OBJECTIVE. The purpose of this study was to evaluate the utility of T1- and T2-weighted MRI signal-intensity ratios and signal-intensity SDs of renal lesions to determine the feasibility of distinguishing between simple cysts, hemorrhagic renal cysts, clear cell renal cell carcinoma (RCC), and papillary RCC. MATERIALS AND METHODS. Pathology records of 53 cases of papillary RCCs between 1 and 5 cm in size were included. Thirty-eight pathology-proven clear cell RCCs, 54 simple renal cysts seen on abdominal MRI, and 59 hemorrhagic renal cysts seen on abdominal MRI were identified. Lesion location and size, T1- and T2-weighted signal intensity, and corresponding SD values for each renal lesion and psoas muscle (from which lesion-to-muscle ratios were calculated) were collected. RESULTS. Analysis revealed a statistically significant difference (p < 0.001) in T1-weighted lesion-to-muscle signal-intensity ratios between simple cysts (mean ± standard error, 0.54 ± 0.05), clear cell RCCs (0.86 ± 0.06), papillary RCCs (1.17 ± 0.05), and hemorrhagic renal cysts (1.95 ± 0.04). The T2-weighted lesion-to-muscle signal-intensity ratios showed a statistically significant difference between all lesion types (p < 0.02) except between hemorrhagic renal cysts and papillary RCCs, where the difference approached significance (p = 0.075). ROC analysis showed an optimal cutoff of T1-weighted lesion-to-muscle signal-intensity ratio of 1.39 to differentiate hemorrhagic cysts (above this value) from RCCs (below this value). Corresponding sensitivity and specificity were 91.2% and 74.6%, respectively. CONCLUSION. T1-weighted lesion-to-muscle signal-intensity ratio is a useful measure to discriminate mildly hyperintense RCCs from more hyperintense hemorrhagic cysts when contrast enhancement is unavailable.

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