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