Adrenal myelolipoma is a rare benign neoplasm composed of mature adipose tissue and a variable amount of hematopoietic elements. Most lesions are small and asymptomatic, discovered incidentally at autopsy or on imaging studies performed for other reasons. Most tumors are unilateral but show no predilection to one particular side. Tumor size varies from several millimeters to more than 30 cm.
Adrenal myelolipoma images are provided below.
Since myelolipomas contain different proportions of fat and myeloid tissue, a definitive diagnosis using computed tomography (CT) scanning or magnetic resonance imaging (MRI) may be difficult, although rarely so, if only a small amount of fat is present.
To distinguish the mass from a well-differentiated liposarcoma, a percutaneous fine needle aspiration via ultrasonographic or CT guidance can confirm the diagnosis. The presence of mature adipose tissue intermixed with hematopoietic elements, including megakaryocytes on cytology or histology, is diagnostic of myelolipoma.
Yip et al in a study of 196 adrenalectomies in 192 patients from 2000 to 2008 found that CT or MR characteristics predicted the presence of benign lesions with 100% specificity. Histopathology confirmed that all 66 adrenal masses with imaging characteristics suggesting benign adenoma were indeed benign lesions and included 61 benign adrenal adenomas and 5 benign nonadenomatous lesions (3 myelolipomas, 1 composite myelolipoma/adenoma, and 1 ganglioliponeuroma). The authors stated that to exclude malignancy, adrenal masses with non-benign imaging characteristics should be resected. 
Lamas et al reported on 2 cases of large mixed adrenal tumors that had a heterogeneous appearance and areas of fat density on imaging and resulted in autonomous cortisol production leading to Cushing syndrome. The patients underwent adrenalectomy, and histology identified adrenocortical adenomas with widespread myelolipomatous metaplasia. The authors noted that although adrenal myelolipomas are usually asymptomatic, nonfunctioning adrenal incidentalomas, there have been a few reports of myelolipomatous masses that are associated with adrenocortical hypersecretion. 
Montone et al presented pathologic and radiographic features of 3 patients with adrenocortical neoplasms. Two patients had imaging findings that were compatible with adrenal myelolipoma. Pathologically, 2 of the lesions were classified as adrenocortical neoplasms of uncertain malignant potential, and 1 lesion was classified as an adrenocortical adenoma. All 3 lesions contained myelolipomatous foci throughout the neoplasm, and 2 of the tumors contained several pure lipomatous foci. The authors noted that imaging studies may result in a false diagnosis of a benign adrenal myelolipoma and, as a result, undertreatment. 
The preferred imaging modality is CT, which shows focal fatty density within the mass. MRI also accurately depicts both microscopic and macroscopic fat using chemical shift imaging and explicit fat saturation technique, respectively. Myelolipomas may be discovered incidentally on ultrasonography (US), which otherwise is not used routinely to characterize adrenal neoplasms.
Limitations of techniques
Occasionally, myelolipomas can enlarge enough to make the organ of origin difficult to discern on CT, resulting in a differential diagnosis that includes renal angiomyolipoma and retroperitoneal liposarcoma or lipoma. In these patients, the multiplanar capability of MRI can help to define the tissue planes and confirm that the mass is adrenal in origin.
Some myelolipomas may have a larger amount of hematopoietic tissue and no recognizable fat, making them impossible to distinguish from well-differentiated retroperitoneal malignancies or other adrenal tumors on CT or MRI. Percutaneous biopsy may be necessary to establish a diagnosis.
If large enough, a suprarenal mass may cause inferior displacement of the kidney. The mass may be radiolucent if it is predominantly fatty and, occasionally, can contain calcifications. (See the images below.) [6, 7, 8, 9, 10]
Degree of confidence
Plain radiographs are nonspecific and not part of the imaging workup of these lesions. Perform cross-sectional imaging using CT or MRI to document the presence of fat.
Smaller myelolipomas are unlikely to exert enough mass effect to be visible on conventional radiographs, thereby constituting false-negative results. Similarly, inferior displacement of the kidney on radiographs can be a result of other adrenal or extra-adrenal lesions. 
CT appearance of myelolipomas depends on their histologic composition.Large amounts of fat often are seen interspersed with higher-attenuation myeloid tissue. The attenuation values are low (-20 to -30 Hounsfield units [HU]), reflecting the mixture of adipose and myeloid elements. The density of the mass may be slightly higher than that of the adjacent retroperitoneal fat secondary to the presence of the myeloid tissue. [12, 13, 14, 15, 16, 17, 18, 19, 20]
The masses usually have a recognizable capsule and may contain calcification in as many as 20% of patients. Calcification may be related to previous hemorrhage. After contrast administration, the mass enhances.  If intratumoral or peritumoral hemorrhage has occurred, high-attenuation or low-attenuation fluid may be present, depending on the age of the blood.
Below are CT images regarding adrenal myelolipomas.
Degree of confidence
CT is sensitive in depicting macroscopic fat in myelolipomas. The key to a CT diagnosis is to find a true focal fat collection in the adrenal mass. This modality is specific and diagnostic if the focal fat density is revealed.
CT can be problematic if the mass is large and the organ of origin is not obvious using axial imaging. MRI may be warranted for further evaluation as a result of its multiplanar capability. Additionally, if little or no macroscopic fat is present, the lesion has soft tissue attenuation.
Adrenal adenomas may have low-attenuation values because of a large amount of intracellular lipid, but the density usually is not less than –20 HU. In almost all myelolipomas, some regions have densities of less than –30 HU and as low as –100 HU. If the mass contains more hematopoietic tissue, it may appear more heterogeneous and mimic a retroperitoneal liposarcoma. A definitive diagnosis requires percutaneous-guided cytologic or histologic sampling.
