Noun
A partition; a septum; specifically, the folds of the pleura (and the space included between them) which divide the thorax into a right and left cavity. The space included between these folds of the pleura, called the mediastinal space, contains the heart and gives passage to the esophagus and great blood vessels.
Source: Webster's dictionaryHowever, MRI findings have no advantage over CT scan findings in the evaluation of the mediastinum. Source: Internet
On the lateral sides (edges), the mediastinum is bound by the membranes that line the lungs. Source: Internet