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Essentials in imaging of the sellar region:

Evaluation of the sella using MRI requires images with optimal spatial and contrast resolution and minimal background noise.
Many lesions in this area are very small. This requires small pixel size and thin slices.
T1-weighted MR images are currently the best method for demonstrating the anatomy of the sella and parasellar regions.
 

 

 
 

The anatomy of the sella turcica and parasellar region is intricate, and the pathology diverse. Many different imaging modalities have been used for the assessment of this area, but for the most part the introduction and widespread use of high-resolution computed tomography (CT) relegated its imaging predecessors, such as plain skull roentgenograms, pluridirectional tomography, and pneumoencephalography, to obsolescence. Magnetic resonance imaging (MRI) is now widely available, and a considerable body of experience has been accumulated in using it to evaluate this region. MRI is now accepted as the imaging procedure of choice in the evaluation of sellar and parasellar pathology. Its major advantages are its superior soft tissue contrast and its capacity for multi planar imaging. Also, there is no artefact from bone, and the patient is not exposed to ionizing radiation. CT is preferred for evaluating calcification and bone detail. Regardless of which imaging tool is used, it is useful to review the normal radiologic anatomy of this area and to survey the more common types of pathologic entities that occur in it.

Equipment and Technique

CT examination of the sella is performed in the coronal and axial planes using contiguous or overlapping thin CT sections (1.5 to 2.0 mm) and targeting the sella to a 160 mm field of view (POV) to yield maximum resolution. Coronal images are most informative and should be performed perpendicular to the planum sphenoidale, but avoiding dental amalgam as much as possible. The examination should extend from the tuberculum to the dorsum sellae. Iodinated intravenous contrast material is used routinely on all patients unless there is a strong history of a relevant allergy. A total of 3.0 to 4.5 g of iodine is injected by rapid bolus and infusion, allowing good visualization of the normally enhancing cavernous sinuses, pituitary tissue, and pituitary stalk.

CT of the sella can be combined with the intrathecal administration of an iodinated contrast agent (CT cisternography), allowing improved visualization of the suprasellar cistern and its contents. This may be useful in the evaluation of parasellar cystic lesions, or in differentiating intra-axial from extra-axial pathology.

Approximately 4 to 6 ml of a non-ionic contrast agent (iohexol, 180 mg/ml or iopamidal, 200 mg/ml) is injected via lumbar puncture, and the patient is then placed in a head-down position with the neck flexed for approximately 1 min to allow the contrast material to enter the suprasellar cistern. High-resolution CT is then obtained in the axial and coronal planes. This procedure is now rarely necessary because most pathology is assessed noninvasively and with better resolution using MRI.

Evaluation of the sella using MRI requires images with optimal spatial and contrast resolution and minimal background noise. In general, a unit with high field strength (1.0 to 2.0 tesla) combined with a dedicated quadrature head coil is best for this purpose. Mid­field units operating between 0.3 and 1.0 tesla can also produce excellent images, but when the finest detail is required without unacceptable background noise, the high-field units are generally superior.

MR images can be obtained in the sagittal, coronal, or axial planes or any combination thereof. Usually, time constraints preclude acquiring images in all three planes. It is generally accepted that coronal images are the most useful, particularly when examining the pituitary gland8. Coronal images allow the gland to be examined for asymmetries, and there is minimal partial volume artefact from the cavernous sinuses and carotid arteries. The sagittal image is useful in demonstrating midline anatomy and the orientation of the sella turcica and pituitary gland relative to the sphenoid sinus. Axial images are of limited usefulness. Generally, only one or two consecutive axial images can be obtained through the pituitary gland, and these may be subject to considerable partial averaging effects from the sphenoid sinus below and the suprasellar cistern above. Axial sections can be useful for evaluating the cavernous sinus and the medial temporal fossa on each side.

Detailed spatial resolution is of the utmost importance because many lesions in this area are very small. This requires small pixel size and thin slices. Most current MR imagers are capable of obtaining images as thin as 2 or 3 mm. In-plane resolution finer than 0.5 mm on a side can be obtained. However, the use of very small pixels results in images that are very grainy. Generally, pixel sizes approximately 0.8 mm on a side are optimal. These can be achieved with a 20-cm field of view and a 256 x 256 matrix.

