Specific Imaging Findings

Pituitary adenoma under 1 cm in size is by convention referred to as microadenoma. Most microadenomas are located laterally within the anterior lobe and may not cause any notable change in the size or contour of the gland. The majority are seen on precontrast T1WI as a round or oval, sometimes triangular hypointensity. Some microadenomas may be T1 bright, presumably due to hemorrhagic transformation. T2 hyperintensity is found in the majority of microprolactinomas. Most growth hormone-secreting adenomas are, however, T2 iso to hypointense. Some microadenomas are depicted on T2WI only and some exclusively on post-contrast images. Dynamic imaging detects an additional 10% of lesions. The tumors typically show a different dynamic pattern, usually of delayed or complete lack of enhancement. In rare cases adenomas accumulate contrast medium earlier than the normal gland, reflecting a direct arterial supply due to dural invasion. Delayed imaging may show prominent adenoma enhancement within relatively dark normal gland. Dedicated pituitary imaging, including dynamic post-contrast scans, may also be performed with CT. Adenomas in Cushing disease tend to be located around the midline and are frequently not visualized on imaging studies. Presence of fluid levels is highly indicative of adenomas, representing degeneration and hemorrhage.

Pertinent Clinical Information

Microadenomas may be asymptomatic and discovered in patients investigated for unrelated reasons. Symptomatic microadenomas are usually prolactin-secreting and are more common in women presenting with infertility, amenorrhea and galactorrhea. In men, microadenomas usually present with impotence, prolactin levels are higher, tumors larger and more invasive, and the outcome is worse. There is a solid correlation between the prolactin blood levels and MRI: concentration over 200 ng/ml practically guarantees tumor detection, while imaging is positive in less than half of cases below 50 ng/ml. T2 hypointense prolactinomas tend to have higher prolactin secretion. Microadenomas may also lead to Cushing disease with ACTH-producing tumors, or acromegaly with GH-secreting adenomas. Endovascular venous sampling (from inferior petrosal and/or cavernous sinus) may be necessary for diagnosis in patients with Cushing disease.