Adenomas are classified radiographically by size. Microadenomas are considered to be less than lcm, and macroadenomas, greater than lcm.
Microadenomas are usually hypointense compared with the normal pituitary on Tl-weighted images. Small incidental pars intermedia cysts located between the anterior and posterior lobes may have a similar appearance or a high intensity signal. On intravenous contrast administration the microadenoma frequently enhances to a lesser degree than the surrounding gland, accentuating its hypointense appearance. Microadenomas are often asymmetrically located in the gland.
Fig. 4.5 CT and MRI scans of a microadenoma. (a) This post-contrast coronal CT scan demonstrates a 1 cm mass of low density; note the elevation of the diaphragma sella on the right side. (b) Coronal and (c) sagittal MRI scans.