There is a wide variety of image pulse sequence options. T1­weighted images appear to be the most sensitive and provide the best anatomic detail for extra-axial structures. The pituitary gland and parasellar region are no exception to this general rule. T1­weighted sequences demonstrate the soft tissue of this area very well as intermediate gray structures within the dark surrounding cerebrospinal fluid (CSF). T1-weighted images can also be obtained in a relatively shorter period compared to conventional T2­weighted spin-echo images. These T2-weighted images, which are extremely useful in evaluating the parenchyma of the brain, generally do not display the anatomy of the base of the brain as well as T1 -weighted sequences. In general, conventional T2-weighted images take longer to acquire, and it is more difficult to resolve the intrinsic anatomy of the pituitary gland, to visualize the pituitary infundibulum, and to see the optic chiasm on them. For these reasons, T1-weighted images are the imaging sequence of choice. Because of the requirement for fine spatial detail, signal averaging is required to produce a visually pleasing and diagnostic image. Imaging time increases in direct linear relationship to the number of signal averages. Optimal T1-weighted images are obtained in a reasonable imaging time using four signal averages. Paramagnetic intravenous contrast agents are now used routinely in the study of intrasellar and parasellar pathology when non contrast MR images are inadequate. The most notable of these is gadolinium­diethylenetriaminepentaacetic acid (gadolinium-DTPA) administered in a dose of 0.5 to 1.0 mmol/kg. It behaves like the iodinated contrast material used in CT in that it accumulates in the vascular and interstitial spaces of tissues that do not have an intact blood­brain barrier. Thus, the normal pituitary gland, cavernous sinus, and pituitary stalk will enhance (brighten) following gadolinium­DTPA administration. Unlike contrast-enhanced CT, however, rapidly flowing arterial blood will not enhance, and the carotid arteries in the cavernous sinus will remain dark ("MR flow void"). Paramagnetic contrast agents are useful in delineating subtle intrasellar lesions. They also better define the extrasellar extent of some lesions by increasing the contrast between enhancing pathologic tissue and the adjacent nonenhancing brain tissue.

Current developments in MRI that may further improve the visualization of the sella include volume-imaging techniques and "fast spin-echo" techniques. Volume-imaging techniques (3-D Fourier transform) allow very thin contiguous slices (1mm) to be obtained, thus improving the detection of very small lesions that might be missed owing to partial volume effects. The main reason these sequences are not widely used is their long imaging times. The combination of volume-imaging techniques with gradient echo sequences and ever-improving gradient systems may ultimately allow very thin slices to be obtained in a clinically acceptable length of time. Fast spin-echo techniques acquire T2­weighted images much more quickly than conventional spin-echo techniques. This decreases the chance that patient motion will degrade the image while maintaining the advantages of a T2­weighted image-that is, better discrimination of pathologic from normal tissues on the basis of variations in water content, and better visualization of cisterns. Further, these shorter imaging times make it possible to improve the image resolution by using expanded image matrices.

In summary, the optimal technique consists of using a head coil, sagittal and coronal T1-weighted images, the thinnest possible slices, pixel sizes approximately 0.8 mm on a side, and four signal averages. This allows the entire region to be covered in two planes, and examinations are kept briefer than 30 min. If necessary to improve the visibility of the lesion, a paramagnetic contrast agent is injected intravenously, and the coronal T1-weighted sequence is repeated. Occasionally, a supplementary T2-weighted fast spin-echo sequence is used if the T1-weighted sequences do not show any abnormality and there is compelling reason to believe that a lesion has been overlooked.

Normal Anatomy

The sella turcica is a midline depression in the sphenoid bone which contains the anterior and posterior lobes of the pituitary gland and the distal portion of the pituitary stalk. It is covered by a dural reflection, the diaphragma sellae. Above this lies the supra­sellar cistern, which contains the supraclinoid carotid arteries and the optic tract, chiasm, and nerves, and through which travels the pituitary stalk, Lateral to the sella turcica are the cavernous sinuses containing the carotid arteries, cranial nerves III, IV, and VI, and the first two divisions of the fifth cranial nerve. Anteriorly, the sella turcica is bound by the tuberculum sellae and anterolaterally by the anterior clinoid processes, Anteroinferiorly, the foramen rotundum conducts the maxillary branch of cranial nerve V. Posteriorly, there are the smaller posterior clinoid processes, the dorsum sellae, and the interpeduncular cistern containing cranial nerves III and IV. Inferiorly, the sella turcica has a thin floor of cortical bone, below which lies the air-containing sphenoid sinus. The sinus is extremely variable in size. Adjacent to the posteroinferior aspect of the cavernous sinus lies Meckel's cave, containing the gasserian ganglion. Immediately below and lateral to the gasserian ganglion, the third branch of the fifth cranial nerve exits through the foramen ovale.

Contrast-enhanced CT scans will demonstrate the arteries of the circle of Willis and the pituitary stalk as enhancing suprasellar structures. The pituitary stalk is usually 1 mm in diameter and is always smaller than the adjacent basilar artery. The optic chiasm is occasionally visualized anterior to the pituitary stalk. Any additional enhancing or calcific suprasellar structures should be viewed with suspicion. CT cisternography makes the structures in the suprasellar cistern more visible by surrounding them with iodinated contrast material. They appear as dark "filling defects" in the opacified suprasellar cistern. Suprasellar mass lesions are also well demonstrated as additional filling defects displacing the normal structures.

The pituitary gland is best seen on coronal projections as a homogenously enhancing intrasellar structure with a flat or concave superior border. The pituitary stalk inserts into the superior aspect of the gland, usually in the midline. The cavernous sinuses are symmetric structures on either side of the pituitary gland and enhance to approximately the same degree.

