Figure 1: Coronal HASTE image demonstrates bilateral adrenal lesions.

Figure 2-A and 2-B: Axial T1 in phase demonstrates bilateral adrenal lesions that are intermediate in signal intensity.

Figure 3-A and 3-B: Axial T1 opposed phase image shows drop out of signal in both adrenal lesions compatible with masses that contain intravoxel fat.

Bilateral adrenal adenomas.

Adrenal masses are a common incidental finding at abdominal CT and are reported to occur in up to 5% of patients. These masses often are adrenal adenomas that are non-hyperfunctioning, however, some adenomas do secrete hormones. Adrenal adenomas have a high cholesterol content and often are hypodense on non-contrast CT images. However, there are cases in which CT is unable to determine the presence of fat. MRI becomes the next test in order to determine the etiology of the adrenal mass.

On MR imaging, adenomas are best studied using the T1 weighted in phase and T1 weighted opposed phase images. The fat and cholesterol that adenomas contain is intravoxel fat, or microscopic fat, not bulk or macroscopic fat. Intravoxel fat within a lesion is best demonstrated by a signal drop out on the opposed phase images. Therefore, on the opposed phase images the adenoma would have a lower signal intensity than on the in phase image. An objective way to determine if there is enough signal drop out on the opposed phase images to be classified as an adrenal adenoma is to use the following equations:

Adrenal-to-Spleen Chemical Shift Ratio:
[(Adrenal opposed phase / Spleen opposed phase )/(Adrenal in phase / Spleen in phase )] <0.71

Adrenal Signal Intensity Index:
[(Adrenal in phase – Adrenal opposed phase )/Adrenal in phase ] x 100>16.5%

References:

1. Israel GM , Korobkin M, et al. Comparison Of Unenhanced CT And Chemical Shift MRI In Evaluating Lipid-Rich Adrenal Adenomas . American Journal of Roentgenology. 2004; 183 : 215 - 219.
2. Webb W., Brant W, and C Helms. Fundamentals of Body CT . Philadelphia : Saunders, 1998.