The mediastinum is the “thoracic space” between the two pleural sacs, the root word originally meaning septum. The mediastinum extends from the thoracic inlet cephalad to the superior surface of the diaphragm. The undersurface of the mediastinum is bounded posteriorly by the anterior longitudinal spinal ligament (dorsal) and the posterior mantel of the sternum anteriorly (ventral). Although lesions of the paravertebral region (costovertebral sulci) are traditionally classified as mediastinal in nature, this area is not formally within the mediastinum.

Multiple surgical and radiologic subdivisions within the mediastinum have been described in the literature. Most commonly, the mediastinum has been divided either into four compartments (superior, anterior, middle, and posterior) or three compartments (anterosuperior, middle or visceral, and posterior).

The anterior mediastinum can be defined as the area bounded by the thoracic inlet superiorly, the sternum anteriorly, and the pericardium and vertebral column inferiorly (see Figure 1). Each compartment extends to the diaphragm inferiorly and is bounded laterally by the mediastinal surface of the respective parietal pleura.