T1-weighted MR images are currently the best method for demonstrating the anatomy of the sella and parasellar regions. The midline sagittal section is extremely useful in delineating a great deal of this anatomy. The pituitary gland is seen in the hemispherical sella turcica. The anterior lobe of the gland is intermediate gray, very similar in intensity to the white matter of the cerebral hemispheres. The posterior lobe is much smaller and is typically nestled in a small depression in the dorsum. It is rather unique in being very bright on T1-weighted sections. The pituitary stalk is seen to angle anteriorly as it descends from the hypothalamus to the pituitary gland. Normally, the stalk is approximately 1 mm in transverse diameter. The size of the pituitary gland is variable. The maximum normal diameter is generally accepted as approximately 9 to I0 mm, although the gland may be up to 12 mm in diameter in late pregnancy and the early postpartum period.

The optic chiasm is seen immediately anterior to the pituitary stalk in the suprasellar cistern. The CSF in the suprasellar cistern is dark gray. The third ventricle and its inferior recesses, the optic and infundibular recesses, are well seen immediately above the optic chiasm and infundibulum, respectively. The floor of the sella turcica, formed from thin cortical bone, is difficult to appreciate, particularly if the sphenoid sinus is large. There is no appreciable contrast interface between dark cortical bone and the dark air­containing sinus. Parasagittal sections are of limited usefulness, primarily because they include the cavernous sinus and carotid arteries, which loop through this area. The presence of these structures causes troublesome partial volume averaging effects, which may lead to misinterpretation of images.

Coronal images are particularly useful for evaluating the pituitary gland and cavernous sinuses. The coronal section through the midbody of the pituitary gland generally demonstrates symmetry about the stalk. The superior surface of the gland may be either flat, concave, or convex. Upward convexity is in itself not indicative of an intrapituitary mass, because such a convexity is often present at the point where the infundibulum inserts into the gland. Above the gland, the fibrous diaphragma sellae is generally not seen as a distinct structure because the CSF of the suprasellar cistern lies immediately above it.

Because these materials have approximately the same signal intensity (dark gray on T1-weighted images), the thin diaphragma is seen poorly if at all. The supraclinoid carotid arteries and their bifurcations are seen as regions of signal void (due to flowing blood) in the suprasellar cistern. The optic chiasm is a biconcave disc in cross section. It has approximately the same signal intensity as the pituitary gland. Immediately above it is the hypothalamus, which forms the inferior and lateral walls of the third ventricle. Only a thin layer of CSF separates the optic chiasm from the hypothalamus, but they can be distinctly seen as separate structures. The clinoid processes, both anterior and posterior, are variable in size and in the amount of marrow they contain. Typically, the anterior processes are larger. They have a high-intensity center surrounded by a dark ring of cortical bone.

Laterally lie the cavernous sinuses. The carotid artery is the most prominent structure in the cavernous sinus. It is circular in coronal cross section and is devoid of signal. The remainder of the sinus is composed of venous channels, septa, and cranial nerves. Cranial nerves III and IV and the ophthalmic branch of V have a consistent anatomic relationship to the carotid artery, being supero­lateral, directly lateral, and inferolateral to it, respectively. They can occasionally be seen in the lateral cavernous sinus wall on MRI The sixth cranial nerve lies in the sinus itself and is generally too small to be seen consistently. The mandibular branch of the fifth nerve is well visualized as it exists from the gasserian ganglion through the foramen ovale. Similarly, the maxillary branch of the nerve can reliably be seen in the foramen rotundum. The venous channels of the cavernous sinus itself have a rather heterogeneous appearance. Regions of flow void are not consistently seen, perhaps because the flow is too sluggish or because the channels are too small. The lateral cavernous sinus wall together with the CSF in the medial middle cranial fossa form a low-signal­intensity boundary between the medial temporal lobe and the sinus. The medial cavernous sinus wall is extremely thin and cannot be resolved as a distinct structure between the pituitary gland and the cavernous sinus proper.

Meckel's cave is immediately lateral to the posterior portion of the cavernous sinus. Frequently, the gasserian ganglion can be seen in it.

Pathology

General Considerations

The most common lesions in this region are the pituitary adenoma, craniopharyngioma, meningioma, carotid aneurysm, and optic and hypothalamic gliomas. Because each of these lesions arises from a relatively distinct anatomic site, a concise differential diagnosis can usually be established by localizing the lesion to a particular structure. The excellent anatomic detail on MRI facilitates this process. Analysis of the signal intensity may then further refine the differential diagnosis.

In addition to these common disorders, many uncommon neoplastic and infiltrative diseases affect this area. These include (but are not limited to) germinoma, lymphoma, leukaemia, chordoma, metastasis, nasopharyngeal carcinoma, sarcoidosis, and histiocytosis X.

Pituitary Adenomas

Pituitary adenomas are common, benign, epithelial tumors that arise from the anterior lobe of the pituitary gland. The clinical presentation and classification depends primarily on whether they are functioning (secretory) or nonfunctioning (non secretory). From the radiologic perspective, it is best to classify adenomas on the basis of size, those under 1 cm in diameter being considered microadenomas and those greater than 1 cm being considered macroadenomas.

