Imaging of the Cavernous Sinus and Skull Base Steven A. Newman, MD

University of Virginia, Charlottesville, Virginia, United States

Learning Objectives: 1. Have a basic understanding of the skull base anato­my. 2. Enumerate the common pathologic processes that involve the skull base. 3. Know when MRI would not be the first choice for imaging of the skull base.

CME Questions: 1. What is the most common lesion involving the cav­ernous sinus?

A. Chordomas B. Pituitary tumors C. Meningiomas D. Neurilemomas

2. Besides detecting infarcts DWI sequences are par­ticularly useful in diagnosing?

A. Meningiomas B. Parasellar aneurysms C. Cholesterol containing lesions D. Arachnoid cysts

3. Which cranial nerve is not located in the lateral wall of the cavernous sinus?

A. Ill (oculomotor) B. IV (trochlear) C. V (trigeminal) D. VI(abducens)

4. Besides the Vlth nerve what other structure runs through Dorello's canal?

A. Inferior petrosal sinus B. Superior petrosal sinus C. Transverse sinus D. Sigmoid sinus

Key Words: 1. Cavernous Sinus 2. Parasellar 3. Central Skull Base

Introduction: For those bom in the imaging era it is hard to

conceive of a time when we could not look non-inva-sively into the cranium or orbit. This is particularly true for those areas of the skull base. This was no man's land where surgery was off limits. The first

advance in imaging, of course, came with Roentgen's description of x-rays in 1895. This permitted analy­sis of bony lesions with plain films and subsequent fluoroscopy. Lesions such as meningiomas that cause hyperostosis and pituitary lesions that enlarge the sella could be visualized on plain films. Contrast; however, was poor and soft tissue pathology could not be truly appreciated.

In 1919, Walter Dandy at John Hopkins improved intracranial contrast by injecting air into the ventricular system. This highlighted pathology that altered, shifted or obliterated the ventricular system. While this had only limited applicability for skull base lesions, pneumoencephalography did demon­strate the extent of large pituitary and other parasellar lesions. In 1923, Muniz, who was subsequently to win the Nobel Prize for his description of frontal lobotomy, introduced intravenous contrast. This inception of angiography permitted analysis of vascu­lar lesions involving the skull base but also outlined mass lesions by their effect on shifting the major ves­sels including the carotid artery, the Circle of Willis, and the distal vertebrobasilar system.

The modern era of imaging was introduced by Hounsfeld in the late 1960s. The first prototype CT scan (produced by EMI - - the Beatle's music compa­ny), which combined a rotating x-ray source, a detec­tor and a computer, was installed at the Atkinson Morley Hospital in London in the early 1970s. This was nothing short of revolutionary. My mentor in neuro-radiology returned from London stating he had seen the greatest advance since Roentgen. Two years later I had the opportunity to see for myself that he was not exaggerating. For the first time soft tissue detail could be determined in and around the sella and sphenoid bone. Detection of pathology was improved by the addition of iodinated contrast which breached the blood/brain barrier and outlined tumors through­out the intracranial cavity and orbit especially those previously inaccessible in the area of the middle skull base.

Currently the most important advance in imaging technology was the development of magnet­ic resonance imaging, which has provided exquisite detail and specific information regarding intracranial

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lesions. It is particularly good for those areas where CT scanning is poor including next to bone, in the area of the orbital apex, and in the posterior fossa. MR also offers the advantage of multiple sequencing which permits more details regarding the specific pathology of intracranial neoplastic and inflammato­ry pathology. Paramagnetic contrast (gadolinium pentalate) further highlights lesions of the skull base by changing the imaging characteristics particularly on T-l weighted images. This was a major advance in permitting assessment of the extent of meningiomas since prior to gadolinium, meningiomas were often iso-intense to gray matter both on T-1 and T-2 weight­ed images. With gadolinium; however, meningiomas could easily be outlined including their frequent dural extensions (dural tails). The impact of CT and MR on our ability to realize pathology in this region can be best appreciated by a statement that, "Before CT and MR imaging the extent of tumor of was determined at the time of surgery, and the morbidity of the extent or approaches needed to stage a tumor was considered unwarranted in many cases." Thus the advent of modern imaging has opened the doors to modern skull base surgery as well as sterotactic radiosurgery.

