Imaging Consequences

Because of the central location within the abdominal cavity and the vicinity of numerous viscera, plain films are rarely informative. Occasionally, a large adrenal mass may be suggested by the downward displacement of the kidney, but this appearance is rarely seen, and differentiation from renal, splenic, pancreatic, gastric, and retroperitoneal tumors requires further investigation.

Calcification may be seen in the adrenal glands and can be idiopathic or result from neonatal causes (infarction, hemorrhage, infection), maternal diabetes mellitus, tuberculosis, histoplasmosis, cyst, tumor, and Addison's disease.46 Retroperitoneal pneumography is now obsolete.

Ultrasonographic visualization of the adrenal is not an easy technique and may produce false-positive results, but an accuracy of 70% has been described. Ultrasonography is considered to be the investigation of choice in the neonate and young child, when the relatively small amount of retroperitoneal fat makes computed tomography (CT) scanning a less satisfactory technique. The adult adrenal gland is slightly more echogenic than the kidney. A left adrenal mass should be distinguished from normal splenic vessels, splenic lobulation, and masses arising from the kidney, spleen, and pancreas. A mass within the right adrenal gland must be differentiated from the right crus of the diaphragm, retrocaval lym-phadenopathy, and masses arising from the liver and kidney.

Ultrasonography is useful in assessing the development of the fetal adrenals. They appear as disklike structures medial to the kidney in transverse scanning through the fetus and as heart-shaped structures of low echogenicity superiorly and medially to the kidney in the longitudinal plane. It is possible to monitor a linear increase of the adrenal area, circumference, and length during the 20th to 40th weeks of gestation.47

CT scans identifies the adrenals in nearly all patients and has a reported accuracy of more than 90% in the diagnosis of adrenal masses. The normal glands have a variable appearance on the CT scan. Usually, the right gland is linear or V shaped, with the medial and lateral limbs posteriorly; the medial limb is more caudal and is larger, measuring up to 4 cm in length. The left adrenal gland is V shaped, is triangular, or has a Y configuration, with its apex anteromedial and its limbs posterior (see Fig. 64-4A).46

Absolute criteria for enlargement of the adrenals on the CT scan do not exist. Convexity of the adrenal outline is significant and should be considered abnormal. By comparison with the right crus of the diaphragm, a normal adrenal gland should not be thicker than the crus.

Fine-needle biopsy under CT or ultrasonographic control can be performed in the diagnosis of incidentaloma (nonfunctioning adrenal tumor identified on a routine scan) after a pheochromocytoma has been ruled out.

Magnetic Resonance Imaging

The normal adrenals are best seen on T1-weighted sequences, in which cortex may be differentiated from medulla. The T2-weighted images appear to be suitable to help differentiate benign from malignant adrenal neoplasms (see Fig. 64-4B).46

Radionuclide Imaging

Iodine 131-metaiodobenzylguanidine (MIBG) and iodine 123-MIBG) concentrate in the adrenergic neurotransmitter vesicles, and this is used for demonstrating pheochromocytomas. Selenium 6-selenomethylnorcholesterol and !3lI-6(3-iodomethylnorcholesterol are used for steroidogenesis and allow imaging for adrenal hyperplasia and adenomas; suppression of the normal tissue with dexamethasone enhances uptake into the adenomas and provides a better image.46

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