Материал: Advanced Imaging of the Abdomen - Jovitas Skucas

SPLEEN

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Adrenals

Technique

Computed Tomography

The current imaging modality of choice to evaluate the adrenal glands is computed tomography (CT), although magnetic resonance (MR) is making strong inroads. Both enlargement and a focal tumor can be detected, assuming it is large enough. Some adrenal abnormalities have a nonspecific CT appearance, although different enhancement patterns and fat content allow differentiation between some benign and malignant lesions.

It is not uncommon to detect adrenal enlargement as an incidental finding when CT is performed for other indications. Once an abnormality is detected, the following specialized techniques are useful for further evaluation: 6b-iodomethyl-19-norcholesterol (NP-59) scintigraphy, unenhanced CT densitometry, opposed phase chemical shift magnetic resonance imaging (MRI), and percutaneous biopsy.

Ultrasonography

Transabdominal US can almost always visualize a normal right adrenal gland, but the left gland is seen in only about two thirds of examinations; endoscopic US, on the other hand, almost always detects the left adrenal gland but

identifies the right gland in only a minority of patients.

Magnetic Resonance

Compared to CT and US, MRI has better soft tissue contrast depiction. A disadvantage is its relative insensitivity for visualizing adrenal calcifications. Normal adrenal glands have an intensity similar to that of liver parenchyma on T1-weighted images. They are not as well seen on T2-weighted images, although fat suppression accentuates their signal intensity compared to the adjacent fat, and this technique is commonly used.

Scintigraphy

Scintigraphy has a distinct role in evaluating functional adrenal tissue. The radiopharmaceutical iodine-131–NP-59 is taken up by normally functioning adrenal cells and evaluates cortical function. Increased adrenal uptake of NP-59 occurs in Cushing’s syndrome, aldosteronism, and some adrenal tumors. This is a complex procedure; uptake lasts several days and imaging is thus delayed. The delay also decreases background activity. Planar scintigraphy and single photon emission computed tomography (SPECT) can be combined in an attempt to increase study sensitivity. NP-59

appears to be cost-effective for evaluating adrenal incidentalomas.

Selenium-75–methylnorcholesterol scintigraphy is similar to NP-59. Cortical adenomas have either normal or increased uptake, while malignancies show decreased activity. Uptake of this radiopharmaceutical shows a direct relationship with the functional state of hyperfunctioning adenomas.

Iodine-131–metaiodobenzylguanidine (MIBG) is a norepinephrine analogue, and a normal adrenal does not accumulate large amounts of this tracer. It shares some of the norepinephrine pathways and is of use in detecting pheochromocytomas, neuroblastomas, and carcinoids. In some hyperfunctioning endocrine conditions both anatomic and functional information are obtained. Thyroid uptake of iodine 131 is blocked with potassium iodide prior to and after the use of this agent.

Indium-111-octreotide binds to somatostatin receptors throughout the body. These receptors are found in neuroendocrine and other structures, including tumors originating from these structures. Increased somatostatin receptors and thus increased octreotide concentrations occur in carcinoids, pheochromocytomas, and neuroblastomas.

Biopsy

Imaging provides guidance for needle biopsy of adrenal tumors. Most biopsies utilize a posterior approach, although anterior, transhepatic, transpancreatic, and transsplenic approaches have been used. A wider artificial window can be obtained by injecting saline into the adjacent paravertebral space and thus displacing the pleura laterally (1); this allows a wider path and potentially safer adrenal access by avoiding puncture of pleura and diaphragm. A CTguided approach is often used, although an open MR scanner and MR fluoroscopy using steady-state free precession sequences, if available, provide considerable advantages (2); MR fluoroscopy permits an oblique paravertebral approach without pleural transgression. A sensitivity of >90% can be obtained in detecting a malignancy. Although most biopsies provide a specific diagnosis, a malignancy obviously can be missed; a biopsy of a benign lesion, such as an adenoma, does not exclude the concomitant presence of a carcinoma.

ADVANCED IMAGING OF THE ABDOMEN

Complications encountered include pneumothorax, perinephric hemorrhage, hepatic hematoma, and needle-track metastases. Adrenal hematomas can also be induced.

