A 35-year-old woman undergoes a high-resolution CT chest for evaluation of persistent cough. On axial imaging at the level of the carina, the radiologist identifies a structure that is anterior to the esophagus, posterior to the sternum, and contains air. Which of the following structures is most likely being visualized?

Explanation

## Correct Answer: Trachea ### Anatomical Relationships at the Level of the Carina **Key Point:** The trachea is the primary air-containing structure in the anterior mediastinum at the level of the carina, positioned anterior to the esophagus and posterior to the sternum. The trachea: - Begins at the larynx (C6 level) and descends through the neck and thorax - Bifurcates into right and left main bronchi at the level of the **carina** (approximately T4–T5 vertebral level) - Is located in the **anterior mediastinum**, anterior to the esophagus and posterior to the sternum - Contains air, making it radiolucent on CT - Has a characteristic C-shaped cartilaginous framework with a posterior membranous wall ### Why the Trachea at the Carina Level? At the carina, the trachea is at its terminal point before bifurcation. The description of an air-containing structure anterior to the esophagus and posterior to the sternum precisely matches the tracheal anatomy at this level. ### Mediastinal Compartments | Compartment | Contents | Boundaries | |---|---|---| | **Anterior mediastinum** | Thymus (in children), fat, lymph nodes, trachea (upper portion) | Anterior: sternum; Posterior: pericardium, great vessels | | **Middle mediastinum** | Heart, pericardium, main bronchi, lower trachea, esophagus (mid portion) | Contains the mediastinal contents proper | | **Posterior mediastinum** | Esophagus (lower), thoracic aorta, azygos system, sympathetic chain | Posterior to the pericardium | **Clinical Pearl:** On axial CT at the carina level, the trachea appears as a round or oval air-filled structure; the esophagus lies immediately posterior and is collapsed or contains fluid/food. ### High-Yield Anatomical Landmarks **Mnemonic: ABCDE of Mediastinal Structures (at mid-thorax)** - **A**orta (posterior) - **B**ronchi (main, at carina) - **C**arina (bifurcation point, ~T4–T5) - **D**esophagus (posterior to trachea) - **E**sophagus (runs behind the heart) **Tip:** When identifying structures on chest CT, remember the anterior-to-posterior sequence: sternum → trachea/great vessels → esophagus → vertebral bodies. [cite:Felson's Principles of Chest Roentgenology Ch 2]