Adenoma characterization: adrenal protocol with dual-energy CT.

PURPOSE: To determine the diagnostic performance of dual-energy computed tomography (CT) by using virtual unenhanced CT to characterize adrenal masses.

MATERIALS AND METHODS: This study is retrospective, HIPAA-compliant, and approved by the institutional review board. Between December 2009 and June 2010, 49 patients with 49 adrenal masses underwent 120-kVp unenhanced CT and 80-kVp and 140-kVp early and delayed contrast agent-enhanced dual-energy CT. Early virtual unenhanced (EVU) and delayed virtual unenhanced (DVU) CT images were composed of data sets of early and delayed contrast-enhanced CT, respectively. Adenomas were divided into lipid-rich adenoma and lipid-poor adenoma on the basis of lesion attenuation values measured according to unenhanced CT and percentage loss of enhancement. Absolute percentage loss of enhancement was calculated with the following equation: (CT(EE) - CT(DE)) × 100/(CT(EE) - CT(UE)), where CT(UE), CT(EE), and CT(DE) are adrenal mass attenuation values at unenhanced CT, early contrast-enhanced CT, and delayed contrast-enhanced CT, respectively. The sensitivity of adrenal protocol adenoma with delayed contrast-enhanced CT was obtained with a reference standard of unenhanced CT, pathologic examination, or size stability on follow-up examination findings. Lesion attenuation values measured on unenhanced CT, EVU CT, and DVU CT images were compared by using repeated measures analysis of variance with post hoc test.

RESULTS: Of 49 masses, 33 were adenomas and 16 were nonadenomas. Adenoma group was 18 lipid-rich adenomas and 15 lipid-poor adenomas. Mean attenuation values of the lipid-rich adenomas on EVU CT images (11.7 HU ± 9.5) were significantly greater than those on unenhanced CT images (0.7 HU ± 7.2) (P = .001) and DVU CT images (6.6 HU ± 8.4) (P = .01). The sensitivities of EVU CT and DVU CT for lipid-rich adenoma were 39% (seven of 18) and 61% (11 of 18), respectively. The sensitivity for adenoma with percentage loss of enhancement values calculated from virtual unenhanced CT and early and delayed contrast-enhanced CT was 100% (33 of 33).

CONCLUSION: Although adrenal protocol with dual-energy CT by using virtual unenhanced CT and washout rate can help diagnose all lipid-poor adenomas, it may miss lipid-rich adenomas that can be diagnosed on unenhanced CT images.