Aberrant thymus--MR assessment.

Aberrant thymic tissue, particularly when it is located posteriorly, can present a diagnostic dilemma. Once a mediastinal "lesion" is defined on the plain film, MR is the most efficient and best diagnostic test for defining the nature of the mass and showing its relationship to other mediastinal structures. Two cases of aberrant thymic tissue, one on the right side and one on the left, are presented to elucidate the unique role of MRI.