The pituitary gland resides in the sella turcica, a depression in the sphenoid bone, and is subdivided into an anterior portion, composed of hormone-secreting cells, and a posterior portion, composed of axons from hypothalamic neurons that secrete oxytocin and vasopressin. The hypothalamic axons form the pituitary stalk (infundibulum) that enters the sella through a perforation in the overlying dura (diaphragma sellae). On unenhanced magnetic resonance imaging (MRI) T1-weighted sequences, the anterior pituitary is isointense to gray matter whereas the posterior pituitary is hyperintense (pituitary bright spot). The anterior pituitary enhances with contrast. The potential space between the anterior and posterior portions of the pituitary is known as the Rathke cleft. Patients with mass lesions confined to the sella (intrasellar lesions) are frequently asymptomatic. However, with a hormone-secreting pituitary microadenoma, symptoms may occur from the excess secreted hormone. The area directly above the sella turcica (the suprasellar region) contains the optic nerves and optic chiasm anteriorly, the hypothalamus with its infundibulum superiorly, and the third ventricle posteriorly. Mass lesions can be purely suprasellar or begin in either the sellar or suprasellar region and grow into the other region. Those that cross between sellar and suprasellar regions frequently show pinching in the middle due to constriction by the diaphragma sellae giving an hourglass appearance. Suprasellar lesions may cause headache secondary to elevated intracranial pressure from involvement of the third ventricle or central or bitemporal vision loss due to involvement of the optic nerve or chiasm, respectively. Lateral growth of sellar or suprasellar lesions can involve the cavernous sinus and may cause pupillary abnormalities, ocular motility deficits, facial numbness in the ophthalmic/maxillary divisions of the trigeminal nerve, or cerebral ischemia from compression of the cavernous carotid artery.
Acute infarction or hemorrhage in the pituitary is referred to as pituitary apoplexy; abrupt headache and visual symptoms suggest the diagnosis. An underlying adenoma is present in most cases. Emergent neurosurgical consultation is indicated.
A Rathke cleft cyst ( Fig. 118.1 ) is a congenital lesion that is nearly always an incidental asymptomatic finding. The cystic fluid is proteinaceous and usually hyperintense on T1-weighted imaging, but can also be T1 hypointense. There is either no enhancement or a small rim of enhancement. Larger Rathke cleft cysts can extend into the suprasellar region. If there is diagnostic uncertainty, follow-up imaging can be performed to ensure stability.
Pituitary hyperplasia is an enlargement of the pituitary gland to 10–15 mm due to either increased physiologic requirements for hormone secretion (e.g., pregnancy) or decreased responsiveness of end-organ glands such as the adrenal glands. Imaging characteristics are the same as normal pituitary tissue. The pituitary gland can also appear enlarged in intracranial hypotension due to increased blood volume.
Pituitary microadenomas are < 10 mm in size, typically isointense or hypointense on T1 sequences, and often have decreased enhancement compared to normal pituitary tissue. A microadenoma causes asymmetric enlargement of the pituitary gland displacing it from the midline, and the infundibulum is often deflected contralaterally, best seen on coronal images.
Meningiomas are frequently found in the parasagittal region, arising from the diaphragma sella, the walls of the cavernous sinus, or the sphenoid or petroclival dura. On MRI, they typically appear iso- to slightly hypointense on T1 sequences and have robust homogenous enhancement. Internal calcifications may be present. An adjacent tapering dural thickening (a “dural tail”) can often be seen. If there is extension laterally to the cavernous sinus, there can be encasement of the cavernous carotid artery limiting surgical resection.
Pituitary macroadenomas ( Fig. 118.2 ), the most common mass in the suprasellar region, are > 10 mm in size with sellar and suprasellar components, and thus frequently exhibit the hourglass sign. They cause asymmetric enlargement of the pituitary gland displacing it and the pituitary stalk from the midline. They are typically isointense or hypointense on T1 sequences and will enhance. If there is extension laterally to the cavernous sinus, there can be encasement of the cavernous carotid artery.