NORMAL ANATOMY

An understanding of sellar and parasellar anatomy and development is essential to interpretation of pituitary imaging studies. The pituitary gland develops from the anterior and posterior lobes that develop separately then fuse. The anterior lobe (adenohypophysis) arises from Rathke's pouch in the primordial stomodium then migrates along the craniopharyngial duct to the sella. The posterior lobe (neurohypophysis) arises from an evagination of the inferior third ventricle then migrates downward into the sella. The sella turcica is a shallow depression in the sphenoid bone bounded anteriorly and inferiorly by the sphenoid sinus, laterally by the cavernous sinuses, posteriorly by the dorsum sellae and superiorly by the diaphragma sellae and suprasellar cistern [Figure 1]. In normal adults the adenohypophysis appears isointense to cerebral white matter on standard T1 weighted images and comprises approximately 70-80% of the total gland volume. A portion of the neurohypophysis may appear hyperintense to the adenohypophysis on T1 weighted images, due to the effect of stored neurosecretory granules on the T1 relaxation time. This hyperintense region, the so-called posterior pituitary bright spot, is evident in up to 90% of children but is less consistently observed in adults.

Figure 1. Normal sagittal anatomy. Unenhanced sagittal image shows the optic chiasm (C) in the suprasellar cistern, infundibulum (arrowhead), anterior (A) and posterior (P) lobes of the pituitary gland, sphenoid sinus (*)

The size and shape of the normal pituitary gland varies. In adult males the gland ranges from 8-10 mm craniocaudad dimension in the sagittal plane. The upper contour of the gland is normally flat or concave. In females, the size of the gland varies with the menstrual cycle and pregnancy. In the post-partum period it may normally exceed 12 mm in height. It is also normal for the gland to have a convex upper border in menstruating or lactating females. During puberty, the gland may also have an convex upper surface in both males and females.

The pars intermedia, a vestigial remnant of Rathke's pouch, is not normally visualized on imaging studies. In some cases, a small cyst may mark the location of the pars intermedia.

The infundibulum appears as a thin stalk that tapers from the floor of the third ventricle to the pituitary gland. It normally tapers from the infundibulum to the upper surface of the gland. The diameter of the stalk normally does not exceed that of the basilar artery or 3 mm in diameter. It enhances after the administration of intravenous contrast medium, because it lacks a blood brain barrier [FIG 1]. Although it usually has a midline position it may normally deviate slightly from the midline.

Parasellar anatomy is seen best in contrast enhanced MR images. The cavernous sinus, which lies directly lateral to the pituitary gland, enhances after the administration of gadolinium containing contrast media [FIG 2]. Its enhancement is normally greater than that of the adjacent pituitary gland so that the border between the gland and the cavernous sinus can be discerned. The internal carotid artery and its bifurcation into the middle and anterior cerebral arteries appears as a signal void on MRI images because the protons within flowing blood don't produce detectable signal on T1 or T2 weighted pulse sequences. On sagittal images the carotid siphon may be seen in profile and in coronal images it is seen in cross section as several round flow voids. Within the enhanced cavernous sinus, portions of cranial nerves III, IV, V and VI may be seen on the coronal or axial images. In coronal images, cranial nerve III lies lateral and slightly superior to the carotid artery. The course of cranial nerve VI parallels the internal carotid artery in axial images. Cranial nerve V is located in Meckel's Cave that appears as a fluid filled structure in the posterior cavernous sinus.

Figure 2. Normal coronal sellar anatomy: a T1 weighted MRI scan through the sella turcica captures the optic chiasm (C), infundibular stalk (I), adenohypophysis (A), loops of the internal carotid artery (I) and the sphenoid sinus (*). Contrast enhancement within the cavernous sinus is seen surrounding the pituitary gland and internal carotid artery flow voids.

MR shows structures immediately above the sella [FIG 1,2]. The optic chiasm is seen anterior to the infundibulum. The optic nerves extend anteriorly and the optic tracts extend posteriorly from the chiasm. The diaphragma sellae is a thin dural membrane overlying the sella that is fenestrated to allow passage of the infundibulum. It normally enhances after administration of intravenous contrast medium because it lacks a blood-brain barrier. The third ventricle is identified above the sella because of its fluid contents. The tuber cinereum is seen as a triangular shaped structure below the third ventricle. The mammillary bodies are seen posterior to the tuber cinereum.

