Differentiation of Rebound and Lymphoid Thymic Hyperplasia from Anterior Mediastinal Tumors with Dual-Echo Chemical-Shift MR Imaging in Adulthood: Reliability of the Chemical-Shift Ratio and Signal Intensity Index

Purpose: To prospectively evaluate (a) effectiveness and limits of dualecho chemical-shift magnetic resonance (MR) imaging for distinguishing hyperplastic thymus from anterior mediastinal tumors in adulthood by using chemical-shift ratio (CSR) and signal intensity index (SII), with proposal of optimal threshold value for each, and (b) whether age affects these indexes. Materials and Methods: Study was institutional review board approved, with informed consent obtained. Ninety-two subjects (53 men, 39 women; age range, 18-84 years) were divided into a rebound and lymphoid hyperplasia group (group A, 30 patients) and a tumor group (group B, 62 patients). MR images were assessed; interrater reliability was evaluated. Differences in CSR and SII were tested with the Mann-Whitney U test and the Kruskal-Wallis test. Discrimination abilities of CSR and SII were evaluated with logistic regression models, and optimal cutoff points were proposed. Quantitative parameters were correlated with age by using Pearson correlation coefficients. Results: Interreader agreement was excellent (intraclass correlation coefficient: CSR, 0.893; SII, 0.898). Mean CSR and SII 6 standard deviation were 0.545 6 0.162 and 46.29% 6 18.41 for group A and 1.045 6 0.094 and 20.06% 6 4.89 for group B, respectively, with significant differences for both indexes between groups (P < .0001). No overlap was found for SII between groups; CSR values overlapped in a few younger adults. Distinguishing hyperplastic thymus from tumors was better with SII than CSR. Respective sensitivity, specificity, and cutoff points were 100%, 100%, and 8.92% for SII and 100%, 96.7%, and 0.849 for CSR. Significant correlation was found for CSR (r = 20.761) and SII (r = 0.821) with age in group A (P < .001). For group B, significant correlation with age was seen for CSR (r = 0.702, P < .001) but not SII (r = 20.196, P =.127). All subjects but one in group A and none in group B had signal intensity decrease at chemical-shift MR imaging. Conclusion: With dual-echo chemical-shift MR imaging, SII and CSR have high accuracy to distinguish thymic hyperplasia from tumors, although overlapped CSR values can occur in early adulthood. (C) RSNA, 2014