Chest computed tomography

Approach to diagnosing mass lesions

The imaging evaluation algorithm consists of three mandatory steps. First, it is necessary to confirm that the mass originates specifically from the mediastinum. On standard radiographs, this is indicated by the absence of the air bronchogram sign, obtuse angles at the border with the lung, and displacement of mediastinal lines.

Next, the intra-mediastinal localization is established (anterior, middle, or posterior). It is important to understand that there are no anatomical tissue barriers between these compartments, so this division is used solely to generate a differential diagnosis list. On CT, the compartment is determined by the center of the mass on an axial slice or by the pattern of displacement of surrounding structures.

The final stage is characterization of the content. Based on X-ray attenuation on CT, fatty, fluid (cystic), vascular, and solid (soft-tissue) lesions are distinguished. For instance, the presence of fat attenuation makes it easy to recognize a hiatal hernia or liposarcoma.

Acute mediastinitis and abscesses

Acute mediastinitis is a rare but life-threatening condition that begins abruptly with fever and chest pain. The most common cause is esophageal perforation (including due to severe vomiting in Boerhaave syndrome) or postoperative complications.

CT visualizes increased attenuation of the adipose tissue, pleural effusion (usually left-sided), and extraluminal gas bubbles. In advanced cases, fluid becomes walled off, forming a mediastinal abscess, which may be accompanied by empyema or myositis of adjacent muscles.

Important rule: the presence of small gas bubbles and fluid collections retrosternally is completely normal within the first 20 days after sternotomy. An infectious process is diagnosed only if gas appears de novo or its volume progressively increases.

Fibrosing mediastinitis

This disease is associated with the proliferation of collagen and fibrous tissue, which threatens obstruction of the superior vena cava or central pulmonary vessels. Two CT patterns are distinguished:

• Focal pattern: usually associated with previous histoplasmosis. It appears as a soft-tissue mass with calcifications in the paratracheal region or lung hila.
• Diffuse pattern: an infiltrating mass without calcifications affecting multiple compartments. Frequently associated with retroperitoneal fibrosis.

Differential diagnosis from lymphoma or metastases is successfully performed using MRI. Fibrosis and calcifications yield markedly decreased signal intensity on T2-weighted images (T2 weighted MRI), whereas malignant tumors typically show high signal intensity. However, MRI by itself detects calcifications less effectively than computed tomography.

Pneumomediastinum

This is an accumulation of free air surrounding mediastinal structures. Causes include trauma, perforations, inhalational cocaine use, or barotrauma in pulmonary infections. Air can spread along perivascular spaces into the neck or downward into the retroperitoneum (retroperitoneum).

Radiographs are characterized by optical effects: the "continuous diaphragm sign" (air accumulation above it) and the "double bronchial wall sign". Computed tomography reveals linear lucencies surrounding the vessels.

Differential diagnosis on CT: pneumomediastinum is fundamentally different from pneumothorax and pneumopericardium. In pneumomediastinum, gas dissects through the adipose tissue, so soft-tissue strands are always visible within the air collections. There is no free adipose tissue in the pleural or pericardial cavities, so gas there appears as a homogeneous lucency without strands.

Specific mass lesions

Intrathoracic goiter (extension of a multinodular goiter from the neck) has a clear spherical contour and a continuous connection with the thyroid gland at the cervical level. Benign goiters typically feature rounded calcifications, while carcinomas show cloud-like ones. The mass usually displaces and narrows the trachea.

Mediastinal liposarcoma is a large tumor mass predominantly of fatty attenuation. Unlike benign lipomas, liposarcoma has thick septa and mural nodules. On PET/CT (PET/CT), its soft-tissue component actively accumulates the radiopharmaceutical (fluorodeoxyglucose).

Class and exam questions

What else to study on this topic

• Detailed classification of mediastinal neoplasms (Table 4.1): thymomas, lymphomas, germ cell tumors.
• Capabilities and limitations of PET/CT in visualizing thymic hyperplasia.
• Anatomic structure and imaging evaluation of the pericardial layers.
• Water-soluble contrast esophagography when Boerhaave syndrome is suspected.
• Signs of descending necrotizing mediastinitis and fasciitis.

Other topics in the «Chest Radiology» section

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