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TERMINOLOGY

Abbreviations
- Adrenal corticotrophic hormone (ACTH)

GROSS ANATOMY

Overview
- Adrenal (suprarenal) glands are part of endocrine and neurological systems - Essentially different organs within same structure, composed of thick outer cortex and thin inner medulla - Lie withinperirenal spacebilaterally, bounded byrenal (perirenal)fascia, above/medial to kidneys - Composed of "body" and 2 limbs (medial and lateral)
Anatomic Relationships
- Right adrenal is usually more apical in location - Lies anterolateral to right crus of diaphragm, medial to liver, and posterior to inferior vena cava (IVC) - Often pyramidal in shape with inverted V shape on transverse section
- Left adrenal is usually more caudal and lies medial to upper pole of left kidney, lateral to left crus of diaphragm, and posterior to splenic vein and pancreas - Often crescentic in shape with λ or triangular shape on transverse section
Divisions
- Adrenal cortex - Embryologically derived from mesoderm - Divided into 3 distinct zones (zona glomerulosa, zona fasciculata, and zona reticularis) - Secretes mineralocorticoids(aldosterone) from zona glomerulosa, glucocorticoids(cortisol) from zona fasciculata, and androgensfrom zona reticularis
- Adrenal medulla - Embryologically derived from neural crest - Part of sympathetic nervous system - Chromaffin cells secrete catecholamines (mostly epinephrine) into bloodstream
- Vessels,nerves, and lymphatics - Arteries - Superior adrenal arteries: Typically 6-8; from inferior phrenic arteries - Middle adrenal artery: 1; from abdominal aorta - Inferior adrenal artery: 1; from renal arteries - Veins - Right adrenal vein drains into IVC - Left adrenal vein drains into left renal vein (usually after joining left inferior phrenic vein) - Nerves - Extensive sympathetic connection to adrenal medulla - Presynaptic sympathetic fibers from paravertebral ganglia end directly on secretory cells of medulla - Lymphatics - Drain to lumbar (aortic and caval) nodes

ANATOMY IMAGING ISSUES

Multimodality Imaging Appearance
- No consensus on "normal" size or thickness of adrenals but average thickness of ~ 3 mm for medial/lateral limbs - While not based on any strong evidence, > 10-mm thickness can be used as threshold for hyperplasia
- MR: Generally isointense to liver on T1 MR and isointense to slightly hyperintense to liver on T2 MR
- Ultrasound: Easiest to visualize in newborns (as result of physiologic enlargement) and become progressively more difficult to visualize with age - Right adrenal gland easier to visualize than left (due to lack of liver as acoustic window and overlying bowel gas) - Adrenal glands in adults usually hypoechoic (juxtaposed against hyperechoic periadrenal fat), although medulla can rarely be discretely seen and appears hyperechoic
Key Concepts
- Adrenal (cortical) adenomas - Very common (at least 2% of general population) but usually cause no symptoms - Mostly "nonfunctioning" but identical to "functional" adenomas that cause Cushing/Conn syndrome - Most adenomas contain abundant lipid (precursor to steroid hormones), allowing definitive diagnosis using CT/MR sequences that highlight lipid - Lipid is intracellular/intercellular (not macroscopic deposits of fat) - Best CT technique: Nonenhanced CT with nodule measuring < 10 HU; or multiphase-enhanced CT with nodule demonstrating "washout" kinetics - Best MR technique: Chemical-shift MR with signal dropout within nodule on opposed-phase images - Standard imaging features for diagnosis of adenoma should be used for nodules measuring < 4 cm, while lesions > 4 cm should raise concern for malignancy
- Pheochromocytoma (tumor of adrenal medulla) - Signs: Headache, palpitations, excessive perspiration - 90% arise in adrenal, 90% unilateral, 90% benign - Similar tumor arising in other chromaffin cells of sympathetic ganglia is called paraganglioma - More common with multiple endocrine neoplasia, neurofibromatosis, and von Hippel-Lindau - Often markedly hypervascular in arterial phase
- Adrenal myelolipoma - Uncommon benign tumor (usually incidental finding) composed of mature adipose and hematopoietic tissue - Characterized by presence of macroscopic fat - May have internal soft tissue component or calcification
- Adrenocortical carcinoma - Highly aggressive malignancy with poor prognosis - Large, heterogeneous mass (often with necrosis, hemorrhage, or calcification) with frequent local invasion, vascular invasion, and distant metastases
- Cushing syndrome (excess cortisol) - Signs: Truncal obesity, hirsutism, hypertension - Causes: Pituitary tumors (→ adrenal corticotrophic hormone), exogenous (medications) > adrenal adenoma > carcinoma
- Conn syndrome (excess aldosterone) - Signs: Hypertension, hypokalemic alkalosis - Causes: Adrenal adenomas > hyperplasia > carcinoma
- Addison syndrome(adrenal insufficiency) - Signs: Hypotension, weight loss, altered pigmentation - Causes: Autoimmune disease > adrenal metastases > adrenal hemorrhage > adrenal infection