The bony structures surrounding the sella include the dorsum sellae, anterior and posterior clinoid processes, and the clivus. The clivus marrow signal intensity is uniformly bright on T1 and T2 weighted images.

PITUITARY IMAGING MODALITIES

Magnetic resonance imaging (MRI) is the preferred imaging modality to evaluate suspected abnormalities of the pituitary gland and pituitary fossa, because of its contrast resolution, multiplanar capability and capability for dynamic contrast enhanced imaging. An MRI examination of the pituitary fossa should include sagittal and coronal thin section T1 weighted images before and after contrast medium administration. The addition of T2 weighted images in the coronal plane may yield more information in some cases. The use of fat saturated T1 weighted images after contrast administration is helpful to separate areas of enhancement from adjacent adipose tissue in cases in which parasellar disease is present. Axial images are also a useful supplement to the MRI protocol when lesions extend into the parasellar structures. Because of its high contrast sensitivity, MRI without the addition of gadolinium containing contrast medium is able to detect 90% of pituitary adenomas. The addition of contrast material to the MRI examination improves the sensitivity for detecting adenomas to 95%.

Dynamic contrast enhanced imaging may be a useful adjunct to the routine images in cases of suspected microadenoma. The dynamic technique consists of a series of images at the same location acquired rapidly after the injection of contrast medium intravenously to monitor temporal changes in signal intensity in intrasellar structures. Signal intensity increases more rapidly and to a greater degree in normal pituitary gland than in microadenomas [Fig 3]. However, 3 to 5 minutes following the injection of contrast the signal intensity of the normal gland and microadenoma may become indistinguishable. Microadenomas also demonstrate a slower washout of accumulated contrast material than the normal gland that may cause them to appear hyperintense on post contrast images obtained following a longer delay. A series of coronal images at 30 second intervals following a contrast bolus injection centered on the sella provides excellent temporal resolution of contrast enhancement [Fig 4].

Figure 3. Time intensity curve comparing the enhancement characteristics of normal pituitary gland with that of a microadenoma. Note the normal gland takes up contrast more rapidly than adenoma tissue and soon begins to wash out. The adenoma curve shows a slower uptake, lower peak enhancement level and prolonged retention of contrast. At time tx the intensity of a microadenoma may equal that of normal gland, potentially obscuring its presence.

Figure 4. Coronal dynamic gadolinium enhanced MRI scan demonstrating superior temporal resolution of contrast enhancement in the normal gland surrounding a hypointense microadenoma in the right lobe. Each scan (A-C) was obtained at the same coronal location at one second intervals following IV contrast administration. Coronal unenhanced (D) and delayed enhanced (E) images of the same patient show the microadenoma in the right lobe of the gland (arrow) that is hypointense relative the the normal pituitary tissue.

Computed tomography (CT) can be used for evaluation of the sella turcica if patients are unable to undergo MRI examination. CT provides better evaluation of the osseous structure of the sella than MRI and greater sensitivity to the presence of calcification. A CT imaging protocol for the sella should include direct, thin coronal scans through the sella with the patient in the prone or supine position after administration of iodinated contrast medium. Alternatively excellent thin coronal images may now be obtained by reformatting the image data acquired in an axial plane with multidetector scanners [FIG 5].

PITUITARY ADENOMA

Pituitary adenomas may be classified according to size and hormone production. By definition, adenomas less than 10 mm in diameter are microadenomas. The most common type of microadenoma, a prolactinoma, appears in MR images as a nearly round circumscribed lesion eccentrically located in the gland that enhances less than the adjacent gland. It may appear hypointense on T1 weighted images without contrast medium and isointense or hyperintense on T2 weighted images [FIG 6]. The presence of a microadenoma may also cause a slight upward bulge in the superior margin of the gland or a tilt of the infundibulum away from the lesion. Compared to normal pituitary tissue, prolactinomas demonstrate a slower rate of contrast enhancement, achieve a lower peak enhancement and retain contrast for a longer period of time. When imaged shortly after contrast administration a microadenoma typically appears as a hypointense focus in the anterior lobe of the gland. However microadenomas may be isointense or even hyperintense to normal gland if delayed contrast enhanced imaging is performed [FIG 3].

