Modalities for Imaging the Adrenal Gland

Evaluation of a patient with an adrenal gland mass is founded on a thorough history and physical examination, followed by appropriate biochemical tests. After the diagnosis has been established, imaging procedures are used for localization and presurgical planning. Improvements of functional and anatomic imaging procedures allow reliable preoperative evaluation of virtually all adrenal masses. Computed tomography (CT) and magnetic resonance imaging (MRI) are the main modalities used to localize adrenal tumors. The best radiologic imaging test is CT scanning, and it is usually the only imaging study required. In this chapter, we review adrenal imaging techniques and discuss their indications and limitations. We also present flowcharts showing how the most prevalent adrenal diseases, including incidentaloma of the adrenal gland, should be approached.

Computed Tomography

CT is the modality most commonly used to evaluate a patient suspected of having an adrenal mass.1 CT accurately delineates the location, size, and configuration of the mass; local invasion; and affected adjacent lymph nodes or distant metastases.2 For routine applications, 1-cm contiguous scans in the adrenal area are usually obtained. For smaller masses (e.g., in primary hyperaldosteronism) thinner scans, such as 0.5-cm slices, are necessary. The normal right adrenal gland is a comma-shaped gland of roughly 1 x 2 x 0.5 cm, and the left adrenal is a lambda-shaped gland of roughly similar size. The normal adrenal gland is approximately the same size as the diaphragmatic stripe seen on CT (Fig. 66-1). The right adrenal gland is usually directly posterior to the inferior vena cava, and the left adrenal is anterior to the upper portion of the kidney and adjacent to the aorta. Although intravenous contrast material is not routinely used, it is useful for differentiating vascular structures from the adrenal or for enhancement characterization of adrenal tumors. Oral contrast media to opacify bowel may be required for extra-adrenal pheochromocytomas and for delineation of adrenal carcinomas. Despite the merits of CT scanning, it lacks specificity. For example, adrenal adenomas, carcinomas, and pheochromocytomas cannot be differentiated by plain CT scanning. Cysts and myelolipomas are the only conditions that are reliably diagnosed by CT. In cases of Addison's disease, CT scanning may reveal atrophy,3 but as exemplified in this condition, much of its capacity to differentiate one lesion from another is based on size rather than specific tissue characteristics.1 False-negative examinations result largely from trying to image tumors smaller than 1 cm in diameter.

Magnetic Resonance Imaging

MRI is increasingly used because it can reveal tissue-specific characteristics, which allows the examiner to differentiate metastases, adrenocortical carcinoma, and pheochromocytoma from adenoma, lipoma, myelolipoma, and cysts.4 Because MRI does not use ionizing radiation, it is an attractive modality for evaluating children and pregnant women.5 T1-weighted images allow relatively fast data acquisition, which may be accelerated by using paramagnetic contrast media such as gadolinium-diethylenetriaminepenta-acetic acid (DTPA), resulting in a reduction of motion artifact and increasing the sensitivity for identifying adrenal lesions. T2-weighted sequences reveal characteristic signal intensities in certain conditions and help with the differential diagnosis.6 Some studies have suggested that MRI can differentiate nonfunctioning from malignant adrenal lesions,6 but because of similar characteristics of some tumors, the results are not reliable enough to use in selecting therapy.6,7

Adrenal Scintigraphy

Adrenal scintigraphy provides localization and functional information and is therefore helpful for differentiating certain adrenal neoplasms. Scintigraphy is often used in conjunction with CT or MRI because it offers much less anatomic information than other modalities. Numerous radiolabeled pharmaceuticals are being investigated to provide improved localization tests for the adrenal cortex and medulla.

