fat; each is a hairline (≤1 mm) that lipomatosis may slightly widen in elderly or obese patients.
• Effacement or focal widening localizes adjacent disease: thymic lesions, thyroid mass, lymphoma, germ cell tumor for the anterior line; esophageal mass, lymphadenopathy, aortic disease, neurogenic tumor for the posterior line.
• Displacement matters as much as effacement – toward the lesion with volume loss (pulmonary hypoplasia, chronic scarring, atelectasis), away from it with anterior pneumothorax or congenital lobar overinflation (usually left upper lobe).
• The retrocardiac opacity is left lower lobe pneumonia, unrelated to the junctional lines; it hides behind the heart on the frontal view and classically produces the spine sign – loss of the normal progressive increase in vertebral lucency – on the lateral view.

References

1. Yoon HK, Jung KJ, Han BK, Cho YA, Park JM, Lee KS, et al. Mediastinal interfaces and lines in children: radiographic-CT correlation. Pediatr Radiol. 2001;31(6):406-12. PMID: 11436887.
2. Feragalli B, Mantini C, Patea RL, De Filippis F, Di Nicola E, Storto ML. Radiographic evaluation of mediastinal lines as a diagnostic approach to occult or subtle mediastinal abnormalities. Radiol Med. 2011;116(4):532-47. PMID: 21424313.

 

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