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Adrenal gland enlargement and nodules


Anatomy, embryology, pathophysiology

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    The adrenal glands are multifunctioning, inverted Y-shaped, retroperitoneal endocrine glands normally located superior to the kidneys in the perirenal space ( Fig. 28.1 ).

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      The adrenal glands mediate the stress response by releasing cortisol and are involved in secondary sex hormone synthesis and blood pressure regulation.

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      Absence of the Y-shape indicates development in the absence of an orthotopic kidney (i.e., agenesis or ectopia).

    Fig. 28.1, Normal adrenal glands. The axial T2-weighted image (A) shows normal bilateral adrenal glands ( arrows ) as thin, linear, hypointense Y-shaped structures. The corresponding coronal T2-weighted image (B) shows the normal position of the adrenal glands surrounded by retroperitoneal fat and their relationship to the kidneys. The out-of-phase (C) and in-phase (D) images demonstrate some out-of-phase signal loss as a result of the presence of fat-containing enzymes and enzymatic precursors. The T1-weighted, fat-suppressed, postcontrast image (E) shows normal avid early adrenal enhancement.

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    The histological zonal architecture of the adrenal gland is divided into two distinct components: the cortex, which is derived from mesothelium, and the medulla, which is derived from neural crest cells.

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      The adrenal cortex is composed of the zona glomerulosa, zona fasciculata, and zona reticularis.

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        The superficial zona glomerulosa is the primary site of aldosterone production.

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        The middle zona fasciculata synthesizes glucocorticoids (cortisol and corticosterone).

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        The inner zona reticularis produces androgens.

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        Adrenal cortical masses include adrenal adenoma, adrenal cortical carcinoma, adrenal myelolipoma, adrenal cysts, and metastasis.

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        Adrenal hemorrhage can involve the cortex.

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      The adrenal medulla is in the centermost portion of the gland and is responsible for catecholamine production.

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        Adrenal medullary masses include pheochromocytoma, neuroblastoma, ganglioneuroma, or ganglioneuroblastoma.

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    Arterial blood supply is via the superior adrenal artery (via phrenic artery), middle adrenal artery (via abdominal aorta), and inferior adrenal artery (via renal artery).

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    The left adrenal vein drains to the left renal vein and the right adrenal vein drains to the inferior vena cava (IVC).

Techniques

Computed tomography

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    Primary modality for evaluating adrenal adenoma

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    Adrenal glands enhance after iodinated contrast administration to approximately 50 to 60 Hounsfield units (HU).

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    Both unenhanced computed tomography (CT) and dynamic contrast-enhanced CT have a role.

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    Benign adenomas and malignant masses show rapid contrast enhancement, but adenomas demonstrate rapid washout of contrast.

Magnetic resonance imaging

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    Alternative to CT when contrast-enhanced CT is contraindicated.

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    Normal adrenal has low to intermediate signal on both T1- and T2-weighted imaging.

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    Chemical shift imaging with T1-weighted gradient echo (GRE) sequences can be used to evaluate for microscopic fat in nodules (see Physics Pearls box).

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    Frequency selective fat saturation can be used to determine presence of macroscopic fat in nodules.

Ultrasonography

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    Normal adult adrenal glands are not seen on ultrasound. Incidental masses near the superior renal poles may be adrenal in origin.

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    Can be used in pediatric evaluation.

Nuclear medicine

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    Malignant adrenal lesions typically demonstrate 18-Fluoride deoxyglucose (18-FDG) uptake exceeding that of normal liver.

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    Whole body 123-Iodine scintigraphy with metaiodobenzylguanidine (MIBG) compounds can detect functional lesions and is superior to other modalities in the detection of extraadrenal lesions.

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    For patients with suspected pheochromocytoma (i.e., elevated urine catecholamine breakdown products) and negative MIBG scintigraphy, FDG-positron emission tomography (PET) or PET/CT may be useful.

Protocols

Suggested computed tomography protocols

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    Unenhanced CT.

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      An adrenal nodule measuring less than 10 HU can be reliably diagnosed as a benign adenoma (98% specific), unless greater than 4 cm ( Fig. 28.2 ).

      Fig. 28.2, An incidentally discovered left adrenal nodule has measured attenuation values of less than 10 Hounsfield units on unenhanced imaging. This is consistent with a lipid-rich adenoma.

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    Adrenal washout CT.

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      Absolute washout protocol: noncontrast, venous phase, and delayed phase (15 minutes) imaging.

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      Absolute Washout = HU v - HU d HU v - HU n       , where HU n , HU v , and HU d are the HU of an region of interest (ROI) in the nodule on noncontrast, venous phase, and delayed phase imaging, respectively.

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      Absolute washout over 60% is diagnostic of benign adenoma, unless greater than 4 cm in size ( Fig. 28.3 ).

      Fig. 28.3, Contrast material washout in an adrenal adenoma. A, Unenhanced computed tomography (CT) scan shows a homogeneous low-attenuation adrenal nodule with attenuation of 6 Hounsfield units (HU) (typical of a lipid-rich adenoma). B, On the portal venous phase of contrast enhancement, the nodule enhances with attenuation values of 86 HU. C, On 15-minute-delayed CT, the mass has decreased enhancement with attenuation of 32 HU. The contrast enhancement during the portal venous phase washed out by 68% by the time of the delayed scans, confirming that this is an adenoma. Absolute washout of 60% or greater is diagnostic of an adenoma.

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      Determination of benignity after noncontrast acquisition may negate need to complete postcontrast series.

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    Relative washout CT.

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      Relative Washout = HU v - HU d HU v       .

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      Relative washout over 40% is diagnostic of benign adenoma, unless greater than 4 cm in size.

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      This protocol can be performed if there is concern for radiation exposure.

Suggested magnetic resonance imaging protocols

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    Chemical shift imaging protocol.

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      Uses in-phase and opposed-phase T1-weighted GRE.

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      Adenomas will demonstrate signal dropout from microscopic fat.

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      Other lesions containing microscopic fat (e.g., well-differentiated adrenocortical carcinoma, clear cell renal cell carcinoma, and hepatocellular carcinoma) may also demonstrate signal dropout on opposed phase imaging.

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    Fat suppression can be used to demonstrate macroscopic fat as seen in myelolipoma.

Specific disease processes

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