Chemical Shift Characterization of Adrenal Masses

Several methods can be used to help differentiate adrenal adenomas from malignant adrenal neoplasms. These include signal intensity index (SII) and adrenal-to-spleen ratio (ASR), among others. These are calculated from chemical shift images using the signal intensity (SI) of the adrenal gland on in-phase images (SI_Ai), the SI of the adrenal gland on opposed-phase images (SI_Ao), and the signal intensity (SI) of spleen (SI_S) on in-phase (i) and opposed-phase (o) images. The liver, which was once used as a reference organ, is not a good comparison, because it can lose signal on opposed-phase images when there is fatty infiltration.

The SII seems to be the most reliable method. All adrenal adenomas had values > 16.5% and all metastatic tumors had a value < 11.2%. An adrenal-to-spleen ratio of less than 0.71 indicates a lipid-rich adenoma. Mnemonic for the ASR formula: O (opposed-phase) Over i (in-phase).

Chemical shift imaging has a sensitivity of ~80%-100% for the differentiation of adrenal lesions, and a specificity of ~90%-100%. No significant difference exists between CT and chemical shift imaging for characterizing lipid-rich adenomas; however, chemical shift imaging may be better for evaluating lipid-poor adenomas with attenuation of up to 30 HU.

False negatives can occur with adenomas with relatively lipid-poor, compact cells. False positives in metastases (renal cell carcinoma, hepatocellular carcinoma, liposarcoma, well-differentiated adrenal cortical carcinoma) that contain fat or envelop periadrenal fat.

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References

• Blake MA, Holalkere NS, Boland GW. Imaging techniques for adrenal lesion characterization. Radiol Clin North Am. 2008 Jan;46(1):65-78, vi. Review.
• Fujiyoshi F, Nakajo M, Fukukura Y, Tsuchimochi S. Characterization of adrenal tumors by chemical shift fast low-angle shot MR imaging: comparison of four methods of quantitative evaluation. AJR Am J Roentgenol. 2003 Jun;180(6):1649-57.
• Bilbey JH, McLoughlin RF, Kurkjian PS, Wilkins GE, Chan NH, Schmidt N, Singer J. MR imaging of adrenal masses: value of chemical-shift imaging for distinguishing adenomas from other tumors. AJR Am J Roentgenol. 1995 Mar;164(3):637-42.