Radiologic evaluation of adrenal masses plays a primary role in
patients with adrenal masses of unknown etiology. Benign adrenal
masses of no particular significance, such as small cysts and most
adenomas, are common, incidentally discovered lesions. These masses
must be distinguished from those masses that may require a specific
histologic diagnosis, such as adrenal metastasis, adrenal cortical
carcinomas, and pheochromocytomas.
Fortunately, the majority of adrenal adenomas can be
distinguished from these entities. This distinction relies on the
fact that most adrenal adenomas contain abundant intracellular
lipid. This lipid can be detected specifically by using so-called
MR chemical-shift techniques or by measurement of attenuation on
unenhanced CT scans. Masses that do not demonstrate such lipid may
require histologic diagnosis, except in those cases where the
clinical situation or endocrinologic findings provide a specific
This review will concentrate on the differentiation of
lipid-containing adrenal adenomas from other adrenal masses,
usually metastasis in oncologic patients. Helpful diagnostic signs
of other primary adrenal neoplasms on MRI will be discussed.
Adenomas vs metastases
A distinction is made pathologically between true adrenal
adenomas and the much more common adrenal cortical nodules.1,2
Adrenal adenomas are grossly distinct masses in the adrenal gland
that are well-demarcated from the surrounding adrenal gland. In
these cases, the surrounding adrenal cortex is either normal or
atrophic due to the suppression by feedback mechanisms from the
overproduction of steroid hormones.1,2
Adrenal adenomas most commonly are hyperfunctioning, producing
either aldosterone or cortisol. Adrenal cortical nodules are much
more common as incidental findings. Most commonly, they are not
hyperfunctioning and not associated with any detectable hormonal
disturbance. Grossly, they are not as discrete as the adrenal
adenomas and are poorly demarcated from the remainder of the gland.
The rest of the adrenal is mildly hyperplastic or has small nodules
scattered within it. Adrenal cortical nodules are common findings
in older patients, especially those with atherosclerotic disease.2
Both adrenal adenomas and adrenal cortical nodules almost always
contain large amounts of intracellular steroid hormone
A variety of MR imaging techniques have been used to evaluate
adrenal masses. On average, many adrenal adenomas will demonstrate
lower signal intensity on T2-weighted images than adrenal
metastases or other solid adrenal masses.3,4 Dynamic
gadolinium-enhanced images also have been used to differentiate
adrenal adenomas from other masses. Many adrenal masses, including
most metastases and pheochromocytomas, will demonstrate fairly
rapid enhancement during a dynamic gadolinium study.5 Most adrenal
adenomas, however, will demonstrate mild enhancement or peripheral
enhancement. T2-weighted images and dynamic studies have some
specificity for distinguishing benign from malignant adrenal
masses; however, there is considerable overlap in appearance
between adenomas and metastases (figure 1), and these studies
should not be relied on for diagnosis.6-8
Chemical-shift imaging techniques are most useful for the
evaluation of possible adrenal adenoma. These techniques are
designed to detect the presence of lipid within adrenal adenomas.
Intracellular lipid is abundant in nearly all adrenal adenomas,
both functioning and nonfunctioning. This lipid is used in the
production of steroid hormones produced by the adrenal gland. If
such lipid is detected by MR techniques in an adrenal mass, the
mass is overwhelmingly likely to be a benign adrenal
Chemical-shift imaging refers to specific MR imaging techniques
where additional diagnostic information is obtained by taking
advantage of the chemical shift or frequency difference between
nuclei. The term "chemical shift" is derived from nuclear magnetic
resonance spectroscopy, where it is used to define a frequency
difference from an internal standard. In MR imaging, it is used
loosely to describe imaging in which the frequency difference
between fat and water is taken advantage of.
