Magnetic Resonance

In some cases, MRI is superior to CT scanning for visualizing the thymus and for differentiating it from the surrounding soft tissue.
In healthy children younger than 5 years, MRI shows the thymus to have a quadrilateral shape and biconvex lateral contours. [9] In older children and adolescents, the thymus is triangular with straight, lateral margins. On T1-weighted images, the thymus appears homogeneous with a signal intensity slightly greater than that of muscle; on T2-weighted images, the signal intensity is close to that of fat.
Mass lesions in the mediastinum have sufficiently different imaging characteristics to allow their distinction on MRI from normal structures and fat, and MRI produces excellent cross-sectional images in the mediastinum without contrast enhancement; with CT, contrast material is often needed to properly identify a mass and to avoid mistaking blood vessels for a mass lesion.
Encasement or invasion of the vasculature, esophagus, and trachea and involvement of the pericardium, myocardium, and pleura are accurately detected with MRI.
In all adults, the thymus is visible on MRI. Distinction between higher intensity mediastinal fat and the relatively hypointense thymus is optimal on T1-weighted images because of the long T1 of the thymus. The progressive decrease in T1 with advancing age is commensurate with the fatty infiltration associated with advancing age. However, the T2 relaxation times of the thymus do not change with aging. The thymus usually appears thicker on MRI than on CT in patients older than 20 years.
On T1-weighted images, thymomas have medium signal intensity that is higher than that of skeletal muscle but that is lower than that of fat. On T2-weighted images, the signal intensity approaches or exceeds that of fat. The mass often appears mostly homogeneous; areas of cystic degeneration appear as areas of variable signal intensity on T1-weighted images. This appearance is the result of variations in protein content or of hemorrhage; however, on T2-weighted images, thymomas appear bright. The hypointense fibrous septa often gives the mass a lobulated appearance.

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