Except for mediastinal pneumatocele and air-containing abscess, chest x-ray can only show changes in the morphology and position of the mediastinum, while CT and MRI can further clarify the cause of mediastinal changes.
X-ray
On chest x-ray film, mediastinal changes mainly include changes in morphology and position. Morphological changes are mostly manifested by widening of the mediastinum, the lesions that cause widening of the mediastinum can be tumorous, inflammatory, hemorrhagic, lymphatic, fatty, and vascular, and mediastinal tumor is most common. Position changes are mostly manifested by mediastinal displacement. Thoracic, pulmonary, and mediastinal lesions can all cause mediastinal displacement. Lung atelectasis and extensive pleural thickening can stretch the mediastinum to the affected side. Large amounts of pleural effusion, huge tumors in the lungs, and hemilateral growth of mediastinal tumors can push the mediastinum to the healthy side.
Figure 1 Mediastinal widening
a. Chest plain film shows the mediastinum widens to both sides (↗); b. CT mediastinal window shows mediastinal widening caused by mediastinal teratoma (↗).
CT
According to CT value, mediastinal lesions can be divided into fatty, solid, cystic, and vascular lesions. Lipoma is more common in the right cardiophrenic angle. Solid lesions can be seen in benign and malignant tumors and lymph node enlargement. Cystic lesions show rounded or subrounded opacities of fluid density; pericardial cyst is mostly in the right cardiophrenic angle; bronchial cyst occurs mainly beside the trachea and esophagus or near the hilum of the lung. Aortic aneurysm can present arcuate calcification of the aneurysm wall.
Contrast-enhanced CT is very valuable for distinguishing vascular masses from non-vascular masses and benign masses from malignant masses. Vascular lesions are often significantly enhanced, and aortic aneurysm, aortic dissection, and mural thrombus can be accurately identified. In solid lesions, benign lesions are mostly homogeneously and slightly enhanced, while malignant lesions are mostly inhomogeneously and significantly enhanced. Cystic lesions can show slight enhancement of the cyst wall. In fatty lesions, only blood vessels inside the lesions are enhanced.
MRI
In solid lesions, tumor is often slightly higher than normal muscle tissue on T1WI and predominantly higher on T2WI in signal intensity. In cystic lesions, simple serous cyst is hypointense on T1WI and significantly hyperintense on T2WI; mucinous cyst or cystic fluid rich in protein is hyperintense on both T1WI and T2WI; cyst contain cholesterol crystals or hemorrhage is also hyperintense on T1WI. Fatty mass is hyperintense on both T1WI and T2WI and hypointense on fat suppression sequences; teratoma often shows focal fat signal on T1WI and T2WI. In vascular lesions, the aneurysm wall has poor elasticity, the bloodstream slows down in flow velocity or forms vortices, and the signal generated by the vortices is mostly inhomogeneous; In arterial dissection, true and false lumen can be easily distinguished depending on the blood flow velocity. Usually, the false lumen is larger than the true lumen, the blood flow in the false lumen is slower and the signal is higher, while the blood flow in the true lumen is faster and usually flow voids can be seen.