Chest radiography, with or without barium esophagography, can be considered the first-line imaging modality used in diagnosing tracheal or esophageal compression, particularly in children. CXR double projection (straight and leaning) can enlighten tracheal compression by nearby structures or tracheal displacement related to aortic arch location. In case of double aortic arch, the CXR image describes the trachea in axis, with two lateral compressions. The CXR with barium esophagography can display a posterior incision on the esophagus due to the presence of anomalous subclavian artery or Kommerell diverticulum, and in the presence of pulmonary sling, the cleft will be on the anterior wall of the esophagus. Moreover, there could be present signs of pulmonary atelectasis or lung hyperinflation in case of bronchial compression. Unfortunately, this technique does not allow direct images of vascular structures or vessels’ anatomy, preventing from an accurate surgical planning.

Bronchoscopy is nowadays an important tool to make a diagnosis when symptoms of respiratory distress are present. This procedure may demonstrate the presence of pulsatile compression and also the degree and precise location of such compression. In cases of pulmonary sling, bronchoscopy shows the length of hypoplastic tracheal segment, the diameter of this segment, and the presence of complete tracheal rings. Bronchoscopy is mandatory after surgical treatment to evaluate the results of surgery and in the postoperative period to follow the outcome of correction.

The echocardiography can be considered the first tool to confirm the suspicion of vascular ring or sling. It is usually able to demonstrate the aortic side; the anomalies of vessels of the aortic arch, such as aberrant subclavian artery or aneurysm of the brachiocephalic trunk; the double aortic arch; and the anatomy of the pulmonary artery. However, echocardiography is a poor imaging tool to either establish or exclude the diagnosis of a vascular ring in case of poor acoustic windows such as in adult patients. In addition, it does not depict the ligamentous structures and hyperinflation of the lungs and the compression of the trachea or esophagus. Echocardiography is absolutely necessary for investigating associated cardiac defects.

Angiography was the first procedure used to perform diagnosis in these patients until the last decade, but currently this imaging modality is rarely used, and it is useful only in selected cases. Angiography may be considered, if the anatomy of the patent vessels must be visualized, including the size of the two arches, in case of a double aortic arch, in order to make a proper selection of the arch that has to be cut. However, angiography is unreliable in demonstrating the tracheal or esophageal compression, and moreover it is an invasive procedure, requiring vascular access and the use of ionizing radiation and contrast agent.

Computed tomography angiography (CTA) allows for an accurate description of vascular and respiratory tract anatomy. Patent vascular channels are evident on CTA as contrast-enhancing segments and are well visualized on reconstructed 3D images. Conversely, atretic vascular segments and ligaments are not evident, but their presence can be inferred from traction on associated vascular structures or compression of the trachea [6]. Inspiratory and expiratory CTA studies allow the dynamic evaluation of tracheal caliber for narrowing or traction, which is particularly important in patients with associated tracheomalacia. CTA scanning times are shorter than magnetic resonance imaging (MRI), and therefore sedation is usually not necessary, which is a significant advantage in a young or respiratory distressed patient. The principal disadvantages of CTA are the need for intravenous contrast agents and the potential late consequences of radiation-dose exposure.