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metastatic disease and are often included in the staging evaluation of tumors arising in the chest and abdomen. Although some tumors such as lymphomas and primary malignancies of the kidney may involve the adrenal glands by direct invasion, most metastatic deposits reach the adrenal glands by hematogenous spread. The most COfllfllOfl primary tumors to produce adrenal metastases are carcinomas of the lung and the breast. The incidence of melanoma is rising; in fact, it is likely that new surveys will include melanoma among the most common primary malignancies seen in patients with a metastatic tumor in the adrenal glands. Benign cortical adrenal adenomas are also common. Commons and Callaway [2] reported an incidence of adrenal adenomas with a diameter of 3 mm or greater as 2.86% based on findings from 7437 postmortem examinations. This high incidence of benign adrenal adenomas has been confirmed by other investigators, although the frequency varies depending on the size cnitenon for the adenomas. Furthermore, the frequency with which this adenoma is found increases with age. If we were to use 1 .0 cm as the size at which adrenal masses are routinely detected, we would likely find that at least 1% of the abdominal CT or MR examinations would reveal an adrenal mass. Many characteristics help to distinguish an adrenal adenoma from a metastasis. Adenomas tend to be small, well-defined, and homogeneous masses with a smooth contour. Although many metastases are large, heterogeneous masses with an irregular contour, these differences are not sufficiently reliable to allow a confident diagnosis of adenoma to be made or to exclude metastatic tumor. With the introduction of MR imaging, meta.stases were shown to have a much higher signal intensity on T2-weighted sequences than adenomas. Initial enthusiasm for this technique as a method to distinguish the two adrenal lesions diminished as reports of significant overlap (20-30%) in signal intensities appeared [3]. Interest then shifted to examining the differences between adenomas and metastases after the IV administration ofgadolinium. Metastases had greater enhancement and more prolonged washout than adenomas, which showed lesser degrees of enhancement and more rapid washout, often with a return to baseline levels only 5 mm after administration of contrast material [4]. Although excellent results were reported, this technique did not gain widespread use. One of the fundamental differences between adrenal adenomas and metastases is the amount ofcytoplasmic lipid present within the cells. Approximately 80% of adrenal adenomas contain an abundance of lipid and are termed “lipid rich’ Others are referred to as “lipid poor,” and some have mixed cellularity. Metasta.ses, on the other hand, do not contain an abundance of lipid. Thus, imaging techniques that can detect lipid may be used to distinguish lipid-rich adenomas from nonadenomas such as metastases [5]. Several investigators have reported the accuracy of CT in distinguishing adenomas from nonadenomas [5-8]. Because both adenomas and nonadenomas enhance with the administuition of contrast material, the density of the adrenal mass on unenhanced scans is used. After selecting a threshold (often 10 H), the sensitivity and specificity of correctly diagnosing an adrenal adenoma can be calculated. MR imaging can also detect adrenal masses with a high lipid content. Lipid-rich adenomas show a decrease in signal intensity between in-phase and opposed-phase images [9-1 1]. Reports suggest that these two techniques, unenhanced CT and chemical-shift MR imaging, have similar accuracies in diagnosing adrenal adenomas [5, 12]. Because some adrenal adenomas are lipid poor or have mixed cellularity, neither unenhanced CT nor chemical-shift MR imaging will be completely sensitive in diagnosing an adenoma. In such cases, initial reports have