The sella turcica houses the pituitary gland in a midline,
shallow depression in the posterior sphenoid bone.
Its appearance is variable and depends on the pneumatization of the underlying sphenoid sinus as they may merge on MR images.
The optic chiasm lies anterior to the infundibulum and visual field deficits are signs of pituitary axis dysfunction.
The pituitary gland arises from two separate sources: the posterior lobe (or neurohypophysis) and the anterior lobe (adenohypophysis).
The posterior lobe is a downward extension of the hypothalamus while the anterior lobe develops from the Rathke pouch.
There is a third portion,
the pars intermedia,
largely vestigial in humans,
prominent in fetal life and in pregnancy and is a remnant of the Rathke cleft.
The adenohypophysis (anterior pituitary) makes up a majority of the bulk of the gland and communicates with hypothalamic cells via a specialized portal venous system whose capillaries are interspersed with hormonally active cells.
It contains 5 principal histologically distinct cell types:
· Lactotrophs that secrete prolactin
· Thyrotrophs that secrete thyroid-stimulating hormone
· Corticotrophs secreting adenocorticotropic hormone
· Somatotrophs that secrete growth hormone
· Gonadotrophs that secrete luteinizing hormone and follicle-stimulating hormone
The neurohypophysis secretes two major hormones: vasopressin (antidiuretic hormone or ADH) and oxytocin,
synthesized in the supraoptic and periventricular nuclei of the hypothalamus,
they descend through the infundibulum and are released by exocytosis.
There is a defined cellular organization in which the somatotrophs and lactotrophs are in the lateral lobes in the anterior part of the gland while the corticotrophs and thyrotrophs are located medially.
Imaging plays a pivotal role in the diagnosis,
surveillance,
and treatment monitoring of patients with pituitary disease.
MRI has long since become the imaging modality of choice over CT for imaging the sellar and juxtasellar regions.
The goal is to indicate the localization and nature of the lesion,
its extension in relation to the various surrounding structures,
its morphology and its enhancement in order to help in the differential diagnosis and treatment planning.
Because MRI enables characterization of lesions containing hemorrhage,
fat,
flowing blood,
mucus,
and cystic structures,
it is sometimes possible to give a specific diagnosis.
MRI evaluation should be done in a 1.5 or 3 Tesla scanner.
Noncontrast T1 and T2,
gadolinium (gadopentetate dimeglumine) enhanced and dynamic sequences as well as gradient echo T2 and diffusion weighted imaging should be included.
The pituitary gland in a normal adult is isointense to the white matter of the temporal lobe on T1 weighted images and the posterior lobe is hyperintense,
seen as a bright spot (Fig. 1).
The signal should be homogenous throughout the gland.
The gland does not exceed the level of the sella entrance.
The pituitary stalk and posterior lobe enhance approximately 20 seconds after injection and the anterior pituitary after 80 seconds (Fig. 2).