Characterization of adrenal adenomas and metastases: correlation between unenhanced computed tomography and chemical shift magnetic resonance imaging

Abstract

To evaluate the correlation of absolute attenuation values of unenhanced computed tomography (CT) with signal intensity (SI) quantitative analysis on chemical shift (CS) magnetic resonance (MR) imaging in differentiating adrenal adenomas from metastases. Forty-one adrenal masses (27 adenomas, 14 metastases) were studied with CS MR imaging and unenhanced CT. MR included T1-weighted breathhold gradient-echo in-phase (IP) and opposed-phase (OP) sequences. The SI index (SI-i) [(SIIP-SIOP/SIIP)] x 100% and chemical-shift ratio (CS-r) relative to the spleen [(SIlesion/ SIspleen)OP/(SIlesion/SIspleen)IP] were calculated for each lesion. CT absolute attenuation values were also determined. The mean attenuation value of metastases was significantly greater than that of adenomas (< 0.0001). On MR, the mean SI-i of adenomas was significantly greater than that of metastases (P < 0.0001) and no overlaps were evident. The CS-r of malignant and benign lesions overlapped considerably, and five adenomas (all with indeterminate Hounsfield Unit values at CT) were misclassified as potentially malignant. CT attenuation values were significantly correlated with both MR quantitative analyses. Since CS MR imaging and CT both depict the presence of lipids within adrenal lesions, absolute attenuation values are highly correlated with MR quantitative analysis. SI-i is the most reliable tool for differentiating adrenal adenomas from metastases, showing better accuracy than lesion-to-spleen CS-r, in particular for adenomas with indeterminate absolute attenuation values.