MR Imaging of Adrenal Lesions

Aoife Kilcoyne, MB BCh BAO, FFR, RCSI; Shaunagh McDermott, MB BCh BAO, FFR, RCSI; Michael A. Blake, MB BCh BAO, MRCPI, FRCR, FFR, RCSI

Disclosures

Appl Radiol. 2017;46(4) 

In This Article

MRI Features of Adrenal Lesions

Adrenal Adenoma

Identifying microscopic lipid within an adrenal mass has remained the cornerstone of characterizing an adrenal lesion as an adenoma (Figure 2).[32] Chemical shift imaging is a rapid and reliable technique that relies on demonstrating lipid within the mass to diagnose an adrenal adenoma.[14] Although non-contrast CT and CSI are used in clinical practice for differentiating adenoma from malignancy, some investigators have reported that between 62% and 100% of adenomas with attenuation of greater than 10 Hounsfield Units (HU) can be characterized confidently using CSI.[21,23] CSI may be preferable to non-contrast CT in characterizing adrenal lesions as adenomas because it demonstrates better sensitivity for intracellular lipid and does not expose the patient to radiation.

Figure 2.

Left adrenal adenoma. Axial T1-weighted in- (A) and out-of-phase (B) images demonstrating signal drop-off on the out-of-phase sequence consistent with an adrenal adenoma with voxels containing both lipid and water protons.

Adrenal Myelolipoma

The imaging diagnosis of myelolipomas is based on the presence of macroscopic fat. MRI characteristics include T1-hyperintense signal that suppresses with frequency-selective fat saturation (Figure 3).[33] Similar to renal angiomyolipomas, the presence of the India Ink (chemical shift) artifact at the myelolipoma-adrenal interface or within an adrenal mass on OOP images should indicate a myelolipoma.[34] Small quantities of macroscopic fat are not completely specific for myelolipoma. In addition to adenomas, a recent retrospective study of 41 adrenocortical carcinomas suggested that 10% contained foci of macroscopic fat.[35] In contrast to renal angiomyolipomas, small amounts of macroscopic fat should be considered characteristic, but not diagnostic, of myelolipomas.[33] Other imaging features (margins, invasion, heterogeneity etc.) should be considered to exclude a rare, fat-containing malignancy. Otal et al[36] reported that a lesion with composition of greater than 50% macroscopic fat can be managed as a myelolipoma.

Figure 3.

Right adrenal myelolipoma. Axial T1-weighted in-phase (A) and fat-saturated (B) images demonstrating signal drop-off on the fat-saturated sequence consistent with intralesional macroscopic fat.

Adrenal Hemorrhage

The causes of unilateral adrenal hemorrhage are blunt abdominal trauma, iatrogenic causes and underlying malignancy. MRI is sensitive in the evaluation of hemorrhage by specific signal characteristics of the blood products. In the acute setting and due to the paramagnetic effect of deoxyhemoglobin, hemorrhage demonstrates isointense or hypointense T1 signal and hypointense T2 signal.[27] In the subacute setting, the signal characteristic of the hemorrhage is mostly determined by intracellular or extracellular methemoglobin with hyperintense T1 signal and hypointense or hyperintense T2 signal. Chronic hematoma can develop cystic changes with low marginal signal intensity due to peripheral hemosiderin deposition and formation of a fibrous capsule.[37] Contrast-enhanced MRI can be performed to evaluate for any enhancing solid component of the hematoma which would suggest the presence of an underlying lesion that has bled.

Adrenal Cysts: Simple Cysts, Pseudocysts, Lymphangiomas

Adrenal cysts are rare. The frequency of adrenal cysts seen at autopsy ranges from 0.064% to 0.18%.[38,39] They can be broadly subdivided into simple cysts, pseudocysts and lymphangiomas.

Simple Cysts

Endothelial cysts/simple cysts are the most common subtype. Simple cysts are hypointense on T1-weighted images and hyperintense on T2-weighted images with no soft tissue component and no internal enhancement.[40]

Pseudocysts

These represent the next most common subtype. Pseudocysts are caused by a prior episode of hemorrhage or infarct; the cyst wall is composed of fibrous tissue. Although low signal, the pseudocyst can be more complex and may present with thicker walls, internal septations and calcifications. MRI is the best method for characterization due to its increased sensitivity for detection of hemorrhagic elements and superior septation characterization (Figure 4).[33,41]

Figure 4.

Adrenal pseudocyst. 15 x 17 x 20 cm mass within the right adrenal gland. The mass is isointense on the axial T1-weighted sequence (A) and mildly T2 hyperintense on the axial T2-weighted FRFSE sequence (B).

Lymphangiomas

These are rare and typically symptomatic.[42] Adrenal lymphangiomas are usually thin-walled cystic lesions with low signal intensity at T1-weighted imaging and high signal intensity at T2-weighted imaging without substantial internal enhancement.[43–45]

Pheochromocytoma

The most common MR imaging appearance of pheochromocytoma is a mass with low signal intensity at T1-weighted imaging and with high signal intensity at T2-weighted imaging.[46] The signal pattern is variable and the high T2 signal in particular is not always present.[47] However pheochromocytomas do typically enhance avidly on T1-weighted imaging following administration of gadolinium based contrast material.[12,47] Usually, pheochromocytomas appear still more hyperintense on fat-suppressed, T2-weighted images due to a signal intensity rescaling effect, which reflects the reduced signal intensity of background fat (Figure 5).[46] As mentioned above, a pheochromocytoma cannot be excluded on the basis of a lack of high intensity on T2-weighted MR imaging. Conversely, non-pheochromocytomas, including some metastases may be incorrectly classified as pheochromocytomas due to high T2 lesion signal intensity.[46] Punctate signal voids representing tumor vessels in paragangliomas create a salt-and-pepper pattern characteristically seen on T1- and T2-weighted images.

Figure 5.

Right pheochromocytoma. Axial T2-weighted sequence with fat saturation (A) demonstrating homogeneous signal hyperintensity. Axial T1-weighted sequence with fat saturation pre- (B) and postintravenous contrast (C) demonstrating avid enhancement postcontrast.

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