Evaluation of the Suprasellar Cistern by Computed Tomography JONATHAN J. DUTTON, MD, PhD,* TERENCE G. KLINGELE, MD,* RONALD M. BURDE, MD,*t MOKHTAR GADO, MD:j:

Abstract: The normal suprasellar cistern is a five- or six-pointed, star­shaped, fluid-filled structure as demonstrated by computerized axial tomography (CAT). At various levels the normal suprasellar cistern con­tains the major intracranial vessels and their anastomotic channels, the optic nerves, chiasm, and infundibular stalk. The existence of lesions, either intrinsic structures of or extrinsic structures contiguous to the su­prasellar cistern can be detected by their effect on the normal anatomy of the suprasellar cistern or by filling defects produced when studied with metrizamide cisternography. [Key words: computerized tomography (CT), optic chiasm, pituitary gland, pituitary stalk, suprasellar cistern.] Ophthalmology 89: 1220-1225, 1982

Computed tomography (CT) is a safe, noninvasive radiologic technique that has revolutionized the clini­cal evaluation of intracranial structures. Ov~r the past decade it has proven to be equal or superior to other methods of examination with a diagnostic accuracy for intracranial mass lesions of some 70 to 93%.1-4 It is currently the method of choice as the initial study for central nervous system lesions. 5-9 For the ophthal­mologist, CT is most useful in the evaluation of orbital disease and perichiasmal pathology. Visual distur­bances may be observed in 70 to 90% of patients with suprasellar mass lesions. 4 ,10 Therefore, it 'is essential that the ophthalmologist be familiar with radiologic techniques of evaluating this region.

The CT appearance of the suprasellar area, both in the normal and pathologic states, has been described in the radiologic literature, 1,3,10-12 but detailed descrip­tions have not been generally available to the ophthal­mologist. It is the purpose ofthis paper to illustrate the normal suprasellar cistern as visualized by modern CT scanning techniques.

From the Departments of Ophthalmology,* Neurology and Neurological Surgery,t (RMB), and Radiology,:j: Washington Uni­versity School of Medicine, St. Louis, Missouri.

Supported in part by a grant from Research to Prevent Blindness, Inc., New York, New York (Department of Ophthalmology).

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SCANNING METHODS

Computed tomography provides reconstructed vi­sual imaging of intracranial structures based on density averaging of tissues included within the scanned sec­tions. For optimal evaluation of the optic chiasm and suprasellar cistern, both axial and coronal sections are desirable. 12- 14 Axial sections taken parallel (at zero degrees) to Reid's baseline (defined as the orbital­meatal plane), as used for orbital and some intracranial scans, frequently do not provide adequate imaging of the suprasellar area. In such cuts the superimposition of the adjacent dorsum sellae and sphenoid sinuses results in artifactual image degradation from partial volume density averaging. 8 Therefore, it is necessary to obtain axial sections at +25° to +30° to the orbital­meatal plane as used for routine brain scans. In the latter planes the chiasm is surrounded by cerebrospinal fluid within the suprasellar cistern, thus improving its visualization. The optic nerves are best seen in -10° sections, and the optic tracts in zero to +30° cuts. 12

Coronal sections are essential for the evaluation of extension of mass lesions extrinsic to the chiasml2,14 and for visualization of the cavernous sinus. 13 The pi­tuitary gland itself is difficult to observe but is best demonstrated with very thin (2- to 4-mm) coronal sec-

0161-6420/82/1100/1220/$1.10 © American Academy of Ophthalmology

DunON, et al • CT EVALUATION OF SUPRASELLAR CISTERN

tions. 13 The thickness of sections is important for image quality and, therefore, resolution of small jux­tasellar lesions. In general, adjacent thin sections of 1.5 to 8.0 mm provide excellent visualization of these areas. 12-14

Contrast enhancement provides valuable informa­tion in the evaluation of specific suprasellar lesions and for delimiting the anatomic details of vascular malformations, aneurysms, and vascularized tu­mors. 1,4,10,I1 , 13 The use of metrizamide cisternog­raphy further increases the capability of CT scanning techniques by allowing better visualization of normal intracisternal structures such as optic nerves, chiasm, infundibulum, and tuber cinereum, as well as out­lining abnormal intrinsic or extrinsic mass lesions in­volving the cistern. 14