Embryology: The bones of the skull base are derived from

the chondrocranium with a smaller contribution from the desmocranium. The chondrocranium is made up of endochondral bone from cartilaginous precursors. These are condensations of mesechyme along the notochord with contributions from the neural crest. The individual centers divide to make up the orbital bones, sphenoid bone (hypophyseal ossification cen­ters) and the basi-occipital bone. The sphenoid is a product of laterally placed cartilage developing from the basal mesoderm. The lesser wing of the sphenoid develops from the orbitosphenoid cartilage and the alisphenoid cartilage center produces the medial por­tion of the greater wing of the sphenoid155.

Development from cartilaginous precursors begins around the 40"' day of gestation155. As the developing nerves are contained within this primitive structure the foramina form within the protosphenoid bone. In addition to the chondrocranial bones of the central skull base, membranous bone form the desmocranium makes up an additional small compo­

nent of the skull base. In particular the occipital bone, parietal bone, temporal bones and frontal bones are all formed from the desmocranium. The desmocranium begins to form around the developing brain at the end of the first month of gestation. The pterygoid plates form from membranous bone and fuse to the body of the sphenoid. By birth there is still a cartilaginous element to the sphenoid bone, the spheno-occipital and sphenopetrous junctions, the petrous apices (fora­men lacerum) and the occipital bone.

Other factors influencing the development of the skull base include the relationship of the terminus of the notochord close to the oropharyngeal mem­brane which separates the ectoderm of the stoma-todeum from the endoderm of the primitive pharynx. The presence of the notochord influences the devel­opment of Rathke's (hypophyseal) pouch which in turn will form the pituitary gland. Pathology in devel­opment can lead to multiple congenital anomalies of the skull base155. Most of the later growth of the skull base is at the spheno-occipital synchondrosis which is the last suture at the base of the skull to fuse.

Anatomy: Understanding the role of imaging requires an

intimate knowledge of anatomy. It is impossible to over-emphasize the critical importance of recognizing the anatomic relations of the bones up the skull14. Following the bones of the cranium the anatomy of the skull base is divided into 4 areas. This includes the anterior skull base which makes up the roof of the orbit, roof of the ethmoid sinus and cribriform plate. The central skull base consists mainly of the sphenoid bones and adjacent bones including the petrous por­tion of the temporal bone. The posterior skull base is made up of the occipital bones ending in the foramen magnum. The lateral skull base includes the tempo­ral bone and its petrous extension bounding the later­al extent of the middle cranial fossa.

In this review we will concentrate only on the central skull base. Lesions in this region may origi­nate and involve the bone but also may primarily involve the bony lining consisting of the dura which covers the frontal, temporal, and occipital lobes as well as the cerebellum and brain stem. Extensions of the dura include the tentorium, the diaphragma sella, and the falx.

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The central skull base is primarily composed of the sphenoid bone, which is divided into 4 sec­tions12*. The lesser wing of the sphenoid is defined by the superior orbital fissure laterally, the body of the sphenoid medially, and terminates in the anterior cli-noid. Therefore it is in intimate contact with the optic nerve and carotid artery as it turns to enter the dura just above and lateral to the optic nerve. The lesser wing of the sphenoid also makes up a small portion of the posterior roof of the orbit and the and thus the rear extension of the anterior skull base.

The greater wing of the sphenoid makes up a large portion of the lateral wall of the orbit. It is con­tiguous with the lesser wing on the opposite side of the superior orbital fissure and extends inferiorly to begin to form the floor of the middle cranial fossa. It is also in contact with the temporal bone laterally and the petrous apex posteriorly.

