12.3 Chest CT — Mediastinum, Airway, Chest Wall & Esophagram
Key Takeaways
- ARRT names mediastinum, airway, chest wall, and esophagram as four separate Chest leaf items, and each has its own compartment, technique, or contrast-selection rule distinct from the cardiac and lung topics covered elsewhere in this chapter.
- The mediastinum is divided into anterior/prevascular, middle/visceral, and posterior/paravertebral compartments, and compartment localization narrows the differential diagnosis for a mediastinal mass before any biopsy is planned.
- A chest CTA for suspected aortic dissection covers from the aortic root through the iliac bifurcation and looks for an intimal flap separating a true lumen from a false lumen.
- The chest-level 'airway' item covers the trachea and central bronchi — evaluated with thin-slice virtual bronchoscopy and, for suspected tracheomalacia, paired inspiratory and expiratory scans showing dynamic collapse of the airway lumen.
- When esophageal perforation or a postsurgical anastomotic leak is suspected, a water-soluble (not barium) oral contrast agent is used first, because leaked barium in the mediastinum can cause severe mediastinitis.
Why This Topic Is Tested
ARRT's Chest subcategory lists mediastinum, airway, chest wall, and esophagram as four independent leaf items, each getting exactly one line in the outline. Because each is compact, it is tempting to under-study them relative to the five-item cardiac cluster or the lung/screening topics — but four separate outline items still means four separate opportunities for a question, and each one tests a genuinely different skill: compartment anatomy for mediastinum, dynamic airway physiology for airway, trauma/reformatting technique for chest wall, and a contrast-safety decision for esophagram. Treat this section as four short, high-yield topics rather than one long one.
Mediastinal Compartments
The mediastinum is the central thoracic compartment between the two lungs, and CT localizes disease within it using a three-compartment model built on axial cross-sections:
| Compartment | Contents | Common pathology |
|---|---|---|
| Anterior (prevascular) | Thymus, fat, lymph nodes, ectopic thyroid/parathyroid tissue | Thymoma, lymphoma, germ cell tumor, substernal goiter |
| Middle (visceral) | Heart, great vessels, trachea, main bronchi, esophagus, lymph nodes | Lymphadenopathy, foregut duplication cyst, esophageal mass |
| Posterior (paravertebral) | Paravertebral soft tissue, descending aorta, sympathetic chain, proximal nerve roots | Neurogenic tumor, extramedullary hematopoiesis, paravertebral abscess |
A newer model from the International Thymic Malignancy Interest Group (ITMIG) divides the mediastinum the same conceptual way but defines the posterior border of the middle (visceral) compartment more precisely, as a line roughly 1 cm behind the anterior margin of each vertebral body — anything posterior to that line is paravertebral. Either model achieves the same clinical goal on the exam: naming the correct compartment for a described mass narrows the differential dramatically, since an anterior mediastinal mass in an adult raises very different possibilities (thymoma, lymphoma) than a posterior paravertebral mass (neurogenic tumor).
Chest CTA for aortic dissection is also tested in this context: the study covers from the aortic root through the iliac bifurcation in a single arterial-phase acquisition, since dissection can propagate the full length of the aorta, and the defining CT finding is an intimal flap separating a true lumen from a false lumen (the false lumen is often larger, with slower flow and a 'cobweb sign' or thrombus).
Airway: Trachea and Central Bronchi
The chest-level airway leaf item is distinct from the neck's larynx/airway item covered in Section 12.1 — this one concerns the trachea and central (mainstem/lobar) bronchi. Thin-slice raw data can be postprocessed into a virtual (CT) bronchoscopy, a 3D endoluminal rendering used to evaluate central airway tumors, strictures, or a suspected foreign body aspiration without an invasive scope. A specific dynamic protocol is used for suspected tracheomalacia (abnormal tracheal wall flaccidity): paired inspiratory and end-expiratory scans are obtained, and a reduction in tracheal cross-sectional area of roughly 50 percent or more on the expiratory series, compared to full inspiration, supports the diagnosis. This paired-phase technique parallels the inspiratory/expiratory HRCT approach used for small-airways disease in Section 12.4, but here the target is the large central airway rather than the lung parenchyma.
Chest Wall
Chest wall CT evaluates the ribs, sternum, sternoclavicular joints, and soft tissue of the thoracic wall. In blunt chest trauma, a dedicated bone algorithm/kernel reconstruction combined with 3D volume-rendered reformats is used to map rib fractures more completely than axial images alone, since oblique or minimally displaced fractures running roughly parallel to the axial plane are easy to miss on standard slices. This mapping matters clinically because flail chest is defined as three or more contiguous ribs each fractured in two or more places, creating a free-floating segment that moves paradoxically with respiration — a diagnosis that is far easier to confirm on a curved or volume-rendered chest wall reformat than by counting fracture lines on individual axial slices. The sternoclavicular joint is a frequent trauma-protocol addition because overlapping bony structures make radiographic evaluation of anterior/posterior dislocation unreliable — CT resolves this ambiguity directly. Chest wall CT also evaluates soft-tissue and osseous masses (lipoma, desmoid tumor, metastasis) when a palpable abnormality is not explained by radiography.
Esophagram
A CT esophagram (esophagography) uses oral contrast administered just before or during chest CT acquisition to evaluate the esophagus for perforation, anastomotic leak after esophagectomy, achalasia, or to help stage esophageal cancer alongside cross-sectional findings such as wall thickening and periesophageal fat stranding. The critical contrast-selection rule the exam tests: when perforation or a postoperative leak is suspected (for example, Boerhaave syndrome or a post-esophagectomy anastomotic leak), a water-soluble iodinated contrast agent is used first, because leaked barium in the mediastinum can incite severe, difficult-to-treat mediastinitis. Barium is reserved for cases where perforation has already been excluded or is not a clinical concern, since it provides superior mucosal coating and detail for subtler abnormalities.
Additional Procedures in the Chest
ARRT's cross-cutting "Additional Procedures" focus theme — vascular studies, biopsy, drainage, aspiration, trauma, and surgical planning — applies across all four topics in this section. A CT-guided biopsy of an anterior mediastinal mass is typically approached through a parasternal or paravertebral window chosen to avoid the great vessels and internal mammary vessels, while a posterior mediastinal or paravertebral mass biopsy is planned to avoid crossing lung parenchyma and pleura wherever possible, since a transpulmonary needle path raises the risk of pneumothorax. These planning decisions are the same "shortest safe path" logic tested for other body regions in Chapter 15, applied here to the chest.
Exam Scenario
A patient one week after esophagectomy develops fever and chest pain, and an anastomotic leak is suspected. The correct oral contrast choice for the CT esophagram is a water-soluble iodinated agent, not barium — using barium in this setting risks converting a contained leak into a severe mediastinitis if contrast escapes into the mediastinum.
An adult patient has an anterior (prevascular) mediastinal mass identified on chest CT. Which diagnosis is most consistent with this compartment location?
A patient with suspected tracheomalacia undergoes paired inspiratory and end-expiratory CT of the central airway. Which finding on the expiratory series supports the diagnosis?