CLINICAL IMPLICATIONS

Clinical Importance
- Rich adrenal blood supply due to endocrine function - Results in adrenal glands being common site for hematologic metastases (lung, breast, melanoma, etc.)
- Adrenal glands respond to stress (trauma, sepsis, surgery, etc.) by secreting ↑ cortisol and epinephrine - Overwhelming stress may result in adrenal hemorrhageor acute adrenal insufficiency (addisonian crisis)

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References

Selected References

Images

Adrenal Vessels and Relations

The adrenal glands rest atop the kidneys with an interposed layer of fat. Reflecting their critical role in maintaining homeostasis and responding to stress, the adrenal glands have a very rich vascular supply. The superior adrenal arteries are short branches of the inferior phrenic arteries bilaterally. The middle adrenal arteries are short vessels arising from the aorta. The inferior adrenal arteries are branches of the renal arteries. The left adrenal vein drains into the left renal vein, while the right adrenal vein drains directly into the inferior vena cava (IVC). Note the size of the adrenal glands is somewhat exaggerated in this graphic to facilitate demonstration of the vascular anatomy.

Adrenal Axial Anatomy and Relations

The right adrenal is often more cephalic in location and lies above the right kidney, while the left adrenal lies partly in front of the upper pole of the left kidney. The left adrenal lies directly posterior to the splenic vein and body of pancreas and lateral to the left crus of the diaphragm. The right adrenal lies lateral to the crus, medial to the liver, and directly behind the IVC.

The adrenal gland is essentially 2 organs in a single structure. The cortex is an endocrine gland, secreting primarily cortisol, aldosterone, and androgenic steroids. All of these hormones are derived from cholesterol, which imparts the characteristic lipid-rich appearance and imaging characteristics of the gland. The adrenal medulla is part of the autonomic nervous system and secretes epinephrine and norepinephrine.

Axial CT, Normal Adrenal Anatomy

First of 3 axial CECT images shows normal adrenal glands bilaterally. The right adrenal is usually suprarenal, touches the back of the IVC, and lies lateral to the right crus and medial to the liver. The left adrenal usually lies ventral to the upper pole of the left kidney and behind the splenic vein. The left adrenal often appears as an inverted Y shape, while the right is more like an inverted V.

Both limbs of the right adrenal are seen on this section.

The lowest portions of the adrenals are seen on this section.

Coronal CT, Normal Adrenal Anatomy

First of 2 coronal CECT images shows the adrenal glands in their suprarenal location, which accounts for their alternate name of "suprarenal glands."

More anterior coronal CT shows the adrenal glands and their relation to adjacent structures, such as the crura of the diaphragm.

Axial and Coronal, Normal Adrenal Anatomy

First of 3 CT images of a subject with normal adrenal glands shows conventional anatomy.

More caudal section shows both adrenal glands with an inverted Y appearance. There is mild thickening of the left adrenal gland at the confluence of the medial and lateral limbs, a normal finding.

Coronal view demonstrates the relationship between the adrenals and adjacent organs.

Axial MR, Normal Adrenal Anatomy

First of 2 axial T1 C+ MR images demonstrates the bilateral adrenal glands. At this level, the 2 limbs of each adrenal gland are not yet discretely visible.

Axial T1 C+ MR at a slightly more caudal level now brings into view the twin limbs of each of the 2 adrenal glands. Each limb is thin and regular (without thickening or a nodule). Note that the right adrenal gland contacts the posterior margin of the IVC and is medial to the right diaphragmatic crus. The left adrenal gland is located slightly ventral and above the left kidney.

Axial T2 HASTE MR demonstrates a normal appearance of the adrenal glands on these images acquired without fat saturation. The adrenal glands in this image appear relatively isointense to the liver (although the relative signal can vary depending on application of fat saturation).

Adrenal Venogram

First of 2 images shows selective catheterization of the adrenal veins in a young woman with hyperaldosteronism, but no definite mass is seen on CT. Selective adrenal vein sampling was requested to assess unilateral excess aldosterone secretion. A catheter has been inserted through the right femoral vein, and the tip was advanced into the opening of the right adrenal vein. The adrenal veins are very fragile and could be easily ruptured by a forceful injection of contrast medium. The angiographer must know the vascular anatomy and gently probe the venous orifice, confirming the location with a small bolus injection of contrast medium, as shown here.