The body of the sphenoid contains the sphe­noid sinus centrally and the sella posteriorly. Aeration of the sphenoid sinus begins at birth and is usually complete by age seven or eight5''. Septations within the sphenoid sinus are quite variable and usu­ally asymmetric. The sella is bounded in the most posterior aspect by the clivus, which extends superi­orly, terminating in the posterior clinoids and dorsum sellae and anteriorly where it forms the tuberculum sellae containing the chiasmatic sulcus. This is not the normal location of the chiasm but a groove sepa­rating the presphenoid and postsphenoid ossification centers. The body of the sphenoid continues posteri­orly as the clivus to terminate in the occipital bones at the foramen magnum, making up a portion of the pos­terior skull base. The fourth portion of the sphenoid complex are the pterygoid extensions extending infe­riorly. The medial and lateral pterygoids are fused anteriorly. The lateral surface of the medial plate forms the medial wall of the pterygoid fossa. The sphenoid bone is bounded by the frontal and zygo­matic bones anteriorly.

Multiple foramina transverse the cranial bones, particularly at the skull base. Those of partic­ular importance to the central skull base include the optic canal which traverses the lesser wing of the sphenoid and sphenoid body. This contains the optic nerve and the ophthalmic artery. The canal is ovoid vertically at its orbital end and horizontally at its

intracranial end. The superior orbital fissure divides the lesser and greater wings of the sphenoid and con­tains cranial nerves III, IV, and VI, as well as the oph­thalmic division of the trigeminal nerve, the superior ophthalmic vein and variable branches of arteries including a recurrent meningeal artery. The sympa-thetics also enter through the superior orbital fissure in conjunction with the trigeminal nerve. The cribi-form plate anterior to the central skull base and part of the anterior skull base transmits the terminal branch­es of the olfactory nerve contiguous with the nasal vestibule. This is also a source of pathology as it is the origin of esthesioneuroblastomas.

Laterally, the middle cranial fossa contains the foramen rotundum just below the superior orbital fis­sure, transmitting the maxillary division of the trigeminal nerve. The foramen ovale is slightly later­al and posterior at the base of the lateral pterygoid plate and transmits the mandibular division of the trigeminal nerve including the motor units of the trigeminal nerve to the masseter and pterygoid mus­cles. Lateral to the foramen ovale is the foramen spinosum which transmits the middle meningeal artery, one of the terminal branches of the external carotid artery along with the internal maxillary artery. Running almost parallel to and below the foramen rotundum is the pterygoid canal, which transmits the vidian nerve from the area of the foramen lacerum to the pterygomaxillary fossa. The sympathetics tra­verse the pterygomaxillary fossa while the parasym­pathetics entering the Vidian canal from the greater superficial petrosal nerve synapse in the sphenopala­tine ganglion. The autonomic fibers exit the sphenopalatine fossa through the inferior orbital fis­sure and the palatine foramen to the nasopharynx and posterior nasal cavity. In addition there are a variable number of lesser palatine foramina (usually two on a side).

At the posterior aspect of the middle cranial fossa, the foramen lacerum is located at the base of the medial pterygoid plate just beneath the curve of the carotid artery as it exits the carotid canal within the petrous bone to extend superiorly before entering the cavernous sinus at its posterior aspect. This fora­men transmits some various recurrent arteries and veins (often off the ascending pharyngeal artery)151

and is a major source of access for nasopharyngeal

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carcinoma entering the area of the middle cranial fossa.

Meckel's cave is posterolateral to the cav­ernous sinus, making up a portion of the floor of the middle cranial fossa (an indentation in the petrous apex). This has direct continuity with the subarach­noid space and thus contains CSF. The Gasserian ganglion represents the intermediate portion of the trigeminal nerve, which subsequently divides into three divisions (ophthalmic, maxillary, and mandibu­lar) that exit through the superior orbital fissure, the foramen rotundum, and the foramen ovale respective­ly"- l32.