The role of diagnostic imaging in the evaluation of microadenomas is to confirm the clinical diagnosis, to localize the tumor, and to determine the involvement of adjacent structures. The CT signs of microadenoma include (1) a focal low-density area in the pituitary gland; (2) increased gland size (>9 to 10 mm vertical height); and (3) asymmetric convexity of the superior surface of the gland with accompanying deviation of the pituitary stalk to the opposite side. An occasional microadenoma may show increased density owing to enhancement or, very rarely, to calcification. Erosion or remodelling of the pituitary floor is of limited help since it may be a normal finding. In patients with a known pituitary microadenoma, erosion or remodelling of the pituitary floor is a sign of inferior extension.

Thin-section T1-weighted spin-echo MR images are now accepted as the best way to image the patient with a clinically and biochemically suspected pituitary microadenoma. The typical microadenoma appears as an area of focal hypointensity within the intermediate intensity of the anterior lobe. It is usually well defined, with a distinct border. Most microadenomas are laterally situated. They may be associated with superior displacement of the upper gland surface (focal convexity upward) and/or displacement of the stalk to the opposite side. There may also be focal remodelling of the sella floor beneath the adenoma. These latter three signs are less reliable because quite often normal glands display similar features. The most reliable sign is that of the focal hypointensity. Quite often the difference in intensity between the small adenoma and the normal gland is slight. This often necessitates photographing the images at narrow windows or carefully reviewing the images on the operator's console. The use of paramagnetic contrast agents increases the sensitivity of MRI for the detection of micro adenomas. Although both the normal pituitary gland and the microadenoma are perfused and therefore enhance with paramagnetic contrast agents, they are not perfused initially to the same degree. Therefore, on immediate postinjection images (0 to 10 min after injection), the relative hypointensity of the microadenoma compared to the normal gland is increased. If delayed images are obtained, the contrast agent appears to slowly permeate into the lesion. The adenoma may then be hyperintense to the pituitary gland.

To date, there are no reliable ways to distinguish among the various types of microadenoma. Prolactin-secreting adenomas appear identical to ACTH- and growth hormone-secreting adenomas; their intensity depends in no way on the hormone they secrete. Approximately half of all hormonally active microadenomas are prolactin-secreting, and the remainder consist about equally of ACTH- and growth-hormone-secreting tumors. A small minority (less than 10 percent) are rarer types (TSH-. FSH-, or LH-secret­ing). The one pattern that is apparent is that the ACTH-secreting adenomas present at a smaller size, almost certainly because they cause significant clinical problems while they are small. This has been well established in the CT and surgical literature and has been the experience with MRI as well.

There has been general disagreement in the literature as to how accurate MRI is in the detection of microadenomas. A few earlier small series reported MRI sensitivities varying from 55 to 100 percent. It is now widely accepted that MRI is superior to CT in the assessment of pituitary microadenomas, particularly when a paramagnetic contrast agent is used.

It is probable that most false-negative examinations will continue to involve small adenomas, particularly in the case of Cushing's disease. One surgical series, reporting on the size of ACTH­secreting microadenomas, noted that the median size was 3 mm. Conventional spin-echo MRI, currently limited to 2 or 3 mm in slice thickness, cannot reliably detect lesions that are smaller than the slice is thick. It is hoped that volume-imaging techniques will ultimately overcome this problem by providing very thin slices that are clinically acceptable.

False-positive examinations for microadenomas may result from both CT and MRI. Small pars intermedia cysts, clinically silent pituitary infarcts, and foci of necrosis may be incidental findings and may be confused with microadenomas by both imaging techniques.

Macroadenomas are well seen on MRI, as they are on CT. The goals in imaging patients with presumed pituitary macroadenoma include diagnosis, differential diagnosis, staging, and evaluation of adjacent anatomic structures. Contrast-enhanced CT will show a large, enhancing soft tissue mass originating in the sella with extension into the suprasellar cistern. The sella turcica is usually enlarged. Enhancement is usually homogenous, with non­enhancing "cystic" foci also seen occasionally. Erosion and expansion of the sellar floor as well as inferior extension into the sphenoid sinus may also be seen. CT is better than MRI for evaluating bony structures adjacent to the adenoma and for detecting calcification (rare) in association with a macroadenoma. Whereas the detection of larger adenomas by CT is straightforward, the differential diagnosis can be difficult. Occasionally, the CT appearance of a pituitary adenoma is indistinguishable from that of a meningioma, a parasellar aneurysm, or even a craniopharyngioma.

The advantages of MRl over CT are better visualization of the carotid arteries and optic chiasm and direct multi planar display of the tumor in relation to parasellar structures. In most patients, the MRI appearance of a macroadenoma is that of a soft tissue mass that is centered in the pituitary fossa and is of intermediate signal intensity (slightly darker than gray matter) on T1-weighted sequences. The mass usually becomes hyperintense on T2-weighted images and enhances diffusely when paramagnetic contrast material is injected intravenously. The important clinical distinction between an adenoma and an aneurysm is easily made on the basis of the MRI appearance of blood and blood vessels. The aneurysm is either black, owing to rapidly flowing blood, or very bright if it is thrombosed. Therefore. with MRI it is possible to confidently exclude the presence of a significant vascular abnormality that would preclude trans-sphenoidal surgery.

