Mediastinal imaging in myasthenia gravis: correlation of chest radiography, CT, MR, and surgical findings

Chest radiographs and CT and MR images of the mediastinum were studied in 16 patients with myasthenia gravis who underwent thymectomy (two with a final diagnosis of thymoma, seven with hyperplasia, and seven with a normal thymus). The anterior mediastinum was analyzed on imaging studies for thymic morphology and the presence of mass lesions, and the findings were then correlated with the results of surgical resection and pathologic examination. The chest radiographs detected an anterior mediastinal mass in two patients consistent with thymoma on subsequent CT and MR examinations. Chest radiographs in the other 14 patients were normal. In seven patients with a final diagnosis of thymic hyperplasia, both CT and MR demonstrated normal thymic morphology in five, an enlarged thymus in one, and a small thymus in one that was easily identified on CT but was difficult to define on MR. In the other seven patients with a normal thymus on pathologic examination, both CT and MR showed an involuted thymus in four, a normal thymus in two, and an enlarged thymus in one. While both CT and MR were superior to chest radiography for studying the thymus, CT provided better spatial resolution and thymic definition in a much shorter scanning time than MR did. This study suggests that CT should remain the procedure of choice when further imaging of the thymus is needed after the initial chest radiographs in patients with myasthenia gravis.