Chemical shift and fat suppression magnetic resonance imaging of thymus in myasthenia gravis.

Myasthenia gravis (MG) is an autoimmune neuromuscular disease leading to fluctuating muscle weakness. Approximately 70% of patients with MG have thymic hyperplasia, and 10–20% of them have thymoma. In MG patients without thymoma, thymectomy should be performed only in selected patients. In contrast, thymectomy should be performed in all patients with thymoma. In addition, thoracoscopic thymectomy can be used in nonthymomatousMG but not in MGwith thymoma, since it has limited exposure. Therefore, it is essential to differentiate thymic hyperplasia from thymoma in MG patients so that physicians can determine the need for thymectomy together with the best approach to use. In most previous studies, imaging characteristics of thymic hyperplasia and thymoma overlapped considerably on both computed tomogram (CT) and conventional magnetic resonance imaging (MRI). However, by incorporating new sequences, MRI has shown recent promise. Inaoka et al. found that chemical shift MRI could reliably differentiate thymic hyperplasia from thymus gland tumors. Furthermore, Popa and colleagues confirmed the ability of chemical shift technique in differentiating thymic hyperplasia from thymoma in MG. In addition, Kuhnt et al. suggested that MRI with fat suppression sequences was superior to CT in thymic imaging. Although the above studies are valuable for the application of MRI in MG, they did not use chemical shift and fat suppression techniques together and directly compare their performance. Moreover they did not consider the influence of age. The proportion of fat tissue in thymus changes significantly over a lifetime; after 40 years-of-age it is predominantly replaced by fat. On the other hand, chemical shift and fat suppression techniques performed differently according to the ratio of fat in the tissue. The performance of these MRI techniques on thymus imaging may change across different age groups as the ratio of fat in thymus changes. The purpose of this study was to assess the role of chemical shift and fat suppression MRI techniques in thymic imaging of patients with MG and to determine whether their performance changes with age.