The ability to obtain images in multiple planes is useful for determining the extrasellar extent and involvement of adjacent structures. Superior extension is the most common route of spread outside the sella turcica. Usually a waistlike constriction can be seen around the tumor at the point where it extends through the diaphragma and above the sella turcica. The relationship of the optic chiasm and the supraclinoid carotid arteries to the superior aspect of the tumor is well seen.

Although the superior extent can be delineated precisely, lateral extension is more problematic because the medial cavernous sinus wall cannot be reliably visualized. Because of this, it is difficult to confidently confirm or exclude the extension of an adenoma into the cavernous sinus, unless such extension is gross, in which case the diagnosis is readily evident. In gross cavernous sinus extension, prominent lateral bowing of the lateral cavernous sinus wall is observed.

In cavernous sinus extension, the relationship of the carotid artery to the tumor and the effect of the tumor on the vessel lumen is easily seen owing to the high contrast provided by the flow void in the carotid artery lumen. Although pituitary adenomas quite often extend into the cavernous sinus, it is rare to see constriction of the carotid lumen, the more common finding being displacement or encirclement of the vessel without constriction.

Inferior extension of an adenoma causes either remodelling of the floor of the sella turcica or frank erosion through the floor into the sphenoid sinus or sphenoid bone. It may be difficult if not impossible to determine whether the inferiorly extending adenoma is eroding through the bone of the floor or merely remodelling it, because there is no contrast between the cortical bone and the air-containing sinus. Inferior extension is seen as asymmetric downward protrusion of a soft tissue mass from the inferior aspect of the gland.

There is a subgroup of tumors, the invasive adenomas, that preferentially grow through the rigid bony floor or into the cavernous sinus rather than taking the path of least resistance into the suprasellar cistern. This type of invasive biological behaviour is well recognized but unexplained. Many of these are microadenomas. They carry a worse prognosis because complete surgical resection is difficult. They do not have any distinguishing imaging features other than their pattern of growth.

Hemorrhage into adenomas is a frequent observation on MRI. Whereas previously it was thought that intrapituitary hemorrhage was seen only in the clinical syndrome of pituitary apoplexy, it is apparent that hemorrhage into an adenoma may be clinically occult. Very often portions of adenomas, both large and small, can be seen to have hemorrhagic foci. These were previously unappreciated but are now being detected with increasing frequency because of the high soft tissue contrast afforded by MRI. In true cases of pituitary apoplexy, a large hemorrhage typically oc­curs in a relatively large adenoma. The diagnosis is evident both clinically and on the MRI scan, which shows an area of high signal intensity representing hemorrhage into the gland. Pituitary apoplexy is also recognized to occur in the postpartum female.

An area of continued difficulty in pituitary and parasellar imaging is that of the postoperative examination in search of residual or recurrent pituitary adenoma. In these cases, it may be very difficult to distinguish postoperative granulation tissue, scarring, or graft material from the normal gland or residual adenomatous tissue. This is especially true in the first 6 months after surgery. In these cases, progressive growth of a soft tissue mass on sequential post­operative MR scans is the best imaging sign of recurrent tumor. This sign must be interpreted in conjunction with endocrinologic markers.

Craniopharyngiomas

Craniopharyngioma is another common benign tumor of the para­sellar region. There is a well-known bimodal age distribution. Children are affected most often; there is a second, smaller incidence peak in the fifth and sixth decades of life. These tumors arise from epithelial remnants of Rathke's cleft. Their gross pathology is variable even in the same tumor, where there may be solid tissue, various types of cyst, and calcification. This is reflected in their CT and MRI appearance, where virtually any pattern is possible. Most are closely related to the pituitary infundibulum, usually on its anterior aspect. Most of these tumors are suprasellar in location, but they may extend down into the sella turcica. A minority are entirely intrasellar. Rarely they occur purely within the third ventricle. They may be invasive locally, and it is sometimes impossible to see clean tissue planes between the tumor and adjacent structures, such as the optic chiasm, hypothalamus, uncus, and pituitary gland.

On CT, craniopharyngiomas usually appear as a mass of heterogeneous density centered in the suprasellar cistern. They often contain cysts of variable density (usually low density), as well as calcifications. The calcifications may take the form of focal clumps or of calcifications in the margins of cystS.9 Contrast enhancement typically occurs in the solid soft tissue components of the mass or at the periphery of cysts. These lesions show variable signal intensities on MRI, reflecting the various tumor constituents. Usually a cyst is seen, but the intensity of the cyst may be high or low on the T1-weighted images. Low intensity indicates the presence of a serous type of fluid or, as reported in one case, keratin. The cyst is more often extremely bright. This high signal intensity is found in the "machine oil" cysts. The cause of the high signal intensity is thought to be hemorrhagic products and/or highly proteinaceous fluid. Solid tumor components do not have any particularly unique characteristics. If calcification is present and is dense, it will be seen as a region of signal dropout. However, MRI is relatively insensitive to the presence of calcium, and lesser degrees of calcification may be entirely overlooked.

