Comparison of CT Histogram Analysis and Chemical Shift MRI in the Characterization of Indeterminate Adrenal Nodules

Abstract

Adrenal nodules having an attenuation of more than 10 H on unenhanced CT are considered indeterminate. The purpose of this study was to compare CT histogram analysis and chemical shift MRI in the characterization of indeterminate adrenal nodules. Thirty-nine adrenal masses that were indeterminate on CT were identified in 37 patients. Histogram analysis was performed on unenhanced CT from a region of interest (ROI) that recorded mean attenuation, number of pixels, and percentage of negative pixels. MR signal intensity drop between the in- and opposed-phase images was measured for the masses. Analyses to determine whether correlations existed among the mean CT attenuation, percentage of negative pixels, and MR signal intensity drop were performed. A final diagnosis was obtained by pathology results, follow-up of more than 6 months, or adrenal washout CT findings. Negative pixels were present in 25 of 28 adenomas and nine of 11 nonadenomas. A threshold of more than 10% negative pixels for the diagnosis of adenoma provided a sensitivity of 46% and specificity of 100%. A threshold of more than 20% MR signal intensity drop yielded a sensitivity of 71% and specificity of 100%. An increase in the percentage of negative pixels was correlated with a decrease in mean CT attenuation. Using MRI, observers characterized seven additional nodules as adenomas compared with CT histogram analysis (McNemar test, chi2 = 5.1429; p = 0.023). CT histogram analysis with a threshold of a 10% negative pixel presence increases sensitivity for the characterization of adenomas compared with analysis of the mean CT attenuation alone. The use of chemical shift MRI with a threshold of 20% signal intensity drop results in a higher sensitivity than CT histogram analysis.