Iodocholesterol-labeled analogs such as 131I-6(3-iodo-methyl 19-norcholesterol (NP-59) and 75Se-6fJ-selenomethyl cholesterol are used to scan the adrenal cortex.8,9 Studies using NP-59 make use of the fact that adrenal lesions can be distinguished on the basis of intact steroidogenesis pathways and

FIGURE 66-1. CT images of normal adrenal glands. A, The medial limb of the right adrenal gland (arrow) is dorsal to the inferior vena cava and ventral to the upper pole of the kidney. B, The left adrenal gland (arrow) has an inverted V appearance. (From Davidson AJ, Hartman DS. Radiology of the Kidney and Urinary Tract, 2nd ed. Philadelphia, WB Saunders, 1994, p 716.)

FIGURE 66-1. CT images of normal adrenal glands. A, The medial limb of the right adrenal gland (arrow) is dorsal to the inferior vena cava and ventral to the upper pole of the kidney. B, The left adrenal gland (arrow) has an inverted V appearance. (From Davidson AJ, Hartman DS. Radiology of the Kidney and Urinary Tract, 2nd ed. Philadelphia, WB Saunders, 1994, p 716.)

the presence of abundant intracellular cholesterol. However, imaging requires several days, which is unacceptable in some cases. Moreover, in several comparative studies, CT required less time to perform and interpret, cost less, used less ionizing radiation, and provided similar diagnostic accuracy.10

Metaiodobenzylguanidine (MIBG) is the most frequently used radionuclide for imaging the adrenal medulla. MIBG is an analog of guanethidine. It is taken up by adrenergic granules and adrenal medulla cells because of its structural similarity to norepinephrine. However, it has essentially no pharmacologic effect.11 Because MIBG is concentrated in catecholamine storage vesicles, it allows functional assessment of adrenal medullary tissue and is diagnostic for pheochromocytomas.12 Iodine 131 MIBG is the most commonly used isotope, but 123I-MIBG has resulted in more accurate delineation of pathologic tissues than 13II-MIBG and provides superior dosimetry.13 MIBG studies take 3 days to complete, and their spatial resolution is poorer than that provided by CT scans.

Ultrasonography

Controversy exists regarding the efficacy of ultrasonography in the evaluation of adrenal tumors. Ultrasonography offers the particular advantages of being less expensive, being noninvasive, and involving no ionizing irradiation, which makes it attractive for evaluating children and pregnant women. For children, ultrasonography has been very effective, whereas in adults, visualization of the right adrenal is successful in approximately 90% of cases and for the left the success rate is only 75%.14 Ultrasonography delineates lesions of 2 cm or larger and is helpful in differentiating cysts from solid masses and in evaluating involvement of large vessels and liver metastases.15 Ultrasonography is an ideal screening modality for adrenal neoplasms and for following the progression of adrenal masses. Ultrasonography is not as accurate as CT. It requires an experienced interpreter and is notoriously operator dependent.2

Arteriography and Adrenal Venography

Considered invaluable tools in differentiating hyperplasia from carcinoma in the 1980s, arteriography and adrenal venography have almost completely been replaced by CT and MRI. In general, these invasive methods should be reserved for the rare instance in which CT or MRI provides insufficient information. Arteriography in patients with pheochromocytoma may be hazardous, and venography may be dangerously invasive, especially in children; it has been associated with significant morbidity in pediatric cases.15 Selective arteriography, however, may be helpful if it is difficult to determine whether a mass on CT is suprarenal or renal in origin.16

Venography, often used in combination with selective venous sampling, is employed more often than arteriography.17 Although the method successfully approaches the left adrenal gland in virtually all cases, this cannot always be accomplished for the right gland and demands an experienced radiologist. Venography with selective adrenal sampling is useful in examining patients with hyperaldosteronism or Cushing's syndrome when the clinician cannot discriminate by CT or MRI between hyperplasia and adenoma, and it is occasionally useful for determining the source of ectopic corticotropin (ACTH) production. Complications occur in about 5% of patients and consist mainly of contrast extravasation and hematoma and rarely of adrenal vein thrombosis and adrenocortical insufficiency.17

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