Practical errors in measurement of the pituitary at CT.

Direct visualization of the pituitary gland is now possible with computed tomography (CT). It is hoped that this method will enable earlier detection and / or accurate localization of pituitary microadenomas. After contrast administration, it appears that microadenomas can be of the density of intravascular blood , or of higher or lower density [1]. Other criteria have been proposed to evaluate the pituitary gland for microadenomas. On the basis of CT scans on normal patients, a frontal or lateral height greater than 5 mm in males and 7 mm in females is considered abnormal [1]. A gland that exhibits a convex upper margin is also considered abnormal. However, no reference has been made to the possible effect on these criteria of intrasellar vascular channels . Intercavernous vascular connections occur frequently within the sella. They are named for their position, either anterior, inferior, or posterior, relative to the pituitary gland. Their presence and size are variable . The anterior intercavernous sinus was present in 76% of an autopsy series, and the posterior intercavernous sinus in 32% [2]. On CT, enhancement within these sinuses is indistinguishable from enhancement of the pituitary gland [1]. Consequently, error in the measurement of true pituitary gland height would occur if it is measured from the bony floor of the pituitary fossa to the superior surface of the pituitary gland, since an isodense inferior intercavernous sinus would increase the apparent height of the pituitary gland. This could yield a falsely large value for the height of the gland, possibly raising the suspicion of the presence of a microadenoma. Perhaps a bolus injection with dynamic scanning would enable a more accurate measurement of pituitary height. The width of the pituitary gland is usually equal to or greater than either the depth or length. In 14% of cases, the intracavernous parts of the carotid arteries are medial. This can cause lateral compression of the pituitary gland [2]. The gland could compensate for this loss in lateral dimension by increasing its height and even possibly forming a convex upward configuration. The superior surface may become triangular as a result of being compressed laterally and posteriorly by the carotid arteries [2]. Since th e carotid arteries are isodense with the pituitary gland after infusion, these variations can cause a false measurement of the width and height of the gland . The use of high resolution scanners with 1.5-mm-thick sections enables visualization of small , low or high density lesions within the pituitary gland. However, if a part of a pixe l contains both a low density microadenoma and an isodense-to-pituitary gland intercavernous sinus, volume averaging could cause an artifactual loss of visualization of the abnormality , leading to a false-negative exami87