Normal Posterior Junction Line Thickness on Radiography
The posterior junction line is a thin mediastinal stripe visible on posteroanterior (PA) chest radiography, formed where the posterior walls of the right and left upper lobes contact each other behind the esophagus and anterior to the spine. It projects above the aortic arch and courses obliquely across the midline. Accurate recognition of this line is important because deviation or thickening may indicate underlying mediastinal or pulmonary disease.
Normal Reference Values
| Orientation | Measurement |
|---|---|
| Posteroanterior | <1 mm |
Clinical Significance
On a standard PA chest radiograph, the posterior junction line should measure less than 1 mm in thickness. Because it represents only the apposed pleural layers and a negligible amount of connective tissue, any measurable thickening or obliteration warrants further evaluation. The line is normally seen as a straight or gently curved opacity projecting over the trachea.
Abnormal thickening, displacement, or absence of the posterior junction line may indicate pathology in the posterior mediastinum or adjacent lung. Conditions that can alter this landmark include:
- Posterior mediastinal mass (e.g., neurogenic tumor, foregut duplication cyst)
- Mediastinal lymphadenopathy (lymphoma, metastatic disease, sarcoidosis)
- Aortic aneurysm or dissection involving the descending thoracic aorta
- Esophageal pathology (megaesophagus, esophageal carcinoma)
- Extramedullary hematopoiesis
A key pitfall is confusing the posterior junction line with the azygoesophageal recess interface or the paravertebral stripe; familiarity with normal mediastinal lines reduces overcalling. CT is the appropriate next step when radiographic findings are equivocal.
Reference: Gibbs JM, Chandrasekhar CA, Ferguson EC et al. Lines and stripes: where did they go? – From conventional radiography to CT. Radiographics. 27 (1): 33-48.
Imaging Notes
The posterior junction line is assessed on the PA projection of the chest radiograph. It appears as a thin vertical or oblique stripe projected over the tracheal air column, typically extending from the level of the clavicles to the aortic arch. Optimal visualization requires adequate inspiratory effort and appropriate exposure technique. The line is seen in the majority of adults but may not always be visible in normal individuals, which is not itself pathological.
When the line is identified, its thickness, course, and continuity should be noted. Any focal convexity, lobulation, or width exceeding 1 mm should prompt further evaluation with CT of the chest with contrast to characterize adjacent mediastinal structures. Lateral radiographs are of limited value in assessing this specific interface and add little diagnostic information for this structure alone.