Congenital

Bilateral adrenal agenesis is incompatible with life. In unilateral agenesis the contralateral gland hypertrophies. The rare infant with congenital adrenal hypoplasia requires replacement therapy for survival.

Accessory adrenal rests are usually of little significance. An intratesticular location is not uncommon. Most accessory glands contain only cortical tissue, while a heterotopic gland contains both cortex and medulla.

A horseshoe-shaped adrenal gland was reported in an infant with asplenia (3).

Congenital Adrenal Hyperplasia

Congenital adrenal hyperplasia is an autoso- mal-recessive condition leading to impaired hormone synthesis. A number of such adrenogenital syndromes have been described; they are based on specific hormone synthesis impairment, with 21-hydroxylase deficiency being the most common. Some infants with congenital adrenal hyperplasia also do not synthesize aldosterone and have salt wasting, a potentially fatal condition.

Clinically, girls and women develop virilization and boys have precocious puberty, but keep in mind that similar findings also occur with virilizing tumors.

Imaging of congenital adrenal hyperplasia reveals large adrenals that are cerebriform in outline. Adrenal rest tissue in other locations also enlarges. This condition should be suspected in an infant with enlarged adrenal glands, although not all infants with congenital adrenal hyperplasia have gland enlargement.

Acquired adrenogenital syndrome is most often due to an adenoma, and less often to an adrenocortical carcinoma.

Wolman’s Disease

Wolman’s disease is an autosomal-recessive condition caused by a deficiency of lysosomal

ADRENALS

acid lipase. It is characterized by abnormal storage of cholesteryl esters and triglycerides. Onset is after the first month of life and manifests as hepatosplenomegaly, abdominal distention, and failure to thrive. It is usually fatal in infancy.

Computed tomography in these infants shows hepatosplenomegaly, with the liver being hypodense, and bilateral adrenal calcifications. These imaging findings, in the appropriate clinical setting, should suggest the diagnosis.

Beckwith-Wiedemann Syndrome

Beckwith-Wiedemann syndrome is a congenital overgrowth syndrome having a sporadic occurrence and variable expressivity. These children develop cysts and various childhood solid tumors, including adrenocortical carcinomas. The kidneys are more often affected than the adrenals. Bilateral multilocular cystic adrenal tumors developed in a neonate with BeckwithWiedemann syndrome (4); resection revealed benign hemorrhagic macrocysts. Unilateral cysts are more common.

Trauma

Trauma to the adrenal and surrounding structures most often leads to hemorrhage and a hematoma. The right side is more often involved, and trauma to the overlying ribs and liver is common. Associated bleeding often is not confined to the adrenal gland but also involves adjacent extraperitoneal tissues. In some patients a hematoma is asymptomatic and is discovered only later, when trauma is already forgotten.

An adrenal injury classification scale, devised by the American Association for the Surgery of Trauma, is outlined in Table 16.1. Adrenal hemorrhage/hematoma is discussed later in this chapter.

Enlargement

Visualization of normal-appearing adrenal glands essentially excludes a neoplasm. An abnormal gland is identified as a focal bulge, diffuse gland enlargement, or both. A focal tumor can be either neoplastic or hyperplastic. In general, a functioning primary tumor is asso-

Table 16.1. Surgical adrenal injury scale

* Advanced one grade for multiple injuries, up to grade V. Modified from Moore et al (5).

ciated with either an atrophic or normal-size contralateral gland. Most small hyperplastic and neoplastic nodules are isodense on CT and isointense on MR to normal adrenal parenchyma on precontrast images and are identified by their contour abnormalities.

Determining whether a tumor is in the adrenal or is extraadrenal in location is usually straightforward with imaging. The presence of a normal adrenal gland adjacent to a tumor establishes their relationship. Especially when large, however, some extraadrenal tumors invade and obliterate the adrenal gland; the reverse is also true—an adrenal tumor can involve adjacent tissues. With both scenarios it may not be possible to establish the site of tumor origin.

Hyperplasia

A distinction between a normal-sized gland and an enlarged one is a borderline of gradation. With diffuse enlargement the usual adrenal shape is preserved, although on occasion some degree of nodularity to the gland outline is observed. Bilaterally enlargement signifies hyperplasia. It should be kept in mind, however, that some patients with clinical evidence of adrenal hyperfunction have normal-sized glands.