These tumors can attain a very large size and may extend laterally into the middle fossa, superiorly to invaginate the third ventricle, or posteriorly into the interpeduncular and prepontine cisterns. As a general rule, a large suprasellar mass with a variety of components is most likely to be a craniopharyngioma. The classic case is a suprasellar mass in which a high-intensity cyst is combined with small amounts of solid tissue and/or calcification. Cases in which there are both suprasellar and intrasellar components and in which no calcifications can be seen may be difficult to distinguish from a pituitary macroadenoma.

Rathke's Cleft Cysts

The nomenclature relating to cysts derived from Rathke's cleft is confusing. The simple epithelial cyst, the colloid cyst of the pituitary gland, and the pars intermedia cyst all have very similar histology. These cysts all have an epithelial wall with a single cell layer. For the purposes of this discussion, they may be considered.

Meningiomas

Meningiomas of the parasellar region may arise from the tuberculum sellae, planum sphenoidale, anterior clinoid, medial sphenoid wing, or posterior clinoid. On CT, they appear as isodense or mildly hyperdense extra-axial mass lesions that are well marginated and enhance intensely and homogenously following the injection of an iodinated contrast agent. They frequently have a broad­based dural attachment and show dural thickening at their margins, and they occasionally show foci of calcification. This calcification may be clumpy and amorphous, diffuse ("psammomatous"), or circumferential. Meningiomas often cause sclerosis of bone (enostosis) along the site of dural attachment, which is best appreciated on noncontrast CT.

The early experience with using MRI for evaluation of meningiomas was unfavourable, owing to the propensity of these lesions to be isointense with brain on most conventional spin-echo sequences, and thus difficult to distinguish from normal adjacent brain. Most meningiomas are isointense or slightly hypointense on T1-weighted sequences. Approximately 50 percent remain isointense on T2-weighted sequences, and 40 percent become hyperintense. The remaining 10 percent show hypointensity on T2­weighted sequences, which often correlates with a very fibrotic or calcified lesion. With isointense meningiomas, one must often rely on indirect signs for detection of the tumor, such as mass effect, dural thickening, buckling of adjacent white matter, white matter edema, and hyperostosis. A black rim may be seen around the lesion, which is thought to represent compressed veins or a CSF cleft. MRI will often demonstrate enlarged blood vessels in the tumor or at its periphery. Occasionally, the dural vascular supply to these lesions at the point of dural attachment may also be seen.

Lacking a blood-brain barrier, these tumors will usually enhance homogenously and intensely following the injection of paramagnetic contrast material. They have a tendency to encase arteries and invade venous structures. As well as encasement, meningiomas have a tendency to constrict the lumen of the vessel, a pattern very rarely seen in other parasellar neoplasms. An enhancing rim of thickened dura ("dural tail sign") can often be identified at the margins of the broad-based dural attachment of the mass. Although highly suggestive, this sign is not specific for meningiomas.

Although MRI has become the imaging method of choice in the investigation of parasellar tumors, differentiating pituitary macro­adenoma from parasellar meningioma remains sometimes quite difficult. Occasionally, the pituitary gland may be visualized directly as a separate intrasellar structure, separated from the supra­sellar meningioma by the diaphragma sellae. Albeit not pathognomonic, the features of homogenous enhancement, an epicentre in the suprasellar cistern, and an enhancing dural margin all favour meningioma over macroadenoma.

Aneurysms

Aneurysms that involve the parasellar region usually arise from the cavernous or supraclinoid portion of the internal carotid artery. On occasion, a basilar tip aneurysm may project into the suprasellar region. A CT scan may show a well-defined mass of slightly increased density, often having curvilinear calcification in its walls. The patent lumen of the aneurysm will enhance brightly following the injection of an iodinated contrast agent, and continuity with a known vascular structure can often be identified. Portions of the aneurysm may show lack of contrast enhancement, indicating a mural thrombus.

Vessels and vascular abnormalities are extremely well delineated on MRl because of the natural contrast between the signal void of flowing blood and the higher signal intensities of adjacent tissue. On conventional spin-echo MRI, a non thrombosed aneurysm is black, has well-defined margins, and is contiguous to a vessel. Thrombus in an aneurysm usually appears as an area of multilamellated high signal intensity on the T1-weighted images and usually has a dark rim on the T2-weighted sections. This dark rim is thought to be due to hemosiderin in the wall of the aneurysm or in the adjacent brain. It is also possible to create "flow-sensitive" sequences on MRI usually using gradient echo or phase contrast techniques. These sequences will display areas of high signal intensity (brightness) in rapidly flowing arterial structures, thus confining a vascular lesion. The flow-sensitive images can be further combined and interpolated by three-dimensional computer reformation techniques to create a "magnetic resonance angiogram" (MRA). The MRA images increase the sensitivity for detection of aneurysms and aid in further defining their relationship to adjacent vascular structures. Currently, however, a normal MRI or MRA examination cannot exclude a small parasellar aneurysm, and selective cerebral angiography remains the procedure of choice for these lesions.