Vascular anatomy can be divided into arterial and venous anatomy. The arterial anatomy of the cen­tral skull base involves proximal branches of the internal carotid artery as well as some of the more ter­minal branches of the external carotid artery. The internal carotid artery enters the skull base in the pos­terior lateral aspect of the carotid canal within the petrous bone running anteromedially to the area of the foramen lacerum. There it turns superiorly, entering the cavernous sinus from below before turning to run anteriorly just lateral to the wall of the sphenoid body.

At the posterior loop of the carotid artery the meningo-hypophyseal trunk rapidly divides into three sub-branches. The tentorial artery (the artery of Bernasconi and Cassinari) supplies the tentorium. A second branch is the inferior hypophyseal artery which supplies portions of the pituitary gland. The third branch, the dorsal meningeal artery, supplies portions of the dura in the area of the posterior cav­ernous sinus and clivus. Within the cavernous sinus, a second branch off the proximal internal carotid artery is the inferolateral trunk. This also subsequent­ly divides into several branches, including an antero-medial branch, which extends to the superior orbital fissure, an antero-lateral branch which extends to the foramen rotundum, a posterior branch which extends to the foramen ovale, and a superior branch which supplies portions of the IV nerve. Subdivisions off all of these arteries supply to varying degrees the ocular motor nerves as well as the branches of the trigeminal nerve.

The venous system is centered on the variable venous plexus that makes up the cavernous sinus. While there are two cavernous sinuses, it actually is a

single unit linked through the intra-cavemous plexus that traverses the sella and around the back side with the venous plexus of the clivus extending down to the foramen magnum. In addition, the cavernous sinus is connected inferiorly via the emissary veins to the pterygoid venous plexus below the middle cranial fossa.

The major venous input to the cavernous sinus is the superior ophthalmic vein, usually as a single unit, although on occasion, the inferior orbital vein will enter the cavernous sinus separately. Variable cortical branches also drain into the cavernous sinus. Normal outflow from the cavernous sinus other than the collaterals as mentioned involves the superior and inferior petrosal sinuses, which extend laterally along the edge of the sphenoid wing to the transverse sinus and inferiorly to the jugular bulb respectively. Blood flow within all of these venous channels may be reversed secondary to elevated intravenous pressure as seen in a carotid cavernous fistula or ruptured carotid aneurysm or related to obstruction of one of the venous outflows following venous thrombosis. The paired cavernous sinuses are also connected via an anterior and posterior intercavernous connection through the sella. This is responsible for unilateral carotid cavernous fistulae that present with bilateral or contralateral findings.

Beneath the posterior aspect of the orbital apex is the pterygomaxillary area. This fossa is pos­terior to the back wall of the maxillary sinus. It is connected superiorly to the orbit by the inferior orbital fissure, laterally to the masticator space and infratemporal fossa, medially to the nasal cavity via the sphenopalatine foramen, and posteriorly through the foramen rotundum and the Vidian canal to the middle cranial fossa and the subtemporal space. The pterygomaxillary fossa is traversed by the distal braches of the parasympathetics entering the area from the Vidian canal and moving from the sphenopalatine ganglia to join the lacrimal nerve and innervate the lacrimal gland. The pterygomaxillary area also contains the maxillary division of the trigeminal nerve and the terminal branches of the internal maxillary artery prior to its dividing into the palatine artery with branches to the dura. An impor­tant additional component of this area is a high con­centration of fat. Tumor infiltration often obliterates

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this fat signal. Anterior to the central skull base is the poste­

rior nasal vestibule centrally and the orbit laterally. Beneath the central skull base is the nasopharynx cen­trally separated from the oropharynx inferiorly by the palate. The nasopharynx is suspended as a fibromus-cular sling from the central skull base17. The roof of the nasopharynx is formed by the floor of the sphe­noid sinus and the basisphenoid which slopes posteri­orly to also form the posterior aspect of the nasophar­ynx. Laterally the skull base is bordered inferiorly by the pterygoid muscles.