A subsequent image shows the catheter repositioned. The tip has been advanced through the left renal vein to enter the left adrenal vein. No attempt is made to opacify the smaller venous tributaries.

Fetal Adrenal and Kidney

Graphic shows the appearance of the adrenal and kidney in the fetus and neonate. The adrenal is much larger, relative to the kidney, than in an adult. The kidney has a lobulated appearance reflecting the ongoing fusion of the individual renal lobes, each composed of 1 renal pyramid and its associated renal cortex.

Neonatal Adrenals and Kidney

First of 3 ultrasounds of a neonate shows the characteristic prominence of the adrenal gland and the lobation of the renal surface in early infancy. This sagittal image shows the large adrenal gland adjacent to the upper pole of the kidney.

Sagittal ultrasound shows the prominent limbs of the right adrenal gland.

Sagittal ultrasound of the kidney shows its lobulated contour, a normal finding in the fetus and neonate.

Adrenal Adenoma CT

Axial NECT demonstrates a low-density nodule within the left adrenal gland with a Hounsfield attenuation of 14 HU, compatible with an adrenal adenoma. Patients with adenomas may be symptomatic (e.g., signs of excess cortisol or aldosterone) or asymptomatic. Most subjects have the adrenal lesion discovered incidentally on a CT performed for some other reason and have no clinical symptoms or signs. In this setting, the adenoma is said to be nonfunctional.

Axial CECT demonstrates a low-density left adrenal nodule that demonstrated a noncontrast attenuation of 3, an absolute washout of 77.3%, and a relative washout percentage of 74.4%, features compatible with a benign adrenal adenoma (calculated based on a formal multiphase adrenal protocol CT).

Axial NECT demonstrates a low-density, lipid-rich adenoma with Hounsfield attenuation of 6-8 HU, characteristic of a lipid-rich adenoma. Adenomas can be either lipid rich (common between 70-90%) or lipid poor. Lipid-rich adenomas, due to their high lipid content, are of low density on NECT.

Adrenal Adenoma MR

Axial GRE in-phase MR demonstrates a homogeneous rounded left suprarenal mass.

Axial GRE opposed-phase MR through the same level shows marked loss of signal within the adrenal mass, confirming the presence of intravoxel/intracellular lipid. This finding is diagnostic of an adrenal adenoma.

Axial T2 FS MR demonstrates that the adrenal adenoma is relatively low in signal, similar in signal to the normal-appearing left adrenal gland. Note that this is unlike most malignant adrenal masses, which tend to be T2 hyperintense.

Adrenal Hyperplasia

First of 3 axial CECT images of a 40-year-old woman with congenital adrenal hyperplasia shows diffuse enlargement of both adrenal glands but preservation of their normal shape.

Each limb of the adrenal is in excess of 1 cm in diameter, one criterion used to diagnose or suggest adrenal hyperplasia. Most patients with adrenal hyperplasia have less markedly enlarged glands due to pituitary (or ectopic) production of excess adrenal corticotrophic hormone. In many cases the adrenal glands may appear normal by imaging.

The striking enlargement of the adrenals is again evident on this image.

Adrenal Myelolipoma

Axial NECT demonstrates a fat-density mass arising from the left adrenal gland. The presence of macroscopic fat within an adrenal mass is virtually diagnostic of an adrenal myelolipoma.

Transverse ultrasound demonstrates a left adrenal mass, which is markedly echogenic and without internal color flow vascularity, representing a myelolipoma.

Axial CECT demonstrates a left adrenal mass with internal solid enhancing components, but with clear evidence of macroscopic fat, compatible with a myelolipoma. Myelolipomas can demonstrate enhancing components and calcification, but the presence of macroscopic fat is essentially diagnostic of this entity.

Pheochromocytoma

Coronal T2 FS MR demonstrates a T2-hyperintense mass arising from the left adrenal gland in a patient with symptoms of malignant hypertension. Pheochromocytomas are classically said to be light bulb bright on T2 MR. While not true in every case, the majority of pheochromocytomas are T2 hyperintense.

Coronal arterial-phase CECT demonstrates a markedly hypervascular right adrenal mass, an appearance that is very suggestive of a pheochromocytoma. These lesions tend to show marked enhancement on arterial-phase imaging, making multiphase imaging (either CT or MR) critical in their imaging diagnosis.

Axial CECT demonstrates an avidly enhancing mass with subtle central necrosis arising from the right adrenal gland, compatible with a pheochromocytoma, in this patient with malignant hypertension and palpitations.