Figure 6. Pituitary microadenoma. Coronal unenhanced (A) and enhanced (B) images show the hypointense rounded microadenoma on the left deforming the upper contour of the gland

Microadenomas of the type that secrete other hormones may have a different appearance. ACTH, TSH, FSH and LH producing tumors tend to occur in the midline of the anterior lobe. HGH producing adenomas tend to occur eccentrically as do prolactinomas. Some of these microadenomas, especially HGH secreting microadenomas, are less well circumscribed, so that the MR detection of them is less accurate.

Adenomas greater than 10 mm in diameter are by definition macroadenomas. While microadenomas present because of their endocrine effects, macroadenomas typically present with clinical symptoms related to their effect on parasellar structures. A macroadenoma is typically seen on MR scans to enlarge the sella turcica or to extend into the adjacent suprasellar cistern, cavernous sinus or sphenoid sinus [FIG 7]. It may show a characteristic "figure of eight" appearance in the case of a macroadenoma that extends superiorly into the suprasellar cistern, because of the effect of the diaphragma sella on the tumor [FIG 8]. MR typically shows uniform contrast enhancement within a macroadenoma after intravenous contrast medium administration. In some macroadenomas with cystic change or necrosis within it, MR shows a non-enhancing region within the tumor. In cases in which hemorrhage occurs within the tumor MR may show signal intensity changes due to the paramagnetic effects of blood products or fluid levels within the tumor [FIG 9]. MR shows cavernous sinus invasion as a region of diminished enhancement in the cavernous sinus. MR has a sensitivity of about 50% for such invasion. Encasement of the carotid artery is reliably depicted as a soft tissue mass which enhances less than the cavernous sinus and which surrounds and often narrows the lumen of the carotid artery [FIG 7].

Figure 7. Regional extension of pituitary macroadenoma. Sagittal T1 (A) axial T2 (B) axial T1 (C) and enhanced axial T1 (D) weighted images showing extension of the tumor into the suprasellar cistern, sphenoid sinus and cavernous sinus bilaterally causing carotid artery encasement.

Figure 8. Pituitary macroadenoma: Coronal T1 weighted image showing suprasellar extension of a macroadenoma through the diaphragma sellae causing a “figure of eight” appearance.

MENINGIOMA

MR accurately demonstrates meningiomas arising within the cavernous sinus, from the clinoid process, or from the diaphragma sellae in the sella and suprasellar region. These meningiomas are the second commonest parasellar lesion detected in adults surpassed only by pituitary adenoma in frequency. MR demonstrates a well defined mass with signal intensity typically isointense to brain on T1 and T2 weighted pulse sequences and marked contrast enhancement after administration of intravenous contrast medium [FIG 10,11]. The enhancement of meningioma usually exceeds that of normal pituitary gland. It may show enhancing tumor extending into the sphenoid sinus, skull base or suprasellar cistern [FIG 11] In the meningiomas involving the cavernous sinus, MRI or digital subtraction angiography shows narrowing of the internal carotid artery as the tumor encases it [FIG 12]. MR angiography or digital subtraction angiography may be useful for detecting this encasement. In the case of meningiomas involving the pituitary fossa, MR may show an enhancing mass displacing the pituitary gland [FIG 13]. Unlike adenomas, meningiomas rarely cause enlargement of the sella. In meningiomas, but rarely in adenomas, MR may demonstrate a "dural tail" or thin tapering margin of enhancement where the tumor contacts the dura [FIG 11]. MR may demonstrate thickening of the low signal intensity representing cortical bone due to the presence of hyperostosis in some cases of intrasellar and parasellar meningioma. CT shows the hyperostosis more effectively than MR [FIG 12].

Figure 10. Meningioma. Axial T1 (A) and T2 (B) unenhanced images show an isointense mass centered in the right cavernous sinus encasing the internal carotid artery and invading the sella turcica.