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Background: Several reports show that MR imaging, especially using the chemical-shift sequence, provides highly reliable differentiation of adrenal adenomas from non-adenomas. The aim of the study was to evaluate the ability of MRI to distinguish adenomas from other adrenal masses using quantitative analysis of the tumors' signal intensity. Material/Methods: Fifty-four patients with 57 adrenal masses underwent MRI. The tumors were determined during surgery as pheochromocytomas (8), metastases (5), adrenal cortical carcinoma (1), and adenomas (24). Nineteen masses were diagnosed as adenomas on the basis of stability on imaging followup and the absence of clinical and endocrinological dysfunction. Chemical-shift-weighted images (T1TFE sequence) and T2-weighted images (TSE sequence) were used for quantitative analysis which included the T2 index (adrenal mass SI to liver SI ratio) and the CSI ratio (the adrenal mass SI on the in-phase image minus the adrenal mass SI on the opposed-phase image divided by the adrenal mass SI on the in-phase image). Statistical analysis was performed with the Mann-Whitney U test. Receiver operating characteristic (ROC) analysis of the calculated parameters was performed. Results: Significant differences in T2 index between adenomas (mean: 1.43±0.50) and pheochromocytomas (2.66±0.67) as well as between metastases (1.64±0.22) and pheochromocytomas were noted (p<0.05). The Mann-Whitney U test revealed no significant difference in T2 index for adenomas vs. metastases (p=0.1). The CSI ratio was significantly different for adenomas (0.36±0.18) vs. pheochromocytomas (-0.15±0.16) as well as for adenomas vs. metastases (-0.23±0.26). No significant difference occurred in the CSI ratios between pheochromocytomas and metastases. ROC analysis showed that the discriminatory ability of adenoma diagnosis with the CSI ratio is better than with the T2 index (areas under the ROC curve: 0.980 vs. 0.867). Conclusions: Quantitative methods using signal intensity ratios and indexes calculated from MR images are helpful in differentiating adrenal adenomas from non-adenomas. In the presented investigation the CSI ratio appeared much more reliable than the T2 index as a quantitative means of distinguishing adenomas from other adrenal tumors.