MIDDLE MEDIASTINAL MASSES

The middle mediastinum is bounded

• Anteriorly by the pericardium• Posteriorly by the pericardium and posterior tracheal wall • Superiorly by the thoracic inlet• Inferiorly by the diaphragm.

Its contents include •the heart and pericardium•the ascending and transverse aorta •the superior vena cava (SVC) •inferior vena cava (IVC) •the brachiocephalic vessels •the pulmonary vessels•the trachea and main bronch; •lymph nodes •the phrenic, vagus, and left recurrent laryngeal nerves

Masses in children• Lymphadenopathy• Fore gut duplication cysts• bronchogenic cysts• oesophageal duplication cysts• neurenteric cysts• Cystic hygroma• Hiatus hernia• Achalasia

Masses in adults• Lymph nodes• Carcinoma of bronchus• Bronchogenic cyst• Aneurysm

Bronchogenic cyst

• Round or oval, unilocular water density mass. It may cause airway obstruction and secondary infection.Communication with air way resulting in cavity is rare.

Oesophageal duplication cyst• Less common than bronchogenic cyst.• Usually larger and to the right of midline.• May contain ectopic gastric mucosa, which cause

ulceration, haemorrhage or perforation.

ANEURYSM OF THORACIC AORTA

Widening of the mediastinum.

A round or oval soft tissue mass in any part of mediastinum with a well defined outline.

Sometimes a peripheral rim of calcification

May also involve adjacent bones producing:

1. Pressure erosion of the sternum.

2. Anterior scalloping of one or two vertebral bodies

Presentation:Chest pain radiating to back (dissecting).Hoarseness of voice due to traction on recurrent laryngeal Nerve.Dysphagia.

CT findings

• Shows a dilated aorta containing a central lumen of blood of high attenuation (80-100 H.U.) due to enhancement of the blood pool with water soluble contrast medium (true).

• A peripheral layer of clot of lower attenuation which may contain calcification (false lumen). The subintimal flap and false lumen of a dissecting aneurysm can be demonstrated by CT.

Knowledge of the local anatomy of an interrupted mediastinal line is much more helpful in identifying a possible alternative diagnosis.

Aortopulmonary Window The aortopulmonary (AP) window is a middle mediastinal

space bounded • Superiorly by the inferior margin of the aortic arch • Inferiorly by the superior margin of the left pulmonary

artery• Anteriorly by the posterior wall of the ascending aorta • Posteriorly by the anterior wall of the descending aorta • Medially by the trachea, left main bronchus, and

oesophagus• Laterally by the left lung.

The AP window contains lymph nodes, the left recurrent laryngeal nerve arising from the vagus nerve, the left bronchial arteries, the ligamentum arteriosum, and fat.

AP window reflection. On a posteroanterior chest radiograph, the AP window reflection (arrowhead) extends from the aortic knob to the left pulmonary artery and has a normal concave appearance

An abnormal convex contour of the AP window suggests a mediastinal abnormality, most commonly lymphadenopathy although such a contour may occasionally represent a normal variant caused by the accumulation of fat. Vascular abnormalities such as an aortic arch aneurysm can also distort the AP window.

AP window lymphadenopathy: 

Chest radiograph shows the AP window with an abnormal convex border (arrow). 

AP window lymphadenopathy.   CT scan demonstrates lymphadenopathy (arrow), which accounts for the distortion of the AP window.

Aneurysm of the aortic arch. 

 CT scan reveals an aneurysm (arrow) arising laterally from the aortic arch, a finding that accounts for the abnormality.

Right Paratracheal Stripe

The right paratracheal stripe is seen projecting through the SVC. It is formed by the trachea, mediastinal connective tissue, and paratracheal pleura and is visible due to the air–soft tissue interfaces on either side.

The right paratracheal stripe should be uniform in width, with a normal width ranging from 1 to 4 mm, a right paratracheal stripe 5 mm or more in width is considered widened.

Posteroanterior chest radiograph shows the right paratracheal stripe (arrow).

CT scan shows the right wall of the trachea with medial and lateral air–soft tissue interfaces caused by air within the tracheal lumen and right lung (arrow). These interfaces create the right paratracheal stripe

On the PA film there is a lobulated paratracheal stripe on the right.

On the lateral radiograph there is a density overlying the ascending aorta and filling the retrosternal space.These findings indicate a mass in the anterior as well as in the middle mediastinum.

The CT confirms the presence of lymphomas in both the anterior and the middle mediastinum.

Pitfalls in Assessing the Middle Mediastinum

A variety of normal vascular variants may be mistaken for middle mediastinal disease at chest radiography.

• A right-sided aortic arch, seen in 0.5% of the general population , may mimic paratracheal lymphadenopathy because it obliterates the right paratracheal stripe; however, the absence of the aortic knuckle on the left should help correctly identify this variant.

Right-sided aortic arch.  Posteroanterior chest

radiograph demonstrates an abnormality in the right paratracheal region (arrow) with loss of the paratracheal stripe. Note, however, the absence of the aortic knuckle on the left.

CT scan shows a right-sided aortic arch (arrow)

A left-sided SVC may create an additional mediastinal line lateral to the aortic arch at radiography . This variant courses anterior to the left hilum and drains into the coronary sinus.

Left-sided SVC. 

Posteroanterior chest radiograph shows an additional line (arrow) lateral to the aortic arch.

Venogram demonstrates a left-sided SVC, which explains the finding 

CT scans obtained at the levels of the aortic arch.

CT scan at the level of pulmonary trunk

show the left-sided SVC (arrow), which drains into the coronary sinus.

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