As previously mentioned the carotid canal transverses the petrous bone (medial temporal bone) exiting at the foramen lacerum. Inferiorly the petrous bone is traversed by the internal auditory canal, which conveys the two branches of the vestibular nerve, the cochlear nerve, and the facial nerve containing the nervus intermedius which carries the parasympathet­ic innervational fibers to the face and the lacrimal gland. The stylomastoid foramen, although not essential to the central skull base carries the terminal branches of the motor facial nerve. Exiting posterior to the mastoid, the facial nerve turns to traverse the parotid gland before dividing into five divisions innervating the facial muscles.

Inferiorly the hypoglossal canal carries the CN XII to the tongue. The foramen magnum forms the opening at the base of the skull, which allows access to the spinal cord.

The bones of the face contain varying amounts of active marrow and fat. The medullary space of the infant is predominantly red marrow but this is gradually replaced by inactive yellow marrow2

usually by early teens. This has particular implica­tions for signal characteristics both on CT and MR imaging. Orbital tissue, of course, includes optic nerves, the extraocular muscles, fat of the orbit, as well as the small nerves, arteries and veins of the orbital apex. The soft tissue of the orbit is contiguous through the superior orbital fissure with the cavernous sinus, which actually is an intracranial but extradural extension of the orbit.

The cavernous sinus itself is a variable venous plexus (varying amounts of trabeculations) containing the extradural internal carotid artery141. In about 10 percent of cases, fat is also found within the cav­

ernous sinus. The roof of the cavernous sinus is con­tiguous medially with the diaphragma sella. The lat­eral wall of the cavernous sinus is made up of a dou­ble layer of the medial temporal dura laterally and a variably continuous sheath made up of a condensation of the sheath of CN III, IV and the first division of CN V. This double layer permits a surgical approach to the lateral wall of the cavernous sinus without enter­ing the venous plexus by splitting the superior orbital fissure.

It is also important to distinguish whether tumors extend into the cavernous sinus or only involves the lateral wall. Within the cavernous sinus body, the carotid artery runs posteriorly to anteriorly surrounded by the branches of the sympathetics. The abducens nerve enters the posterior aspect of the cav­ernous sinus through Dorello's canal formed by the petroclinoid ligament (Gruber's ligament) extending from the apex of the petrous bone to the posterior cli-noid. Within the cavernous sinus the abducens nerve runs in conjunction with the carotid artery and the sympathetics. The carotid artery is often in close approximation to the medial wall of the cavernous sinus and thus the lateral wall of the sphenoid bone where it may easily be injured by fractures that affect the body of the sphenoid.

The dura is contiguous with the periosteum lining the outside of the bones through the various fis­sures accessing the intracranial cavity. In addition to that, the periosteum is contiguous with the periorbita, lining the orbit, meeting the dura at the superior orbital fissure and orbital apex.

Imaging Techniques: Although to a large degree, CT scan has been

supplanted by MRI, CT still plays a major role in imaging of the central skull base** '"'. CT provides the ideal way of assessing the foramina of the skull base. It is the diagnostic procedure of choice following trauma152187 . Fractures are easily detected. It is very sensitive to intracerebral or subarachnoid bleeding. CT is also ideal for presumed retained metallic for­eign material as this is a contraindication to MRI. CT is also extremely sensitive to calcium containing objects such as calcified tumors, old aneurysms, and various congenital anomalies. CT imaging of the skull base should be done with thin section (1.5-3mm) scans'1*.

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In the past, it was important to specify direct coronal images to truly evaluate the area of the cav­ernous sinus as well as the orbit and orbital apex. This is becoming less of an issue with the advent of multi-detectors CT scanners, which obtain a continu­ous image set that may be displayed in all three planes without loss of quality. It is still important to specify thin sections, minimally 5 mm, but better at 1.5 to 3 mm. Unless looking for blood or suspected fracture alone, most imaging is obtained with iodinated con­trast enhancement. Because of the significant poten­tial for allergic reaction, a previous history of allergy to iodinated contrast should be sought and renal func­tion must be assessed prior to ordering scans. Changes in windowing strategy permits attention to either bone structure or soft tissue. Since this may be done a posteriori it is not necessary to specify con­cerns up front.