Optic Gliomas and Hypothalamic Gliomas

Optic and hypothalamic gliomas may be indistinguishable from one another radiologically and surgically. Optic gliomas may extend superiorly into the hypothalamus, and hypo thalamic gliomas may extend downward to incorporate the optic chiasm or tract. When lesions are smaller, the optic chiasm and hypothalamus can be separated from one another and the diagnosis is easier.

Optic gliomas occur in children and adults. They may involve the intraorbital or intracranial portions of the nerve. In children, they are usually seen in patients with neurofibromatosis and are often bilateral. CT and MRI demonstrate focal or diffuse enlargement of the optic chiasm and/or optic nerves. When CT is used, these lesions are best seen when outlined with intrathecal contrast (CT cisternography). However, the advent of MRI has made this unnecessary. The lesions enhance to a variable extent following the injection of a contrast medium. Calcification is uncommon and is best seen on CT. The thickened optic nerve and/or chiasm is usually isointense on T1-weighted MR images. On T2-weighted sections it is of higher signal intensity than normal nerve. Oblique images directly along the long axis of the optic nerve are extremely beneficial in determining the transition between normal and abnormal optic nerve. There are often abnormalities in the optic radiations of such patients. In many cases this represents tumor extension along the optic radiations. However, it has become apparent over the past several years that patients with neurofibromatosis often have multiple white matter abnormalities in anatomic areas remote from the optic radiations. These are presumed to be either hamartomas or dysplastic white matter. It is therefore difficult to stage the posterior extent of optic gliomas in neurofibromatosis patients, because it is impossible to distinguish what is presumably a hamartoma from an infiltrating glioma. The adult optic glioma tends to have a more aggressive behaviour, but the imaging findings are identical to those seen in childhood.

The hypothalamic glioma may be histologically identical to the optic glioma. It too can present as a suprasellar mass, and when it does so, it is difficult to separate from the optic nerve. Useful differential imaging features are that hypothalamic gliomas do not show the propensity of optic gliomas to grow along the optic pathways or posteriorly along the optic radiations. Clinically, hypothalamic gliomas present primarily with hypothalamic dysfunction, and optic gliomas with visual loss. This may be the most useful differentiating point. MRI is helpful in delineating the extent of a hypothalamic tumor in the brain and suprasellar cistern

The Empty Sella Turcica

An "empty" sella turcica is actually a large sella that is filled with CSF extending downward from the suprasellar cistern. It develops in response to a larger than normal hiatus in the diaphragma sellae, which exposes the sella turcica to the transmitted pulsations of CSF in the suprasellar cistern. This may cause enlargement of the sella and flattening of the pituitary gland. The findings on MRI, and on CT for that matter, are a large sella turcica occupied by CSF. The pituitary gland is flattened along the floor of the sella turcica, usually in the posteroinferior portion. The pituitary stalk can be seen to traverse this CSF space from the median eminence of the hypothalamus down to the flattened pituitary gland. This is an important feature to ascertain, because it virtually excludes the possibility that the sella turcica is occupied by a space-occupying cyst. Cysts and other space-occupying lesions deviate the stalk away from its normal course.

Chordomas and Chondrosarcomas

A cranial chordoma is a tumor derived from primitive notochordal remnants and usually presents as a destructive, locally infiltrative mass originating in the clivus. It is typically a midline mass, which expands the clivus and can eventually break through the posterior clival cortex to extend intracranially and compress adjacent anatomic structures. Chordomas can also extend laterally into the cavernous sinuses, anteriorly into the sella, or inferiorly into the nasopharynx. The lesions commonly show foci of sequestered bone or irregular calcification amid the destroyed clival marrow, all best examined on noncontrast CT. Chordomas are isointense or hypo­intense on T1-weighted MR scans, replacing the normally bright clival marrow. They enhance to a variable degree following contrast injection, and are heterogeneously hyperintense on T2­weighted scans. Fibrous connective tissue strands may be seen occasionally and give the tumor a "lobulated" appearance. Areas of calcification appear on MRI as dark foci in the soft tissue components of the tumor.

Chondrosarcomas of the clivus may be radiologically indistinguishable from chordomas. They are more apt to occur laterally, often centered on the petroclival suture.

