Chemical Shift MR Imaging of the Adrenal Gland: Principles, Pitfalls, and Applications

Abstract

Adrenal lesions are a common imaging finding. The vast majority of adrenal lesions are adenomas, which contain intracytoplasmic (microscopic) fat. It is important to distinguish between adenomas and malignant tumors, and chemical shift magnetic resonance (MR) imaging can be used to accomplish this distinction by depicting the fat in adenomas. Chemical shift imaging is based on the difference in precession frequencies of water and fat molecules, which causes them to be in different relative phases during the acquisition sequence and allows in-phase and opposed-phase images to be obtained. It is important to acquire these images by using the earliest possible echo times, with the opposed-phase echo before the in-phase echo, and by using a single breath hold to preserve diagnostic accuracy. Intracytoplasmic fat is depicted as signal drop on opposed-phase images when compared with in-phase images. Both qualitative and quantitative methods for assessing signal drop are detailed. The appearances of adrenal adenomas and other adrenal tumors on chemical shift MR images are described, and discriminatory ability at chemical shift MR imaging compared with that at adrenal computed tomography (CT) is explained. Other adrenal-related conditions in which chemical shift MR imaging is helpful are also discussed. Chemical shift MR imaging is a robust tool for evaluating adrenal lesions that are indeterminate at nonenhanced CT. However, it is important to know the advantages and disadvantages, including several potential imaging pitfalls. The characterization of adrenal lesions by using chemical shift MR imaging and adrenal CT should always occur in the appropriate clinical setting.

Imaging of Pheochromocytoma and Paraganglioma

“…This sign has been described more with head and neck paragnagliomas than with trunk paragnagliomas, and is also neither sensitive nor specific, as it can be seen with any hypervascular tumor, namely, in angiofibromas (19). Chemical shift imaging can diagnose intralesional microscopic fat such as that typically seen in adrenal adenomas, but it does not accurately differentiate adenomas from pheochromocytomas because the latter can contain intralesional fat from lipid degeneration (24,25). Signal heterogeneity within the tumor due to hemorrhage, cystic transformation, and calcifications remains a helpful feature in differentiating pheochromocytomas from benign adenomas ( Figure 5).…”

Fat Quantification in the Abdomen

“…Detection of fat in the adrenal glands is important for distinguishing adrenal adenomas, which often contain fat, from malignant tumors 12,14 . However, there is not an obvious clinical need for the quantification of fat in the adrenal glands.…”

“…Advanced CSE methods are now being applied in investigational studies to assess PDFF in the pancreas but are not yet validated for this purpose 11 . Although CSE has been used to identify the presence of fat in the adrenal glands to characterize adrenal nodules and differentiate lipid-rich adenomas from metastases, PDFF has not been investigated as a quantitative imaging biomarker of adrenal fat yet 12–15 .…”

How I do it – Inzidentalom der Nebenniere

“…Auch hier bedient man sich des Lipidgehalts. Mithilfe des Chemical Shift Imaging zwischen fett-und wassergebundenen Protonen kann das Nebeneinander von Fett und Wasser innerhalb eines Voxels nachgewiesen und abgeschätzt werden [45]. Wenn bei identischer Fensterung zwischen "inphase" (IP) und "opposed-phase" (OP) ein Signalabfall vorliegt, enthält eine Raumforderung intrazelluläres Fett und entspricht sehr wahrscheinlich einem Adenom.…”