Magnetic Resonance Imaging
The fat in an adrenal myelolipoma is of high signal intensity on T1- and T2-weighted sequences and is key to diagnosis. The hematopoietic tissue has low signal intensity on T1-weighted images and moderate signal intensity on T2-weighted images. The nonuniform admixture of fat and marrow elements also may result in a heterogeneous appearance on T2-weighted sequences. [13, 22, 23, 24, 25, 26, 27]
Explicit (chemical) fat-suppressed T1-weighted sequences show a focal loss of signal intensity in the fatty part of the mass, which confirms the diagnosis. The presence of myeloid tissue or hemorrhage results in persistent areas of increased signal intensity on fat-suppressed images. A water-saturated T1-weighted sequence shows the fat as high signal intensity on a background of low signal intensity.
MRI images are provided below that depict adrenal myelolipomas.
Chemical shift gradient-echo imaging depicts microscopic (intracellular) fat that is contained in the same voxel as a water proton. Two scans are obtained through the mass using parameters that are identical except for the echo time (TE), which is selected so that the fat and water protons are either in phase or out of phase. Out-of-phase images show signal loss, since the fat and water protons in the same voxel cancel each other out. This decrease in signal intensity indicates the presence of microscopic fat. Adrenal myelolipomas enhance after gadolinium administration as a result of the presence of hematopoietic tissue.
Gadolinium-based contrast agents (gadopentetate dimeglumine [Magnevist], gadobenate dimeglumine [MultiHance], gadodiamide [Omniscan], gadoversetamide [OptiMARK], gadoteridol [ProHance]) have been linked to the development of nephrogenic systemic fibrosis (NSF) or nephrogenic fibrosing dermopathy (NFD). The disease has occurred in patients with moderate to end-stage renal disease after being given a gadolinium-based contrast agent to enhance MRI or MRA scans.
NSF/NFD is a debilitating and sometimes fatal disease. Characteristics include red or dark patches on the skin; burning, itching, swelling, hardening, and tightening of the skin; yellow spots on the whites of the eyes; joint stiffness with trouble moving or straightening the arms, hands, legs, or feet; pain deep in the hip bones or ribs; and muscle weakness.
Elsayes et al described 3 categories of myelolipomas based on their MRI features.  These categories are (1) homogeneous, hyperintense masses on T1-weighted imaging indicating that they are composed mainly of fat; (2) heterogeneous masses composed of an admixture of fatty and myeloid elements; and (3) focal enhancing masslike areas composed mainly of myeloid cells.
Degree of confidence
MRI is as sensitive and specific as CT in depicting macroscopic fat and confirming the diagnosis. In evaluating an adrenal myelolipoma, explicit (chemically selective) fat saturation sequences show more signal loss than chemical shift imaging, since the presence of macroscopic fat is typical of a myelolipoma. In contrast, a lipid-rich adrenal adenoma would reveal greater loss of signal intensity on chemical shift imaging as it contains intracellular (microscopic) fat. An added advantage of MRI is the ability to image directly in the coronal and sagittal planes, confirming that the origin of a large mass is adrenal and not hepatic or renal.
Lesions, such as adrenal adenomas, metastases, and primary cortical carcinomas, rarely have been reported to contain focal fat and can be misdiagnosed as myelolipoma. However, these lesions are rare enough that the detection of macroscopic fat should not prevent the diagnosis in almost all patients. Conversely, some adrenal myelolipomas (eg, giant myelolipomas) have been shown to contain minimal fat, resulting in difficulty in making a definitive diagnosis by MRI. As with CT, markedly heterogeneous masses may not be distinguished easily from well-differentiated retroperitoneal malignancies, such as liposarcoma, and require tissue sampling for definitive diagnosis.
Sonographic appearance varies with the composition of the neoplasm. If the tumor contains predominantly fatty components, it appears uniformly hyperechoic. If the tumor contains a predominance of myeloid cells, it may appear heterogeneous or hypoechoic. [29, 30]
The classic appearance is an echogenic suprarenal mass, which suggests the diagnosis. The ipsilateral hemidiaphragm artifactually appears as a disrupted line, a finding unique to fat-containing masses. The margins of the lesion are difficult to define because of the lack of contrast with the adjacent retroperitoneal fat.
Degree of confidence
Although a predominantly hyperechoic adrenal mass strongly suggests the diagnosis of myelolipoma, US is not as specific as CT or MRI, either of which should be obtained for further evaluation. US is more nonspecific if the tumor contains only a small quantity of fat and appears hypoechoic, thus mimicking other benign or malignant adrenal neoplasms.
Small lesions are more difficult to diagnose by US, since they may be masked by adjacent echogenic retroperitoneal fat. The finding of apparent disruption of the diaphragmatic echoes is appreciated only for tumors larger than 4 cm; however, the absence of this finding does not mean that a hyperechoic mass does not contain fat.
Lesions containing a predominance of myeloid cells appear more hypoechoic and mimic an adenoma, hematoma, metastasis, or primary cortical carcinoma. The presence of hemorrhage or calcification within the tumor alters the sonographic appearance and further confounds the issue.
As a result of vascular specificity, an angiogram can confirm that a mass is adrenal in origin rather than arising from the liver or kidney. The mass is predominantly avascular, with a peripheral rim of vascularity from branching vessels. 
Degree of confidence
The procedure is fairly specific in determining the organ of mass's origin but is otherwise nonspecific regarding its tissue characteristic. With the advent of cross-sectional imaging modalities, little indication exists for routine angiography.
Other adrenal masses, such as adenomas, lipomas, cysts, and metastases, can have a similar angiographic appearance.