Dermoids and Epidermoids

Dermoids and epidermoids are benign, slow-growing "inclusion tumors." They are thought to result from the growth of epithelial remnants that became entrapped during neural tube closure. Epidermoids are cysts of stratified squamous epithelium containing waxy desquamative keratin products, whereas dermoids also contain more complex dermal appendages and fat. Epidermoids are usually isodense to CSF on CT and do not enhance with contrast. As they expand, they cause compression of neural structures, often insinuating into the recesses and crevices of the cisternal spaces. Epidermoids are typically slightly hyperintense to CSF and heterogeneous on T1-weighted MR images, becoming very bright on T2-weighted sections. They may mimic CSF very closely and are often best seen on intermediate ("proton density") spin-echo sequences. The presence of fat or calcium demonstrated by either CT or MRI is more suggestive of a dermoid. These lesions occasionally rupture into the subarachnoid space, giving rise to a chemical meningitis. In these cases, MRI may show the cyst contents disseminated in the subarachnoid space.

Neuromas

Intracranial schwannomas or neurofibromas may arise from the cranial nerves of the cavernous sinus, usually the fifth cranial nerve. They appear as well-defined, slowly expansile paracavernous soft tissue masses that follow the course of their nerve of origin. They may extend from the cavernous sinus into the orbit, through the skull base, or into the posterior fossa. Neuromas commonly show well-corticated remodelling (erosion) of the adjacent bone indicating slow growth, and may enlarge bony foramina. They are usually solid and enhance brightly following contrast administration on CT or MRI. Enhancement may be heterogeneous, particularly because the tumor can undergo cystic degeneration or central necrosis.

Germinomas

Germinomas are tumors of germ cell origin usually seen in children or young adults. They present with diabetes insipidus or visual disturbances. Although these tumors are most commonly located in the pineal region. a minority of them present as suprasellar masses, either in isolation or combined with a pineal lesion.

These suprasellar germinomas are typically midline lesions centered at or just behind the pituitary infundibulum. They are usually homogenous on CT or MRI and only rarely contain calcium or cystic components. On MRI, their signal intensity varies only slightly from that of the brain; they are mildly hypointense on T1-weighted sections and hyperintense on the T2-weighted series. They enhance brightly and homogenously with administration of a contrast material. Gadolinium-enhanced MRI is most useful in demonstrating the extent of local invasion or dissemination to the leptomeninges. Other, rarer germ cell tumors (e.g., teratomas, embryonal carcinomas, etc.) show more heterogeneity owing to their more varied histologic elements.

Inflammatory Lesions

The most common inflammatory lesions of the parasellar region involve the leptomeninges of the suprasellar cistern and are usually part of a disseminated basal meningitis. Tuberculosis, fungal infec­tions, sarcoidosis, histiocytosis X, and other chronic basal meningitides may affect the suprasellar cistern. Each of these can cause thickening of the pituitary stalk, a suprasellar mass, or focal lesions in the inferior aspect of the brain. Contrast-enhanced MRI is the most sensitive imaging modality in these cases, often showing diffuse or patchy enhancement of the basal leptomeninges. This pattern is nonspecific and can also be seen in leptomeningial spread of tumors. Parasitic cysts, most commonly cysticercosis, can also affect the suprasellar cistern.

Pituitary abscesses are uncommon, and their imaging features are often indistinguishable from those of adenoma, making a correct preoperative diagnosis difficult. They usually present as a sellar mass that shows rim enhancement following contrast administration.

Lymphocytic hypophysitis is a rare, presumably autoimmune, inflammatory disorder usually associated with the peripartum stages of pregnancy. CT and MRI show diffuse enlargement of the anterior lobe of the pituitary gland.

The Tolosa-Hunt syndrome refers to painful ophthalmoplegia caused by an inflammatory process in the cavernous sinus. MRI may show asymmetry of the cavernous sinuses. The syndrome is exquisitely responsive to steroid administration.

Optic neuritis can simulate an optic nerve tumor. Both conditions cause nerve enlargement. They can be distinguished by interval follow-up: the untreated glioma will continue to grow, albeit usually slowly, whereas in neuritis, the nerve will return to normal in size and may actually become atrophic.

Metastases and Other Neoplasms

A variety of other tumors occur in the sphenoid bone and clivus. These include skull base metastasis, nasopharyngeal or sphenoid sinus carcinoma, lymphoma, and plasmocytoma. The intensities of these lesions are not sufficiently distinct to permit confident differentiation radiologically.

Nasopharyngeal carcinoma may extend upward into the bones of the skull base. Usually the primary lesion can be seen in the nasopharynx, and for this reason the nasopharynx must be closely scrutinized when abnormalities are seen in the sphenoid bone and clivus. An extra-axial lymphoma may present as a solitary parasellar mass or with diffuse leptomeningial dissemination, which is best seen on contrast-enhanced MRI. Metastases may affect any bone in the skull base. The primary tumors most often associated with skull base metastases are carcinomas of the lung, breast, and kidney. High-resolution CT with bone reconstruction algorithms are best at defining the anatomic extent of cortical bone destruction. Although MRI is limited in its ability to resolve bone, the extent of marrow infiltration and any intracranial extension is well demonstrated. All metastatic tumors that affect the skull base have certain MR imaging features in common. T1-weighted sequences typically demonstrate low-signal tumor tissue replacing the normally bright marrow fat. On T2-weighted sections, the tumor is usually hyperintense to marrow, although the degree of hyperintensity varies.

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