Imaging of the Pituitary Gland

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Barrow Quarterly - Volume 18, No. 3, 2002


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Imaging of the Pituitary Gland


Kirsten Forbes, MD
John Karis, MD
William L. White, MD*

Divisions of Neuroradiology and *Neurosurgery, Barrow Neurological Institute, St. Joseph's Hospital and Medical Center, Phoenix, Arizona

Abstract

This article reviews the magnetic resonance imaging techniques that can be used to assess the pituitary gland and describes the imaging characteristics of normal sellar anatomy and common sellar and suprasellar pathologies.

Key Words: macroadenomas, magnetic resonance imaging, microadenomas, pituitary

 

Undoubtedly, magnetic resonance (MR) imaging is the optimal imaging technique for evaluating the pituitary gland.  Adequate assessment requires dedicated high-resolution imaging through the sellar region.  The coronal plane offers the best single view for assessing the sella and allows the pituitary gland to be distinguished from the surrounding structures.  Sagittal views are particularly helpful for evaluating midline structures.  Because the pituitary gland is small, high spatial resolution images are required.  Slice thicknesses must be 3 mm or less, and the field of view must cover only the sellar and immediate parasellar regions.  T1-weighted sequences, with and without gadolinium, are optimal for evaluating both normal anatomy and underlying pathology.  The amount of gadolinium used is field-strength dependent with a one-half dose typically being used at 1.5 Tesla.  Fat-saturation techniques are useful for postoperative evaluations.[14]


Figure 1. (A) Sagittal precontrast T1-weighted image reveals a normal-appearing pituitary gland. The adenohypophysis is isointense and the neurohypophysis is hyperintense. (B) Sagittal postcontrast T1-weighted image shows normal diffuse enhancement of the gland. (C) Coronal precontrast T1-weighted image facilitates identification of the infundibulum, optic chiasm, and cavernous sinuses. (D) Fat-saturated coronal postcontrast T1-weighted image is useful for assessing the gland for residual tumor or a recurrence after surgery.

Normal Anatomy

The anterior and posterior lobes of the pituitary can often be differentiated on MR imaging.  In adults the anterior lobe is isointense to the remainder of the brain, and the posterior lobe is hyperintense on T1-weighted sequences (Fig. 1).[11]  This finding is thought to reflect the high concentration of neurosecretory granules in the neurohypophysis, but the exact substrate is still debated.  In neonates and during pregnancy, distinguishing the anterior from the posterior pituitary may be challenging because the anterior lobe also appears hyperintense on T1-weighted images.


Figure 2. (A) Sagittal and (B) coronal precontrast T1-weighted images show a hyperintense region just below the hypothalamus. The normal pituitary bright spot is absent in the sella turcica, consistent with an ectopic posterior pituitary gland.

Absence of the normal posterior pituitary bright spot should prompt a search for an ectopic posterior pituitary that has failed to migrate inferiorly from the hypothalamus (Fig. 2).  This finding can also be associated with transection of the pituitary stalk.  However, absence of the pituitary bright spot can be a normal finding or may reflect technical factors.  It should not be interpreted as an absolute indication of pathology.


Figure 3. During puberty and pregnancy the pituitary gland undergoes physiological hypertrophy and shows a convex upper margin, as seen on (A) sagittal T1-weighted and (B) coronal T2-weighted images.


 

Figure 4. The empty sella turcica, shown in this sagittal T1- weighted image, is a normal anatomic variant. An incidental basilar artery aneurysm is present. 
The size of the pituitary gland varies with physiological status.  In the normal state, the gland is less than 8-mm high.  During puberty or pregnancy, it becomes larger and more upwardly convex (Fig. 3).  During puberty it may reach 10-12 mm in females and exhibits even more marked changes during pregnancy.[9]  Chanson et al. report that the pituitary height exceeds 9.0 mm in less than 0.5% of healthy women age 18-35.[3]  They report one healthy 24-year-old female with a normal pituitary gland height of 12 mm and width of 12 mm.  Another female patient with a normal pituitary gland measured 16 mm in width.  They emphasized that physiological pituitary hypertrophy seems to be a frequent cause of incidentaloma and that careful examination of the MRI may help distinguish this entity from pituitary tumors and infiltrating lesions. Physiological pituitary hypertrophy is confirmed by normal baseline pituitary function and extensive hormonal tests.  Identification of these patients is important to avoid unnecessary pituitary surgery.[3]  It is normal for the pituitary gland to be markedly flattened against the floor of the sella, a configuration known as an empty sella turcica (Fig. 4).  The sella largely becomes filled with cerebrospinal fluid (CSF) because of the enlarged diaphragmatic sella hiatus, and it may appear to expand with CSF pulsations.


 


Figure 5. (A) Coronal T1-weighted image shows an upward convexity of the left pituitary gland. Within the gland, a focal area of hypointensity is seen. (B) After intravenous contrast is administered, the microadenoma remains hypointense while the remainder of the gland enhances.

Pituitary Microadenomas

The findings associated with a pituitary microadenoma on MR imaging are often subtle, necessitating a high-quality imaging study (Fig. 5).  With optimal MR technique, about 90% of these small tumors (10 mm or less in diameter) can be identified.[14]  False-negative studies are most often associated with Cushing's disease becau se many tumors that secrete adrenocorticotropic-hormone (ACTH) are small.  A microadenoma can alter the contour of the pituitary gland, creating an outward convexity of its superior or inferior aspects.  Classically, tumors secreting ACTH, thyroid stimulating hormone, luteinizing hormone, and follicle stimulating hormone are found centrally within the pituitary gland while prolactin and growth hormone adenomas occur at the periphery.  Laterally placed microadenomas may cause the infundibulum to deviate from the tumor.  However, a tilted infundibulum can also be a normal finding observed in about 50% of normal subjects.[1]


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