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Radiological Presentation of Mediastinal Abnormalities

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The mediastinum is the central part of the thoracic cavity. Read this article to know more about the radiological examination of mediastinum.

Written by

Dr. Narmatha. A

Medically reviewed by

Dr. Varun Chaudhry

Published At October 14, 2022
Reviewed AtFebruary 8, 2024

Introduction:

The mediastinum is located in the chest cavity. The thoracic or chest cavity is a part of the upper trunk located below the neck and above the abdomen. The mediastinum is bounded laterally by a pleural cavity, superiorly by the thoracic outlet, and inferiorly by the diaphragm. The mediastinum is classified into different compartments.

What Are the Different Compartments of Mediastinum?

1) According to Felson, the mediastinum is classified into:

  • Anterior Compartment - Extends anteriorly to the sternum and posteriorly to the posterior margin of the heart.
  • Middle Compartment - Extends anteriorly to the posterior margin of the heart and posteriorly to a vertical line drawn 1 cm behind the anterior margins of thoracic vertebrae.
  • Posterior Compartment - Extends anteriorly to a vertical line drawn 1 cm along the thoracic vertebrae and posteriorly to the costovertebral junction.

2) According to the International thymic malignancy interest group (ITMIG), the mediastinum is classified into:

  • The Prevascular Compartment - Bounded anteriorly by the posterior surface of the sternum and posteriorly by the anterior aspect of the pericardium. This compartment contains the thymus, fat, lymph nodes, and left brachiocephalic vein. Almost 50 percent of the mediastinal masses are found in the prevascular compartment.
  • The Visceral Compartment - Located posterior to the prevascular compartment. It contains the heart, trachea, esophagus, ascending and descending aorta, superior vena cava, thoracic duct, and pulmonary artery. It is bounded anteriorly by the pericardium and posteriorly by the anterior aspect of the thoracic spine.
  • The Paravertebral Compartment - Located posterior to the visceral compartment. It is bounded anteriorly by the anterior aspect of the thoracic spine and posteriorly by vertical lines along the posterior margin of the chest wall. It contains the thoracic spine, paravertebral soft tissues, sympathetic trunk, and hemiazygos venous system.

What Are the Common Abnormalities of Mediastinum?

1. Anterior Compartment -

  • Lipoma (noncancerous tumor of fat tissues).

  • Thymoma (thymus gland tumor).

  • Thymolipoma (noncancerous tumor of the thymus that contains fatty tissues and thymus cells).

  • Germ cell tumor (tumor of the reproductive cells).

  • A thymic cyst (fluid-filled sac in the thymus).

2. Middle Compartment -

  • A bronchogenic cyst (cyst arising from the primitive foregut).

  • Neurenteric cyst (cyst arises from the remnants of the neurenteric canal).

  • Lipoma.

3. Posterior Compartment -

  • Extramedullary hematopoiesis (abnormal production of blood cells outside the bone marrow).

  • Meningocele (neural tube defect).

  • Schwannoma (tumor of the nervous system).

  • Neurofibroma (nerve tumor).

What Are the Radiological Findings Associated With Mediastinal Abnormalities?

  1. Mediastinal Widening - If the posterior-anterior (PA) view of the chest X-ray shows an increase in the width of the mediastinum by more than 8 cm, it refers to the mediastinal widening. It indicates mediastinal hematoma, aortic aneurysm, or mediastinal tumors.

  2. Hilum Overlay Sign - If the hilar structures are visible through the mediastinal masses, it indicates that the mass might be present anterior or posterior to the hila. It is more common in thymoma.

  3. Right Paratracheal Stripe - It is a line passing through the superior vena cava. When the thickness of the line increases more than 4 mm, it is suggestive of lymphadenopathy in this region.

  4. Silhouette Sign - The silhouette sign refers to the loss of silhouette. Silhouette is the clear differentiation of tissues with different densities. Sometimes, the abnormal mediastinal mass shares its boundary with normal structures of the mediastinum. As the radiodensity of both the normal structure and abnormal masses are the same, the normal structure loses its clear margin, which helps in locating the mediastinal mass.

  5. Paraspinal Lines - This line is located between the lungs and pleura over the vertebral bodies. Left paraspinal lines are more common than right. This line is disrupted by the diseases of vertebrae and intervertebral disc or neurogenic tumors.