In a constant magnetic field, fat and water process at slightly
different frequencies. This difference is 220 Hz at 1.5 Tesla, 160
Hz at 1.0 Tesla, and 80 Hz at 0.5 Tesla. Opposed-phase images are
achieved via T1-weighted gradient recalled echo sequences (e.g., 2D
FLASH) by selecting the TE such that the transverse magnetization
of water and fat are opposed (e.g., TE = 2.1 or 6.3 msec at 1.5
Tesla).12 In these sequences, the signal represents the difference
between the individual fat and water contributions.
Practically, the TE need not be an exact multiple of 2.1 to 2.3
to achieve the opposed-phase effect; imaging constraints sometimes
require a slightly different TE (figure 1). Evidence of adequate
opposed-phase effect can be seen at fat-water interfaces, where a
black-line artifact may be seen, or in bone marrow, where there is
intermixed fatty and hematopoetic marrow. The opposed-phase image
should be compared to an in-phase image, where individual fat and
water contributions are additive (e.g., TE = 4.2 msec at 1.5
Tesla). In order to produce T1 contrast on gradient-recalled echo
pulse sequences, a TR greater than 100 msec and a flip angle of 75°
to 90° typically are selected.
A large number of studies have established that opposed-phase
techniques are helpful for diagnosing a subset of adrenal
adenomas.8,9,13-19 Both quantitative and qualitative assessments of
the opposed-phase images have been performed. In quantitative image
analysis, the signal intensity of the adrenal mass and the signal
intensity of the spleen are measured on the in-phase and
opposed-phase image. Ideally, the sequence parameters (TR, field of
view, acquisition matrix, slice thickness, etc.) are identical
between the two sequences, except for the TE. A chemical-shift
ratio is computed and is defined as the signal intensity of the
adrenal mass on the opposed-phase image divided by the signal
intensity of the spleen on the opposed-phase image divided by the
signal intensity of the lesion on the in-phase image divided by the
signal intensity of the spleen on the in-phase image. A cutoff
value of 0.70 to 0.80 yields high specificity for the diagnosis of
Qualitative assessment of image contrast is highly accurate as
well. For qualitative signal intensity analysis, the relative
tissue contrast between the adrenal lesion is compared to spleen on
the in-phase image and compared to the spleen on the opposed-phase
image to judge whether there is any relative loss of signal
intensity on the opposed-phase image.18,20
The efficacy of opposed-phase MR imaging for distinguishing
adrenal adenomas from other masses is now well
established.11,17,18,20 Intracellular lipid usually is abundant in
adenomas and cortical nodules, but metastases and pheochromocytomas
lack such lipid (figure 2).9-11
Typical adrenal adenomas exhibit a signal intensity that is
similar or greater with respect to spleen or liver on in-phase
T1-weighted images.21 On opposed-phase images performed with
similar parameters, adrenal adenomas show a reversal of tissue
contrast with respect to spleen, demonstrating the presence of
lipid.11,13,14,17,18,20,22-24 The presence of MR-observable lipid
is highly predictive of a benign lesion, but it also is reported in
Adrenal metastases to the adrenal gland contain no such deposits
of intracellular lipid and, therefore, show no relative loss of
signal intensity on opposed-phase images compared to in-phase
images (figure 2). Theoretically, some types of metastases, such as
those from clear cell carcinomas of the kidney, may have some
intracellular lipid that is observable histologically. These types
of metastases, however, have not been reported to show loss of
signal intensity on opposed-phase images.
MRI vs CT
Many studies of noncontrast-enhanced CT support the use of CT
attenuation coefficients (Hounsfield units [HU]) for the diagnosis
of most adrenal adenomas.6,16,27-29 Controversy exists as to what
attenuation coefficient is the most accurate for discriminating
adrenal adenomas from metastasis, but the larger studies suggest a
cutoff value of 18 HU.27 More recent studies suggest that delayed
contrast-enhanced CT may prove equally efficacious in
discriminating adrenal adenomas from metastasis, although other
cutoff criteria must be used.30
Because CT and MRI both depend upon the presence of lipid in the
adrenal adenoma for a specific diagnosis, it is unclear whether CT
or MRI is more accurate for discriminating adrenal adenomas from
metastasis. Indeed, studies suggest that measurements of loss of
signal on an opposed-phase image and attenuation coefficients on CT
are highly correlated and are likely to misclassify the same subset
of lesions.31 That is, an adrenal adenoma that contains very little
or no lipid is likely to be misclassified as a possible malignant
lesion on both types of examinations.
