Mediastinum

Mediastinum : The mediastinum is divided into anterior, middle and
posterior divisions for descriptive purposes. This can be demonstrated on a lateral CXR , but is usually performed with CT.
CT and MRI of normal mediastinum: Computed tomography is the standard method for imaging the mediastinum. MRI is used only occasionally. The cross sectional display and the ability to distinguish between fat, various soft tissues and blood vessels are major advantages of both techniques.
• The mediastinum is composed predominantly of blood vessels. Visualization and differentiation from other soft tissue structures is aided with the use of intravenous contrast medium. At MRI, the larger vessels are readily seen without contrast agent.
The only other normal structures of appreciable size are the thymus, oesophagus, trachea and bronchi.
Normal lymph nodes are small, usually less than 6 mm in diameter (maximum 10 mm), and many are not visible.
The mediastinal structures are surrounded by fat (low density on CT), this enables visualization of other small soft tissue masses which are generally of increased density.