Axial CECT in the arterial phase demonstrates a right adrenal mass, which is markedly hypervascular around its margins and centrally necrotic. The presence of markedly hypervascular soft tissue in an adrenal mass should strongly suggest the diagnosis of pheochromocytoma. This mass was surgically resected and confirmed to represent a pheochromocytoma.

Axial T1 C+ MR demonstrates a large right adrenal mass with a sizable central necrotic component and an avidly enhancing peripheral solid component, found to represent a pheochromocytoma upon surgical resection.

Axial arterial-phase CECT demonstrates a small nodule in the right adrenal gland, which is markedly hypervascular with a Hounsfield attenuation of 150 HU. Adrenal nodules with attenuation values > 110 HU in the arterial phase are highly likely to represent pheochromocytomas, confirmed in this case at surgery.

Adrenal Metastases

Axial CECT demonstrates large, vascular bilateral adrenal masses, compatible with metastases from the patient's primary hepatocellular carcinoma (a portion of which is visible in the liver on this image). As in this case, metastases to the adrenal gland often demonstrate very similar enhancement and texture to the primary tumor site.

Axial CECT demonstrates a large, hypodense, heterogeneous mass arising from the right adrenal gland, representing a metastasis from the patient's primary lung cancer. Lung, breast, and renal cell carcinoma, along with malignant melanoma, frequently metastasize to the adrenal glands due to the rich blood supply of the adrenals.

Axial T C+ MR demonstrates a small hypervascular mass in the right adrenal gland, compatible with a metastasis from the patient's primary clear cell renal cell carcinoma.

Adrenocortical Carcinoma

Axial T2 FS MR demonstrates a large, infiltrative, aggressive-appearing mass arising in the right suprarenal fossa. Any mass with this size and appearance, in the absence of a history of primary malignancy elsewhere, should raise strong suspicion for adrenocortical carcinoma.

Axial CECT demonstrates a large, heterogeneous mass with central necrosis arising in the left upper quadrant. As with many cases of adrenocortical carcinoma, these masses can be quite large, and the site of origin for the mass may be difficult to determine.

Coronal CECT demonstrates a large mass, which is clearly separate from the left kidney and arising from the left suprarenal fossa (ostensibly from the left adrenal gland). Any mass of this size arising in this location is strongly suspicious for adrenocortical carcinoma, a diagnosis that was confirmed at surgery.

Coronal CECT demonstrates a large, heterogeneous, enhancing mass arising from the right suprarenal fossa with tumor directly extending into (and distending) the IVC. Adrenocortical carcinoma commonly invades surrounding venous structures, including the renal vein or IVC.

Axial CECT demonstrates a large right suprarenal mass with direct extension to involve the left kidney as well as extensive tumor thrombus involving the left renal vein. This was ultimately confirmed to represent an adrenocortical carcinoma at resection.

Coronal CECT demonstrates a right-sided adrenocortical carcinoma that directly invades the right hepatic lobe. Note the presence of extensive calcification within the mass, a common feature with these tumors.

Adrenal Hemorrhage

Coronal NECT in a patient who was hypotensive due to blunt abdominal trauma demonstrates enlargement of the left adrenal gland with heterogeneous, high-density material, characteristic of acute hemorrhage. This "nodule" was not present on a prior CT from a few months earlier, aiding in the diagnosis.

Axial NECT in a septic, hypotensive patient in the ICU setting demonstrates a large left adrenal hemorrhage. Notably, a study performed a few days prior had shown a completely normal left adrenal gland.

Axial CECT in a patient who had recently undergone surgery for ovarian cancer demonstrates a new intermediate-density nodule arising from the right adrenal gland. This nodule was new since a recent prior study, compatible with a small adrenal hemorrhage.

Gastric Diverticulum Simulating Adrenal Mass

Axial CECT shows a cystic-appearing "lesion" in the left suprarenal region, simulating an adrenal mass. The mass has the same density as the water-filled stomach, but note the presence of internal gas.

Sagittal CECT in the same patient nicely shows that this "lesion" is actually a gastric diverticulum directly communicating with the adjacent stomach. Gastric diverticula, which often arise in this location, are not infrequently mistaken for adrenal nodules.

An oblique film from a barium upper GI series shows a barium-filled gastric diverticulum projecting posteriorly from the gastric cardia.

Adrenal Insufficiency

First of 3 axial CT sections in a patient with adrenal insufficiency (addisonian syndrome) due to autoimmune disease is shown. The adrenal glands are extremely small and difficult to visualize.

The small adrenals are again evident. If adrenal insufficiency were due to adrenal tumors, bleeding, or infection, the glands would be enlarged.

The adrenal glands have a normal shape and morphology but are extremely small.

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