The genitourinary system

Adrenals

 

Anatomy

Along with kidneys, the adrenal glands lie within the perinephric space (Fig. 117). In most patients, there is sufficient perinephric fat so they are easily seen on CT. The right adrenal glands lies anteromedial to the upper pole of right kidney and immediately posterior to the inferior vena cava. The left gland is anteromedial to the upper pole of the left kidney and posterior to portions of the splenic vein and artery. The right adrenal gland consistently extends above the upper pole of the kidney while the left is more often at the level of the left upper pole and extends to the renal hilus. Both adrenal glands have an inverted Y configuration with the tail of the Y pointing anteromedially. The adrenal glands weigh only about 5 g each and vary from 3 to 6 mm in width. The small size makes it difficult to distinguish between a normal or an atrophic or hypoplastic configuration.

The best imaging modality for the study of the adrenal glands is in most instances CT. In medullary disease scintigraphy is very useful especially for localization of aberrant tissue along the sympathetic chain in the retroperitoneum. It should be remembered that ultrasonography can not exclude adrenal pathology; it can only confirm its presence. MRI is also excellent for adrenal imaging and offers unique advantages (see below).

Pathology

Adrenal diseases are often divided into two major groups: functional (hyper and hypo) and non-functional diseases. Since adrenal hormones can be readily measured, there is seldom doubt as to which category a patient belongs.

Functional adrenal diseases
The adrenocortical functional diseases include Cushing's Syndrome, Conn's Syndrome, Adrenogenital Syndromes, Virilizing Syndromes, Feminizing Syndromes, and Adrenal Insufficiency. Paroxysmal hypertension is the classical functional disorder produced by overactivity of the adrenal medulla.

Cushing's syndrome
Cushing's syndrome is the manifestation of excess glucocorticoids. These steroids may come from either exogenous or endogenous sources. Endogenous Cushing's syndrome is due to overproduction of cortisol by the adrenal cortex. This can be due to an autonomous adrenal tumor, benign or malignant, or due to adrenal hyperplasia from unregulated ACTH production. The most common etiology of Cushing's syndrome is bilateral adrenal hyperplasia, which accounts for approximately 70% of cases. A few of these patients have macronodules, which can be demonstrated by CT. These macronodules measure less than 3 cm and may be less than 1 cm in diameter. Macronodular hyperplasia is due to an ACTH-secreting pituitary microadenoma in the majority of cases. A benign but autonomous adrenal cortical adenoma is the etiology of 20% of cases of endogenous Cushing's syndrome, and a primary adrenal cortical carcinoma

Figure 118. Adrenal tumor (aldosteronoma). Ultrasonography demonstrates a small echo poor tumor cranially to the right kidney in a patient with Conn 's syndrome.

is responsible for about 10% of cases.

Conn's syndrome
Conn's syndrome, or primary aldosteronism, is the result of excess aldosterone produced by the adrenal glands. As with Cushing's syndrome, it may be caused by either adrenal hyperplasia or a primary adrenal tumor. A benign but unregulated aldosterone secreting adenoma is the most common etiology of primary aldosteronism, being responsible for almost 80 % of cases, while hyperplasia is responsible for nearly all of the remainder (Fig. 118). Cortical carcinoma accounts for less than l % of cases. Aldosteronism also occurs in patients with renovascular hypertension. However, this form of secondary aldosteronism is distinguished from primary aldosteronism by measuring the serum renin which is low in Conn's syndrome.

Adrenogenital syndromes
Adrenogenital syndromes are the result of an inborn error in the adrenal enzyme which blocks or impairs the synthesis of cortisol or aldosterone and are manifest at birth. Androgen-producing (virilizing) tumors are rare, may be benign or malignant, and occur in either males or females at any age. Feminizing tumors are even less frequent.

Addison's disease
Adrenal insufficiency may result from inadequate stimulation by ACTH (secondary) or may be due to tissue destruction of the adrenal glands (primary). Primary adrenal insufficiency, or Addison's disease, occurs only after at least 90% of the adrenal cortex has been destroyed.

	Figure 119. Adrenal metastases from lung tumor. Large process cranial to the kidney with mixed echogenicity. Biopsy showed metastases from lung tumor.
	Figure 120. Adrenal cortical carcinoma displacing the right kidney centrally. Mixed enhancement after administration of contrast medium indicating central necrosis.

Idiopathic adrenal atrophy is the most common cause and is most likely an autoimmune disorder. Destruction can also be due to granulomatous disease, usually tuberculosis. However, other causes such as infarction, amyloidosis, hemochromatosis, hemorrhage, or destruction by histoplasmosis, blastomycosis, disseminated fungal infection, lymphoma, and metastatic tumor are possibilities. The radiographic manifestations of adrenal insufficiency depend on the cause of adrenal dysfunction. Calcifications caused by tuberculosis or histoplasmosis made be seen on plain abdominal radiographs, but the most useful examination is CT, since it not only demonstrate calcifications but also the size and shape of the adrenal glands.