THE NORMAL SUPRASELLAR CISTERN

The normal suprasellar cistern is seen on CT scan­ning as a five- or six-pointed star-shaped, fluid density structure located behind the sphenoid sinus, near the floor of the middle cranial fossa. These star points rep­resent cerebrospinal fluid-filled extensions of the su­prasellar cisternal complex and are distinguished eas­ily from the surrounding denser brain parenchyma. Extending in the midline anteriorly is the inter­hemispheric fissure (Fig I). Anterolaterally are the paired Sylvian cisterns and posterolaterally the paired ambient or crural cisterns. In the posterior midline, there is often a shallow depression in the brain stem, the interpeduncular fissure. Forming the solid bound­aries of the suprasellar cistern are the left and right gyrus recti of the frontal lobes anteriorly, the left and right unci of the temporal lobes laterally, and the pons and cerebral peduncles posteriorly (Fig 1). In the ab­sence of pathology these structures are normally bilat­erally symmetrical. Inferiorly the cistern is bounded by the diaphragma sella, sella turcica, and pituitary gland. Superiorly from front to back are the anterior perforated substance, tuber cinereum, mamillary bodies, and lamina terminalis.

Within the suprasellar cistern there are many normal structures that must be recognized as such, thus dis­tinguishing them from abnormal lesions (Fig. 2). Around the perimeter of the cistern are vascular chan­nels best visualized after contrast enhancement. Pos­teriorly, within the interpeduncular fissure, is the basilar artery (Fig 2). Extending posterolaterally from the basilar artery into the ambient cisterns are the paired posterior cerebral arteries. Laterally, at the ori­gin of the Sylvian cisterns, are the left and right carotid arteries giving rise immediately to the middle cerebral arteries that course within the Sylvian sulci. The small posterior communicating arteries join the posterior ce­rebral to the internal carotid artery. Small paired anterior cerebral arteries extend medially and forward from the middle cerebral arteries into the inter­hemispheric fissure. They are joined at the midline

Fig 1. Cut section of brain corresponding to optimal plane of com­puterized tomography (approximately +25 to +30 degrees from orbital-meatal plane) for imaging of suprasellar cistern. I, right gyrus rectus ; 2, right uncus ; 3, brain stem; 4, chiasm; 5, interhemispheric fissure ; 6, Sylvian cistern ; 7, ambient or crural cistern.

before they extend superiorly by the anterior com­municating artery that completes an anastomotic ring within the suprasellar cistern, the circle of Willis (Fig 2).

Several important structures are located within the suprasellar cistern that appear as tissue density masses on routine CT studies (Figs 3B, C, F) or as filling de­fects on metrizamide cisternography (Figs 3D, E, H). Anteriorly are the optic nerves as they enter the mid­dle cranial fossa through the optic canals (Fig 3H). These are best seen on low sections and are normally not well visualized on routine axial cuts. The optic nerves join to form the optic chiasm (Figs 3F, G). On low sections through the cistern the inferior chiasm appears as a V-shaped tissue density (Fig 3G). On midlevel cuts this becomes dumbbell-shaped (Fig 3F), and on high sections the chiasm is boomerang-shaped (Figs 3D, E) with the optic tracts seen extending po sterol ate rally (Figs 3B, C).12 The normal dimensions of the optic chiasm on axial scans are given by Daniels and associates 14 as 1.8 cm (range 1.2-2.7 cm) in the transverse diameter, 0.4 cm (range 0.3-0.6 cm) in the

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OPHTHALMOLOGY • NOVEMBER 1982 • VOLUME 89 • NUMBER 11

Interhemispheric fissure

optic chiasm ----~

~Wf!1'r-"l1l"'- in fun d i b u I u m & tuber cinereum

mammillary body

Fig 2. SchemaLic diagram of normal structures within the suprasellar cistern .

a anter ior cer ebral arter y m m iddle cerebral ar t e r y p posteri or cere bral artery c caroti d artery b basila r art ery

vertical diameter, and 0.8 cm (range 0.4-0.9 cm) in the anteroposterior diameter. On coronal sections they give 1.5 cm (0.9-1.8 cm) as the normal transverse di­ameter. These authors found that intrinsic chiasmal lesions such as a chiasmal glioma increase the vertical and anteroposterior but not the transverse dimension. They feel that any enlargement of the vertical dimen­sion greater than 0.6 cm is suggestive of tumor.

Immediately posterior to the chiasm is the infun­dibulum or pituitary stalk that strongly enhances with contrast (Figs 3E, G). On axial sections this appears as a round mass posterior to the chiasm. On higher sec­tions the infundibulum expands into a broad area of tissue representing the tuber cinereum of the hypothalamus (Fig 3D), and on even higher sections a central lumen (Fig 3C), the third ventricle, may be seen within the substance of the latter structure. Pos­terior to the tuber cinereum on high sections through the cistern lie the paired mamillary bodies (Fig 3C) immediately in front of the basilar artery and pontine eminence.

