Adrenal Gland Imaging

The Correct option Answer: b

Although ultrasonography is the least expensive of the imaging procedures, its value is limited by the relative inaccessibility of the adrenal gland and by the small size of some adrenal lesions. CT is the technique most commonly used to examine patients in whom adrenal abnormalities are suspected. CT reliably detects adrenal tumors greater than 1 cm in diameter. The sensitivity of CT for tumors that are 1 cm in diameter is about 80% and reaches 100% for tumors that are 3 to 4 cm. Although CT is noninvasive and reasonably sensitive, it is nonspecific. It does not distinguish functioning from nonfunctioning tumors, nor benign from malignant tumors with any degree of reliability.

MRI has developed a certain usefulness even after retrenchment from early optimistic predictions. MRI is more expensive and requires greater patient cooperation than CT, but it has greater versatility because of the use of TI-and T2-weighted images. In some cases, the T2-weighted images are capable of providing a differential diagnosis and may distinguish such entities as metastatic or primary carcinoma and pheochromocytoma from adenomas, lipomas, myelolipomas and cysts. In a sense, MRI is complementary to CT in that the latter can better detect the lesion while the former can distinguish one type of lesion from the other. In addition, MRI is probably better than CT for distinguishing anatomic relationships and the extent of involvement of the surrounding tissues.

Two radiopharmaceuticals have proved useful in imaging the adrenal gland. Adrenocortical lesions can be imaged by NP-59 which is taken up as cholesterol in the adrenocortical steroidogenic pathway. The other agent is MIBG, a norepinephrine analogue. It indicates norepinephrine accumulation in storage vesicles and can detect sympathoadrenal tumors at any site in the body. NP-59 can localize the adrenal cortex and any functioning tumors. It can distinguish adrenocortical hyperplasia from functioning adenomas or carcinomas. MIBG is a useful agent in localizing pheochromocytomas throughout the body, especially when the tumors are multiple, extraadrenal, recurrent, or metastatic.

Arteriography, venography and selective venous sampling have become less popular as experience with other imaging techniques has become greater. Disadvantages inherent in invasive procedures using intravascular contrast agents are obvious. Arteriography is specifically dangerous in the study of patients with pheochromocytomas.

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