  6. The Hilum Convergence Sign - The sign used to differentiate mediastinal mass from the enlarged pulmonary artery. If the pulmonary artery branches converge towards the central mass, it indicates an enlarged pulmonary artery. If the pulmonary artery branches converge towards the heart, it indicates a mediastinal tumor.

  7. Cervicothoracic Sign - This sign is used to determine the location of the mediastinal masses. Anterior mediastinum has its superior end near the clavicle. The middle and posterior mediastinum has their superior end above the clavicle. If the mediastinal mass extends above the clavicle, it indicates its location in the middle or posterior mediastinum.

  8. Thoracoabdominal Sign - Any sharply outlined mediastinal mass extending below the dome of the diaphragm must be in the posterior compartment, and any mass that ends at the dome of the diaphragm must be in the anterior mediastinal compartment. This sign is most common in lipoma.,

  9. Right Paratracheal Stripe - The para tracheal stripe is located between the right lung and the trachea. The normal width of the paratracheal stripe ranges from 1 to 4 mm. If the right paratracheal stripe is 5 mm or greater in width, it indicates the widening of the paratracheal stripe.

  10. Azygoesophageal Recess - Azygoesophageal recess is located between the right lung and pleura with an azygos vein posteriorly and an esophagus anteriorly. Deviation in the recess line indicates abnormalities in the middle and posterior compartments.

  11. Anterior Junction Line - This line is seen in the PA chest X-ray view and CT chest. The presence of anterior mediastinal masses can cause obliteration of the anterior junction line. It is formed by the joining of parietal and visceral pleura anteromedially. The thickness of the line varies with the thymus in young patients or increased fat tissues between the pleural layers.

  12. The Posterior Junction Line - This line is located behind the pleura posterior to the esophagus and in front of the vertebral bodies. This line can be seen above the clavicles. The bulging or displacement of the line may indicate mediastinal abnormalities such as neurogenic disease or esophageal malignancy.

What Are the Imaging Techniques Used for the Examination of Mediastinum?

  • Chest Radiograph: The chest radiograph is the initial imaging technique advised for mediastinal masses. For examining the mediastinum, a posterior-anterior (PA) view and lateral view of chest X-rays are taken. For patients in the intensive care unit (ICU), an anterior-posterior view (AP) is taken. The chest X-ray is easily available and cost-effective. It has less sensitivity and specificity when compared to contrast-enhanced imaging techniques.

  • Computed Tomography (CT): CT chest is the most effective imaging modality for mediastinal investigation. Contrast-enhanced CT is commonly performed for the investigation of mediastinal abnormalities. It is performed in a supine position with the arms of the patient above the head level. In emergency conditions, multidetector CT is used to detect mediastinal pathology.

  • Ultrasonography: Ultrasound can detect heart function and mediastinal masses. Anterior mediastinal mass can be indicative of a germ cell tumor. Ultrasound with a frequency range of 5 to 10 MHz is used for infants, and 2 to 4 or 4 to 7 MHz is used for children and adults. Ultrasound has limited effect in the diagnosis of mediastinal abnormalities.

  • Nuclear Medicine: PET-CT (positron emission tomography-computed tomography) is used to detect the malignancy of mediastinal such as lymphoma, esophageal cancer, and lung cancer. It is also helpful for assessing the response to cancer treatment. PET - CT is performed with the help of a radioactive substance called 18-F-fluorodeoxyglucose. I-131 metaiodobenzylguanidine is used for the detection of pheochromocytoma.

  • Magnetic Resonance Imaging: MRI or magnetic resonance imaging is used to detect congenital heart disease and can differentiate thymic hyperplasia from thymoma. This scan is advised for patients who cannot undergo a contrast-enhanced CT scan due to renal failure or contrast allergy. As MRI does not use ionizing radiation, it is considered safe for young patients.

Conclusion:

Chest X-ray and CT chest are the most effective imaging modalities used in the diagnosis of mediastinal pathology. Other techniques, such as PET-CT and MRI, provide only additional details of the mediastinal abnormalities. MRI produces images of the soft tissues with increased resolution than the images of CT. But, an MRI scan is a time-consuming procedure and is not affordable for many patients.

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Dr. Varun Chaudhry

Radiodiagnosis

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