Delayed enhanced CT of lipid-poor adrenal adenomas

OBJECTIVE, Although representing a minority of adrenal adenomas, the lipid- poor variety cannot be accurately identified on unenhanced CT or chemical s hift MR imaging. We compared the delayed contrast-enhanced CT features of l ipid-poor adenomas with those of lipid-rich adenomas and of adrenal nonaden omas to determine whether then were differences in the washout features bet ween these groups of lesions. SUBJECTS AND METHODS. Eighteen proven lipid-poor adenomas, 56 lipid-rich ad enomas, and 40 adrenal nonadenomas underwent CT before, immediately after, and 15 min delay after IV contrast injection. Region-of-interest measuremen ts were made of all adrenal lesions at the three time points. The degree of enhancement. enhancement washout, percentage enhancement washout, and rela tive percentage enhancement washout were calculated for each adrenal mass. Pooled data were analyzed statistically. Optimal threshold values for diagn osing adrenal adenomas were also determined. RESULTS. The mean CT attenuation of lipid-poor adenomas was significantly h igher than that of lipid-rich adenomas at all three phases but not signific antly different from that of nonadenomas. The mean percentage enhancement w ashout on images obtained 15 min after administration of contrast material was similar for lipid-rich and lipid-poor adenomas but was significantly hi gher than that of nonadenomas. The mean relative percentage enhancement was hout was significantly different among all three groups. CONCLUSION. Lipid-poor adenomas cannot be differentiated from adrenal nonad enomas on the basis of a single mean attenuation value. However, lipid-poor adrenal adenomas show enhancement and enhancement washout features nearly identical to lipid-rich adenomas and can be distinguished from nonadenomas on the basis of a percentage washout threshold value of 60% and a relative percentage washout of 40%.