Normal dimensions of the posterior pituitary bright spot on magnetic resonance imaging

Abstract

A n area of T1 hyperintensity is normally observed in the posterior part of the sella turcica on MR images of the brain. This phenomenon, often referred to as the pituitary bright spot (PBS), is thought to result from the T1-shortening effect of stored vasopressin in the posterior lobe of the pituitary 1,12 and is observed in 100% of children 15,17 and in 52% to 100% of adults 5,6,8,19,22 without pituitary disease. The PBS may be absent in up to 48% of normal subjects, more often in older patients.5,23 Individual variations in the appearance of the PBS have been observed. Physiological conditions such as very young age, pregnancy, or lactation may result in an enlarged PBS. 2An abnormal PBS can also be seen in various pathological processes. Central diabetes insipidus (DI) results from low blood levels of vasopressin and may be primary or secondary to a number of causes. In primary central DI, the PBS has been found to be absent in 25% to 100% of cases. 12,15,17 Secondary central DI is a feature of Langerhans cell histiocytosis, germinoma, teratoma, craniopharyngioma, idiopathic giant cell granulomatous hypophysitis, and Wolfram syndrome. When DI was a presenting symptom of one of these conditions, the PBS was always absent. 7,9,12,15,17,18 In conditions in which the posterior pituitary is depleted of its vasopressin granules, the hyperintense T1 signal can also be lost. An absent PBS was observed in 32% of patients with uncontrolled diabetes mellitus 11 and in a patient with severe anorexia nervosa. 21Finally, when the pituitary stalk is interrupted, the vasopressin synthesized in the hypothalamus does not reach the posterior pituitary. The absence of a normal PBS in these cases is often accompanied by an ectopic bright spot due to the storage of vasopressin granules in another anatomical location. A patient with pituitary dwarfism was found to have an absent PBS with an ectopic bright spot. 24Acquired section of the pituitary stalk of traumatic or surgical origin has also been found to result in an absent PBS and ectopic bright spot. Object. The normal pituitary bright spot seen on unenhanced T1-weighted MRI is thought to result from the T1-shortening effect of the vasopressin stored in the posterior pituitary. Individual variations in its size may be difficult to differentiate from pathological conditions resulting in either absence of the pituitary bright spot or in T1-hyperintense lesions of the sella. The objective of this paper was to define a range of normal dimensions of the pituitary bright spot and to illustrate some of the most commonly encountered pathologies that result in absence or enlargement of the pituitary bright spot.Methods. The authors selected normal pituitary MRI studies from 106 patients with no pituitary abnormality. The size of each pituitary bright spot was measured in the longest axis and in the dimension perpendicular to this axis to describe the typical dimensions. The authors also present cases of patients with pituitary abnormalities to highlight the differences and potential ov...

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