Imaging in hypo- and hyperfunctioning adrenal cortical disease
Unless there is calcification or fat in the adrenal the plain radiograph and intravenous urography are usually unrewarding in patients with hyper or hypofunctioning adrenal cortical disease. However, the urogram can be very sensitive in distinguishing between a renal and an extrarenal mass. Although the normal adrenal gland may be difficult to image with ultrasonography, tumors larger than 2 cm can often be detected (Fig. 118). The right adrenal gland is usually easier to study because the liver provides a good acoustic window. Ultrasonography has two major uses in patients with hyper- or hypofunction: to distinguish large tumors (Fig. 119) from other organs due to the availability of the multiplanar imaging and to differentiate a small homogenous adenoma from an adrenal cyst. The single most valuable modality for the adrenal glands is CT (Fig. 120). The perinephric fat present in most patients allows the adrenal gland to be clearly displayed and tumors as small as 5 mm in diameter can be identified. lntravenous contrast is not usually needed but may be used to distinguish an adrenal mass from the upper pole of the kidney. For most patients with hyper- or hypofunction of the adrenal cortex, MRI does not provide additional information after a CT examination. Cortical imaging agents e.g. 131I-19-iodocholesterol have been developed that can localize hyperfunctioning adrenal cortical tumors, particularly in patients with Cushing's syndrome. However, the success of CT in localizing these adrenal tumors made this examination redundant.

Pheochromocytoma
Pheochromocytomas are tumors comprised of chromaffin cells and are usually located in the adrenal medulla. Extraadrenal pheochromo-cytoma (paragangliomas) may lie anywhere between the base of the brain and the epididymis but usually lie along the symphatic chain in the retroperitoneum. They are rare tumors and account for less than 1 % of patients with systemic hypertension. Classically the hypertension is paroxysmal, but it may be sustained and of any magnitude. Approximately 13 % of all pheochromoctymas are malignant. Pheochromocytomas may be associated with other endocrine tumors or with von Hippel-Lindau syndrome and neurofibromatosis. The multiple endocrine neoplasia (MEN) syndrome, Type 2A, includes medullary carcinoma of the thyroid and parathyroid hyperplasia as well as pheochromocytoma. The MEN syndrome, Type 2B, is comprised of pheochromocytoma, medullary carcinoma of the thyroid, and the mucocutaneous manifestations of mucosal neuromas, intestinal ganglioneuromatosis, and a marfanoid habitus. The majority of patients with MEN 2A or 2B syndromes have pheochromocytomas that are bilateral and almost

Figure 121. Malignant phaeochromocytoma. 123I MIBG scintigraphy. a) Normal. b) Large process at the site of the right adrenal gland. c) Lung metastases taking up the radiopharmaceutical. d) Metastases behind the bladder.

always intraadrenal. All manifestations of the syndrome may not occur at the same time. These syndromes are inherited in an autosomal dominant fashion. About 10% of pheochromocytomas are multiple.

As with tumors of the adrenal cortex, CT is the primary localizing examination. Most intraadrenal pheochromocytomas are detected, the sensitivity being around 95 %. Pheochromocytomas are both identified and characterized by MRI. T1-weighted sequences are used to identify the mass while the high signal intensity seen on T2-weighted images helps to characterize it as a pheochromocytoma. This intense T2-weighted signal helps to distinguish a pheochromocytoma from an adrenal adenoma. The analogue of guanethidine - MIBG - is taken up by adrenergic tissue. Labelled with a radionuclide it can localize pheochromocytomas. 1311 is the most frequently used isotope, but 123I has a better detection efficiency and dosimetry. The overall accuracy of MRI, CT and scintigraphy seems to be the same, but scintigraphy has some advantages. With a single injection of the radiopharmaceutical the whole body can be scanned. This is particularly helpful for ectopic or multiple tumors or in the detection of metastatic deposits (Fig. 121). Ultrasonography may be used to localize an intraadrenal pheochromocytoma, but may also identify ectopic tumors lying in the paraaortic area. Pheochromocytomas tend to be more echogenic than normal adrenal tissue. All patients with

Figure 122. lncidentaloma. Small low attenuating process in the left adrenal gland found incidentally in an asymptomatic patient. This probably represents an adenoma.

pheochromocytoma should have either an MIBG scan or MRI prior to surgery to rule out accessory lesions.

Non-functional adrenal diseases
The non-functional adrenal diseases include adenoma, carcinoma, myelolipoma, hemorrhage, cysts, hemangioma, metastases, and lymphoma.

