Contrast-enhanced CT.

1-The problem is that adenomas represent a heterogeneous population: Approximately 70% of them have intracellular lipid but 30% do not. Thus, although nonenhanced CT can be used to identify 70% of adenomas, it does not allow the 30% that do not contain lipid to be reliably differentiated from metastases. In addition, although nonenhanced CT is useful to differentiate adenomas from metastases, the majority of CT examinations in oncology patients use intravenous contrast material. 

2- Adenomas enhance rapidly with intravenous contrast media (either iodinated agents used at CT or gadolinium chelates used at MR imaging) and wash out the agent rapidly (Fig).

Nonenhanced CT scan shows a left adrenal adenoma (arrow), which has an attenuation of 4 HU.

On the dynamic enhanced phase image, the adrenal gland (arrow) enhances vigorously to 54 HU.

On the 10-minute delayed image, the attenuation of the left adrenal gland (arrow) is 23 HU (lower than that of the normal right adrenal gland, kidneys, and liver). There is greater than 50% washout between the dynamic phase of contrast enhancement and the 10-minute delay, which is diagnostic of an adenoma and confirms the finding on the nonenhanced CT scan. Quantitative region-of-interest measurements (in Hounsfield units) are important because degree of enhancement is difficult to quantify with the human eye.

3- Metastases also enhance vigorously with contrast material, but the washout of the agent is more prolonged than with adenomas (Fig). This difference in washout of contrast media has been exploited to further differentiate benign from malignant adrenal lesions. 

Nonenhanced CT scan demonstrates an enlarged left adrenal gland (arrow) with irregular margins and attenuation of 40 HU.

Dynamic enhanced CT scan of the adrenal gland (arrow) obtained 60 seconds after intravenous administration of contrast material demonstrates an increase in attenuation to 53 HU.

Ten-minute delayed image of the left adrenal gland (arrow) demonstrates persistent enhancement of the adrenal gland (56 HU). There is no significant washout of contrast media at 10 minutes, a finding consistent with an adrenal metastasis.

 4-Two features can be measured at delayed CT: the attenuation value of the adrenal gland and the washout of contrast media. A Hounsfield unit of less than approximately 30 at 10 minutes after injection has been shown to be diagnostic of a lipid-rich adenoma; however, most adenomas have an attenuation value higher than 30, and thus it is a specific but not a sensitive test. A more useful parameter is the percentage of washout of contrast material in which the attenuation of the adrenal gland at delayed CT is compared with its attenuation at dynamic CT. Loss of 50% of the attenuation value of the adrenal mass at delayed CT is specific for an adenoma; less than 50% washout is indicative of either a metastasis or an atypical adenoma. Percentage of washout is typically calculated by the following formula: (1 − delayed enhanced HU value/initial enhanced HU value) × 100. Quantitative region-of-interest measurements (in Hounsfield units) are important because degree of enhancement is difficult to quantify with the human eye.

5-It is important to stress that if a lesion in an oncology patient cannot be definitively called an adenoma after CT examination, the patient should undergo further evaluation with MR imaging or an adrenal biopsy to confirm a benign or malignant adrenal lesion. Thus, although an attenuation value of less than 10 HU at nonenhanced CT is diagnostic of an adenoma, an attenuation value of greater than 10 HU is not diagnostic of a metastasis. A lesion greater than 10 HU at nonenhanced CT may be either an adenoma or metastasis.