Characterization of lipid-poor adrenal adenoma: chemical-shift MRI and washout CT.

OBJECTIVE: The purpose of this article is to retrospectively compare the accuracy of MRI and CT in characterizing lipid-poor adrenal adenomas with respect to lesion-attenuation values measured on unenhanced CT.
MATERIALS AND METHODS: Fifty-two lipid-poor adrenal masses measuring greater than 10 HU on unenhanced CT were identified in 52 patients who underwent both chemical-shift MRI and washout CT. Accuracies using the adrenal-to-spleen ratio (< 0.71) or signal intensity index (> 16.5%) for MRI and using absolute (≥ 60%) or relative (≥ 40%) percentage washout for CT were calculated to determine which modality was more accurate for lipid-poor adenoma characterization. Sensitivities of MRI and CT were also compared according to the lesion-attenuation values measured on unenhanced CT. Follow-up imaging or histologic diagnosis was used as the standard reference. The McNemar test was used to compare the accuracies of CT and MRI.
RESULTS: Lipid-poor adrenal masses consisted of 37 adenomas and 15 nonadenomas. The sensitivities and specificities for adenoma on MRI versus CT were 75.7% (28/37) versus 100% (37/37) and 60.0% (9/15) versus 80.0% (12/15), respectively. CT achieved a higher accuracy than did MRI (p = 0.008). The sensitivities for adenomas measuring 20 HU or less on unenhanced CT were 100% (12/12) in both MRI and CT, whereas those measuring more than 20 HU were 64.0% (16/25) and 100% (25/25) in MRI and CT, respectively.
CONCLUSION: MRI is equivalent to CT for characterizing adenomas measuring 20 HU or less on unenhanced CT. However, MRI is inferior to CT for adenomas measuring more than 20 HU because of decreased MR sensitivity.