Adenoma
Benign, nonhyperfunctioning adrenal adenomas are commonly encountered on abdominal CT examinations (Fig. 122). Typically it is a well defined, rounded, homogenous mass; calcification, central necrosis or hemorrhage occurs rarely. MRI may be used to distinguish the adenomas from metastases since metastases typically have a higher signal on T2weighted images. Most adenomas contain lipid and can be identified my MRI fat-detection technique. Large adenomas can be detected by ultrasonography, which then is useful to distinguishing solid tumors from cysts. Sometimes smaller lesions can be found especially if associated with endocrinopathy.

Carcinoma
Primary adrenal cortical carcinoma in an uncommon malignancy with a median age at presentation in the fifth decade. The typical CT -appearance is a large mass with central areas of low attenuation representing tumor necrosis (Fig. 120). Often calcifications are seen. Evidence of hepatic or regional lymph node metastases as well as venous extension are

Figure 123. Adrenal cortical carcinoma. T1weighted image demonstrating a moderately signal intensive process cranial to the right kidney.

well detected by CT. A high signal on T2-weighted MRI supports the malignant diagnosis. The tumors are low or moderate signal on T1-weighted images (Fig. 123). At ultrasonography a suprarenal mass is found; smaller lesions are often homogeneous, whereas larger lesion are inhomogenous representing necrosis and/or hemorrhage. Renal displacement without calyceal distortion at intravenous urography is indicative of a suprarenal mass.

Myelolipoma
A myelolipoma is a hamartoma comprised of mature adipose cells and haematopoietic tissue. CT is the most definitive radiographic examination. A fatty adrenal mass is virtually diagnostic of a myelolipoma. MRI does not add anything. Ultrasonography will show a highly echogenic mass. Small islands of calcium or even bone formation are sometimes present.

Hemorrhage
Adrenal hemorrhage may be spontaneous, traumatic, or related to anticoagulation. Spontaneous hemorrhage often occurs in patients with septicemia, hypertension, renal vein thrombosis, or adrenal pathology such as a tumor. Again CT is the most reliable method of diagnosis. Initially the hematoma has a high attenuation. Follow-up studies usually show resorption of the hematoma and a gradual decrease in density to near water. Also MRI appearance will reflect the evolution from acute to chronic stages with hemoglobin breakdown; initially high signal at both T1- and T2-weighted sequences. If detected, the echogenicity at ultrasonography varies with the state of hematoma.

Cysts
Adrenal cysts are uncommon and their detection does not differ from diagnosis of cysts in other organs. Some adrenal cysts are probably the residua of old hemorrhage ("pseudocysts").

Hemangioma
Adrenal hemangioma is a rare tumor of the adrenal cortex and does not differ from hemangiomas of other organs. The CT -appearance depends on tumor morphology. Typically, a large mass with a thick irregular wall and hypodense centre is seen. Ultrasound demonstrates a complex mass and may reveal cystic areas.

Metastases
The adrenal glands are a common site of metastatic disease. Of patients with an epithelial malignancy 27% will have adrenal metastases with time. Figures for patients with breast cancer and lung cancer are 54% and 36%, respectively. The radiographic appearance of adrenal metastases is unspecific. They may be small, unilateral or bilateral. A metastases is a solid mass on ultrasound, and when less than 3 cm in diameter, is usually homogenous, whereas in larger lesions central necrosis and hemorrhage may occur. CT features suggesting malignancy includes a large size (> 3 cm), poorly defined margins, invasion of adjacent structures, inhomogenous attenuation, and a thick irregular enhancing rim. Evidence of widespread metastatic disease makes the diagnosis easier. MRI demonstrating a high signal on the T2-weighted images is also helpful. However, the definitive method is still percutaneous aspiration biopsy.

Lymphoma
Involvement of the adrenal gland by malignant lymphoma is more common with non-Hodgkin lymphoma than Hodgkin's disease. The adrenal glands are seldom an isolated site of disease, although other involvement may be distant. Adrenal lymphoma can be diagnosed by ultrasound, CT and MRI. On US, lymphoma appears as a well-defined, relatively echogenic homogeneous tissue mass. However, CT provides the best morphologic delineation. The adrenal glands are enlarged with either
rounded mass or more symmetric enlargement, preserving the basic glandular configuration.

Intervention

The most frequent interventional procedure on the adrenals is percutaneoues aspiration biopsy. Cyst aspiration is rarely performed. The most worrisome complication is precipitation of a hypertensive crisis by a pheochromocytoma.

The arterial supply to the adrenal glands comes from many feeding arteries. In contrast each adrenal gland is drained by a single vein. This fact makes the adrenal glands suitable for venous sampling.

 

Henrik S. Thomsen and Howard M. Pollack