For these reasons, no specific recommendations can be given as
to whether MRI or CT is more accurate for classification of adrenal
masses as benign or malignant.
Certain subsets of adrenal masses may give discrepant findings
on CT and MRI.31 For example, an adrenal cyst will show no evidence
of lipid on chemical-shift imaging; therefore, based on these
sequences alone, it may be misclassified as a possible malignant
lesion. In this situation, T2-weighted images are helpful for
correct classification of an adrenal cyst. CT will show low
attenuation to the cyst and correctly classify it as a benign
adrenal mass. On the other hand, hemorrhagic lesions may give
higher attenuation coefficients on CT but be classified correctly
on the basis of MR findings, by virtue of high signal intensity on
There are no specific MRI findings for pheochromocytoma. The
clinical situation, however, often is helpful in that a patient
with biochemical evidence of a pheochromocytoma and an adrenal mass
is highly likely to have a pheochromocytoma. They do not
demonstrate chemical shift evidence of lipid.
Suggestive MRI findings are very high signal intensity on
T2-weighted sequences (figure 3), a pseudocapsule around the
adrenal mass, and, frequently, the presence of internal
hemorrhage.15,32-36 Because of the high signal intensity on T2,
these lesions frequently are seen easily on MRI, and MRI may be the
procedure of choice for identifying them in the abdomen.32-38
Adrenal cortical carcinoma
Adrenal cortical carcinoma may be seen at any age. The clinical
situation usually suggests that a large mass might be an adrenal
cortical carcinoma. They rarely are detected at a small size. On
MRI, these masses have a characteristic appearance, with a high
signal intensity on T1-weighted images representing hemorrhage and
a very heterogeneous appearance on T2-weighted sequences due to
necrosis and hemorrhage (figure 4).26,39-41
MRI is particularly useful for evaluation of these masses
because it accurately can locate the epicenter of the mass in the
adrenal gland with coronal or sagittal images, establishing that
the mass does not arise from the pancreas, the liver, or the upper
pole of the kidney. Furthermore, MRI may detect the presence of
renal vein or IVC invasion. Adrenal cortical carcinoma may contain
areas of lipid that are observable on chemical-shift
Because of the large size of these masses with necrosis, they
rarely are confused with adenomas. The clinical significance of
MR-observable lipid in adrenal cortical carcinoma is unclear,
although undoubtedly it is related to the intracellular lipid
needed for adrenocortical steroid hormone production.
The primary function of MRI or CT in evaluating the incidental
adrenal mass is to diagnose correctly those adrenal cortical
nodules and adenomas that contain lipid. Adrenal masses that do not
have CT or MRI evidence of lipid may require biopsy for diagnosis
if the clinical context does not make the diagnosis clear.
Reasonable MRI and CT criteria to use in the evaluation of
incidental adrenal masses are a chemical-shift ratio of 0.70 to
0.80 or computed tomography Hounsfield units of less than 18 for a
diagnosis of adenomas.
As a group, adrenal adenomas may be distinguished from
metastasis by their T2-weighted signal intensity characteristics
and their dynamic gadolinium enhancement pattern. There are less
data to support using these measures in individual patients,
however, and they should not be relied on for diagnosis of a benign
Other adrenal masses, such as pheochromocytomas and adrenal
cortical carcinomas, have suggestive diagnostic signs on MR
imaging. There is no specific MRI appearance for these tumors,
however, and histologic confirmation is necessary. AR
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