Most clinicians quickly recognize a variety of abnormal patterns of diffuse parenchymal lung disease on conventional chest radiographs. However, anomalies of the upper respiratory tract, some of which can cause acute respiratory failure, are often overlooked during the initial assessment. Imaging of the trachea will be reviewed here. The radiologic manifestations of diffuse parenchymal lung disease, and the management of central airway obstruction, are discussed separately. (See "Evaluation of diffuse lung disease by conventional chest radiography" and "Diagnosis and management of central airway obstruction".)
The trachea extends from the lower border of the larynx (2 cm below the vocal cords) to the carina, where it bifurcates into the mainstem bronchi. The average tracheal length is 10 to 12 cm, and the normal angle of the tracheal bifurcation is 70 ± 20 degrees (image 1A-B). With deep inspiration, the tracheal length increases by as much as 2 cm and the angle of the tracheal bifurcation decreases by up to 10 degrees [1,2].
The trachea is supported anteriorly and laterally by 18 to 22 semicircular incomplete rings of cartilage; this corresponds to 2 cartilage elements per centimeter of tracheal length. Tracheal cartilages can calcify in elderly patients with a predilection for women and patients on longstanding warfarin therapy. The posterior (membranous) tracheal wall consists of longitudinally aligned smooth muscle and fibrous connective tissue.
The normal transverse internal diameter of the trachea ranges between 15 and 25 mm, with a cross-sectional area of 250 to 350 mm2 and a volume of 30 to 40 cm3 at total lung capacity (TLC) [1,3]. The transverse diameter of the trachea increases by 10 percent with inspiration and can decrease by 30 percent with coughing.
The radiographic appearance of the tracheal air column should be assessed routinely when chest radiographs are obtained. If an endotracheal tube is present, proper position must be confirmed (figure 1). Aspiration of sand or other particulates may create a radiopaque outline of the trachea and large airways (image 2).