THE ABNORMAL SUPRASELLAR CISTERN

Of major interest to the ophthalmologist are mass lesions, either intrinsic or extrinsic, that compress the optic chiasm or tracts within the suprasellar cistern.

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When such lesions are present they may be detected on CT by distortions of the normal cisternal anatomy or as filling defects on metrizamide cisternography. The more common lesions affecting the suprasellar cistern frequently result in visual symptoms and in­clude pituitary adenomas, meningiomas, cranio­pharyngiomas, optic gliomas, and aneurysms. Less common lesions include epidermoid cysts, arachnoid cysts, chordomas, and metastatic tumors.

Pituitary adenomas that bulge into the suprasellar cistern are usually associated with enlargement of the sella turcica on plain skull films, and on the CT scan usually present as hyperdense lesions compared to brain tissue but may be iso- or hypo-dense as well. With anterior extension, there may be obliteration of the anterior star points of the cistern, and a filling de­fect on cisternography. With posterior extension there may be flattening of the pons and cerebral peduncles with distortion of the interpeduncular fissure. These tumors often erode the sella, and larger lesions may be visualized extending into the sphenoid sinus. Lobu­lated borders are also present. Pituitary adenomas nearly always become more apparent following intra­venous contrast injections (enhancement). They are rarely cystic or calcified. 10,11

Craniopharyngiomas are mixed density lesions. Sixty percent of these lesions contain areas of calcifi­cation being common in children and extremely rare in adults. Twenty-five percent are partially or even pre-

Third ventricle

Figs 3A-H. Montage of several computerized tomographic studies representing serial cuts from above downward at approximately 1- to 2-mm intervals. A and C are done without contrast material. B, F, and G are done with intravenous diatrizoate sodium contrast solutions. D, E, and H are done following subarachnoid placement of metrizamid contrast medium. Figure 3E-H continued on page 1224.

Fig 3. (Continued) 1224

DunON, et al • CT EVALUATION OF SUPRASELLAR CISTERN

dominantly cystic.l,lo.l1 Enhancement following the use of contrast media is variable, depending upon the nature of the specific lesion. Craniopharyngiomas often cause hydrocephalus.

Meningiomas of the olfactory groove, tuberculum sella, or sphenoid wing are usually well-defined, homogeneous mass lesions indenting the anterior or lateral areas of the suprasellar cistern. They are iso- or slightly hyper-dense with respect to brain tissue. Meningiomas can be calcified and often produce edema of the adjacent brain parenchyma. ILl5 They frequently cause hyperostosis of adjacent bony ele­ments. Meningiomas typically show marked contrast enhancement. 10

Gliomas involving the optic chiasm characteris­tically appear as an isodense enlargement of the chiasm or junctional portion of the optic nerve, as­sociated with splaying of the anterior cisternal star pattern. Extension into the optic tracts aids in distin­guishing these lesions from hypothalamic gliomas with which they may be confused easily. They rarely be­come calcified and usually show little or no enhance­ment with contrast.1.11

Aneurysms may affect any vessel within the su­prasellar cistern. Calcifications in the walls are often a telltale sign when present. These appear as rounded or angular, well-defined, homogeneous mass lesions continuous with a major vessel and show both en­hancement and enlargement with contrast. 11 Throm­bosed lesions may not enhance. Double-dose scans with immediate and delayed scanning as well as dynamic scanning may be helpful in these cases.

Finally, the suprasellar cistern may be deformed by shifting of the normal structures that delimit the cis­tern. Temporal lesions such as tumors or hematomas can shift the uncus into the cistern over the tentorial incisura. Frontal lesions such as meningiomas of the falx can cause bifrontallobe edema and rearward shift ofthe gyrus recti into the suprasellar cistern. 16 In cases of an empty sella, the chiasm may be seen protruding downward.

SUMMARY

While O1ost ophthalmologists may not feel qualified to make radiologic diagnoses, it is certainly advanta­geous to be able to recognize abnormal anatomy on the CT scan of intracranial regions of diagnostic impor­tance to ophthalmic practice. With good scanning techniques the suprasellar cistern should always be vi-

sualized. Although no diagnostic value can be placed on absolute size of the cistern, any deviation in its normal symmetry is highly suggestive of extrinsic compression. Similarly, an unexplained filling defect within the cistern should be considered a pathologic finding, especially if associated with signs of chiasmal compression.

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