Diffusion-weighted MR Imaging of the Thymus in Patients with Generalized Myasthenia Gravis: Usefulness of ADC Values in Distinguishing Lymphoid Hyperplasia, Normal Thymus, and Thymoma

PURPOSE To prospectively evaluate the efficacy of diffusion-weighted (DW) magnetic resonance imaging (MRI) in patients with myasthenia gravis (MG) for distinguishing thymic lymphoid hyperplasia (TLH), normal thymus (NT), and thymoma (THY) by using the apparent diffusion coefficient (ADC), in order to suitably select patients for surgery. METHOD AND MATERIALS The IRB approved the study, and informed consent was obtained. We evaluated 88 subjects (45 male, 43 female) with generalized MG and anti-AchR antibodies that underwent surgery. They were divided into a TLH/NT group (A, 64 patients; TLH, 49; NT, 15) and a THY group (B, 24 patients), based on the histological findings. DW-MRI was performed at b values of 0, 150, 500, and 800 s/mm2. All measures were performed independently by two radiologists and inter-reader agreement was assessed by calculating the intraclass correlation coefficient (ICC). Discrimination abilities were individuated according to the Area under the ROC curve (AUROC) and optimal cut-points were individuated by using the Youden Index (YI). Differences on ADC levels between groups and subgroups of group A were evaluated by Mann-Whitney U test and Kruscal-Wallis test, respectively. RESULTS The inter-reader agreement was excellent (ICC, 0.944). The mean ADC values were significantly different between groups (A, 1.92±0.21 x 10-3 mm2/sec; B, 1.36±0.33 x 10-3 mm2/sec; p<0.001) and between subgroups of group A (TLH, 1.86±0.17 x 10-3 mm2/sec; NT, 2.10±0.23 x 10-3 mm2/sec; p=0.002). AUROC of ADC in discriminating between groups was 0.931, and the optimal cut-point for this distinction was 1.625 x 10-3 mm2/sec (YI, J=0.760) with sensitivity of 96.8% and specificity of 79.1%. AUROC of ADC in discriminating between subgroups of group A was 0.794, and the optimal cut-point for this distinction was 2.01 x 10-3 mm2/sec (YI, J=0.458) with sensitivity of 76.7% and specificity of 79.2%. CONCLUSION DW-MRI is an useful tool in patients with generalized MG. The ADC value can be used for distinguishing TLH, NT, and THY in order to select appropriately patients for surgery (THY/TLH) and patients to treat with medical therapy (NT) since, for non-thymomatous MG, acceptable complete remission rates after thymectomy are observed in TLH but not in NT. CLINICAL RELEVANCE/APPLICATION The ADC value can be used for distinguishing TLH, NT, and THY in order to select appropriately patients for surgery (THY/TLH) and patients to treat with medical therapy (NT).