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.