Hyperplasia of the medulla is less common; such hyperplasia appears to be a precursor to a pheochromocytoma.

Hyperplastic adrenal gland MR signal intensity is identical to a normal adrenal with all imaging sequences.

Infection

Adrenal abscesses are uncommon in adults. Some represent the sequelae of an infected

hematoma. These abscesses can be diagnosed by needle aspiration. At times an infected tumor is in the differential diagnosis.

Adrenal hydatidosis is rare (6). Imaging reveals a similar appearance to that seen in other organs.

Involvement by histoplasmosis or tuberculosis is usually bilateral but also asymmetrical. The imaging findings vary depending on the extent of involvement and the degree of necrosis. Initially, histoplasmosis results in bilateral adrenal enlargement, but eventually the glands become atrophic and develop punctate calcifications. Tuberculosis, a cause of adrenal insufficiency, has a similar adrenal enlargement followed by atrophy and calcification appearance.

Other infections are rare. Anecdotal reports describe adrenal coccidioidomycosis or North American blastomycosis involving the adrenal glands even in an immunocompetent patient; the latter infection can also cause adrenal insufficiency. Computed tomography in a patient with adrenal paragonimiasis detected an enhancing right adrenal tumor, with US revealing a dumbbell-like hyperechoic tumor (7); a multicystic structure filled with creamy material was found at surgery.

Initially with gland enlargement a neoplasm is in the differential diagnosis.With an atrophic, calcified gland, however, a neoplasm is less likely.

ADVANCED IMAGING OF THE ABDOMEN

cyst without a cellular wall, but it has a fibrous capsule. Wall thickness tends to be several millimeters; a thicker or irregular wall should raise suspicion for a neoplasm. These cysts tend to be unilocular and vary in size considerably. Ultrasonography reveals a cyst with internal echoes. Magnetic resonance imaging shows a thickrimmed cyst. Similar to their US appearance, these cysts’ signal intensity varies depending on cyst content. Curvilinear wall or septal calcifications develop in some; an occasional one contains central calcifications.

The CT density of adrenal cysts varies from water to old blood. Some exhibit rim contrast enhancement, but the cystic component should not enhance. Fluid-fluid layering is occasionally found. The relationship of polycystic renal disease and adrenal cysts is not clear.

The differential diagnosis of a cystic adrenal tumor is between one of the above-listed cysts and a cystic or necrotic neoplasm. The presence of soft tissue density nodules suggests a neoplasm. Contrast enhancement of any solid component most often signifies a neoplasm (8). If needed, an aspiration biopsy is obtained, but keep in mind that cysts containing old blood are difficult to drain. If a lesion appears indeed to be a cyst, it can be observed.

One must ensure that a cystic structure is indeed of adrenal origin. For instance, CT or MRI of a gastric cardia diverticulum can minic a left adrenal cyst.

Cysts

Aside from cystic neoplasms, adrenal cysts are uncommon. Cystic neoplasms include a rare carcinoma, pheochromocytoma, or a postinjection therapy adenoma. Nonneoplastic cysts can be classified as parasitic, epithelial retention cysts, endothelial cysts, and pseudocysts due to prior hemorrhage, with a majority of cysts representing the latter two entities. Imaging cannot differentiate between these various nonneoplastic cysts, with a few exceptions. Endotheliumlined cysts consist of lymphatic and vascular degenerative cysts. They tend to be multilocular and are filled with clear or milky fluid.

Some adrenal hemorrhages evolve into a pseudocyst, a poor term because there is nothing “pseudo” about these cysts. They are not related to pancreatic pseudocysts, which have a different etiology. An adrenal pseudocyst is a cortical

Incidental Tumors

In some patients diffusely enlarged adrenal glands are discovered incidentally; most often this finding is of little significance. An incidentally detected focal adrenal tumor (incidentaloma), on the other hand, probably warrants further study. About 10% to 15% of patients with a focal tumor have autonomous cortisol hypersecretion without obvious stigmata of Cushing’s syndrome. Among newly detected single adrenal tumors about half are adenomas, yet even an incidental tumor has about a one-third chance of being a metastasis, even with no known primary.

What is the eventual outcome of incidentally discovered adrenal tumors (incidentalomas)? In patients with a unilateral incidental tumor, 17% had biochemical evidence of adrenal