Characterization of adrenal masses (< 5 cm) by use of chemical shift MR imaging: observer performance versus quantitative measures.

OBJECTIVE

The purposes of this study were to evaluate the ability of chemical shift MR imaging to differentiate 1- to 5-cm adrenal adenomas from metastases and to compare subjective interpretation with several different quantitative measures.

SUBJECTS AND METHODS

Forty-three patients with 46 proved adrenal lesions (28 adenomas and 18 metastases) had MR imaging with a gradient-echo breath-hold technique and echo time varied to obtain in-phase and out-of-phase images. Qualitative analysis of the MR images was done by three experienced observers, who reported their confidence in diagnosing the benignity versus the malignancy of the adrenal masses on the basis of signal loss on out-of-phase images. Quantitative analysis was performed by calculating the difference in signal intensity between in-phase and out-of-phase images by use of regions of interest (signal intensity index) and by use of adrenal-liver, adrenal-spleen, and adrenal-muscle signal intensity ratios. Results between quantitative and qualitative measures were compared by use of receiver operating characteristic (ROC) analysis.

RESULTS

The mean signal intensity was significantly different between adenomas and metastases on out-of-phase images (64 versus 98) (p < .0005) but not in-phase images (130 versus 122) (p = .47). The adrenal-spleen ratio discriminated between adenomas and metastases better than did the adrenal-liver ratio, the adrenal-muscle ratio, or the signal intensity index. No significant difference in interpretation among the three observers was evident (areas under the ROC curves, 0.93, 0.95, and 0.96). The performance of the observers was comparable to the results obtained with the adrenal-spleen ratio measurement (area under the ROC curve, 0.97).

CONCLUSION

Experienced observers were able to differentiate adrenal adenomas from metastases just as well as quantitative measures. The adrenal-spleen ratio is the best quantitative means of distinguishing benign from malignant adrenal masses. Chemical shift MR imaging is a good but imperfect discriminator of adrenal adenomas.