MR Imaging in Differentiating Benign from Malignant Masses

1- In general, metastases and carcinomas contain larger amounts of fluid than adenomas and thus appear bright on T2-weighted images. However, there is significant overlap in T1 and T2 signal intensity between adenomas and metastases, and thus signal intensity is not useful to reliably differentiate between them.

2- Enhancement patterns have also been investigated as a means of differentiating benign adrenal adenomas from metastases, and, similar to their appearance at CT, adenomas vigorously enhance and exhibit early washout of contrast material compared with metastases on MR images. Given the increased cost of MR imaging, CT is probably more cost effective to assess enhancement patterns.

3-  As stated earlier, intracellular lipid is high in most adrenal adenomas and low in metastases. Chemical shift imaging is an MR imaging technique used to detect lipid within an organ and is the most sensitive method for differentiating adenomas from metastases.

4-In out-of-phase images, the adenoma appears darker than on in-phase images (Fig)

T1-weighted in-phase MR image demonstrates a right adrenal mass (arrow).

T1-weighted out-of-phase MR image shows signal drop-off in the adrenal gland (arrow), which is diagnostic of an adenoma.

5- In adrenal masses that do not contain lipid (eg, metastases), there is no significant signal loss on out-of-phase images, and thus the signal intensity of the adrenal gland is the same on in-phase and out-of-phase images (Fig).

T1-weighted in-phase MR image demonstrates a left adrenal mass (arrow).

T1-weighted in-phase MR image demonstrates a left adrenal mass (arrow).

T1-weighted out-of-phase MR image shows no significant signal loss in the adrenal gland compared with that of the spleen. The mass is either a metastasis or atypical adenoma, and biopsy was recommended.

6-  When in-phase and out-of-phase images are compared, an internal standard is useful to visually quantify signal drop-off. In general, the liver is a less reliable internal standard because intrinsic liver disease (eg, steatosis, hemochromatosis) can cause variable hepatic signal intensity on in-phase and out-of-phase images (Fig). We find it useful to compare signal intensity of the adrenal gland with that of the spleen as the internal standard. It is also helpful for the technologists to use one prescan value for both in-phase and out-of-phase acquisitions, since variable prescan values can vary the signal intensity of the adrenal gland. Finally, it is important for the technologist to use the same window and level values on both in-phase and out-of-phase images. 

T1-weighted in-phase MR image demonstrates a right adrenal mass (arrow), which is isointense relative to the liver (L) and slightly higher in signal intensity than the spleen (S).

T1-weighted out-of-phase MR image shows signal drop-off in both the liver (due to steatosis) and the mass. The adrenal mass has clearly lost signal compared with the spleen on out-of-phase images, a finding that is diagnostic of an adenoma.

7- In summary, chemical shift MR imaging is the most sensitive technique for differentiating adenomas from metastases to the adrenal gland. When results of CT examinations are equivocal, MR imaging is the next imaging study of choice for characterizing adrenal lesions.