MRI scan has become the major means of diagnosing and following lesions of the central skull base1. MR is particularly good in those areas where CT is poor, including the orbital apex and next to bone. Since MRI scan produces no signal from the cortical bone, lesions of the cortical bone maybe poorly appreciated. The dura, however, is well visu­alized and, unlike CT, there are no bone artifacts. MRI scanning, particularly the use of T-2 weighted images, is good for differentiating various soft tissue characteristics of neoplasia. Gadolinium has been a major improvement. As gadolinium crosses incom­petent blood-brain barrier, it acts to change the signal characteristics of neoplastic processes into which it leaks. This is particularly true of skull base menin­giomas where, by shortening the T-l characteristics, it produces a bright signal on T-l weighted images. Allergic reactions are rare but recent studies suggest that patients should be pre-screened for renal insuffi­ciency prior to the use of gadolinium contrast.

As opposed to CT where the only variables are windowing and the use of iodinated contrast, imaging sequences on MR are far more variable. As a general rule, the anatomy is best distinguished on T-1 weighted images, usually with gadolinium. A vari­ant of T-l weighted images, fat saturation sequences, reduce the normal high signal of fat without sacrific­ing resolution. This is important for areas where fat is present and high T-l fat signal may obscure pathol­

ogy. An obvious example is the orbital apex and cav­ernous sinus"5 and for looking at the optic nerve and the cranial nerves as they enter and exit the skull base where they are often surrounded by a ring of fatty tis­sue. Obliteration of normal fat signal may be a sign of tumor infiltration.

Since skull base lesions are less frequently due to acute infarcts or demyelinating disease there is less call for the use of diffusion weighted images (DWI) or fluid attenuated inversion recovery images (FLAIR). FLAIR sequences may still be useful in tumors that abut the ventricular system. They are also useful in differentiating cholesterol containing lesions".

Comparison and expected symmetry is an exceedingly important part of any interpretation. Foraminal openings should be the same size on both sides. Any variation between more than 1-2 mm requires interpretation. There may be symmetrical enhancement around cranial nerves due to vascular plexus, and in fact, obliteration of high fat signal might be an indication of expansion of the contained neural branches. Similarly, CSF signal may be helpful as contrast. Within Meckel's cave, CSF signal is expected. Lack of the normal CSF signal may indi­cate infiltration or expansion of lesions within the area of the Gasserian ganglion.

Angiography remains the gold standard for evaluation of vascular abnormalities. These include thrombosis, dissection, aneurysms, and arteriovenous malformations. Angiography also remains essential for therapeutic access, including embolization, bal­loon occlusion, and coiling, either from the arterial or venous side. Original access for cerebral angiogra­phy was by the Seldinger technique with a direct carotid puncture. Today most access is via a femoral approach. When the standard femoral access is not available, angiography may be through a brachial or direct carotid approach. Venous access is also usual­ly via a femoral approach but can be directly through a facial vein approach, the superior ophthalmic vein, or even via percutaneous positioning of a needle with­in the cavernous sinus itself'5.

The advent of non-invasive means of detect­ing vascular anomalies has been a major advance. This includes magnetic resonance angiography (MRA)171 and computed tomographic angiography

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(CTA). MRA is sensitive to aneurysms of 3 mm or greater. Unfortunately MRA often over-calls carotid stenosis. Also not all fistulae are obvious on MRA. MRA is also useful for defining the vascular supply to tumors"'5. Multiple techniques are available for the MRA itself, including time of flight and overlapping images. It is usually advisable that the source images themselves be examined to avoid missing lesions and over-interpreting artifact. More recently MRA has been done with gadolinium contrast. This may be particularly useful in examining the proximal large vessels supplying the skull base.

By timing, the venous outflow system can be highlighted. This use of magnetic resonance images (MRV) may be particularly helpful in defining venous occlusive disease. Transverse sinus thrombosis, cav­ernous sinus thrombosis, and his venous outflow abnormalities may be more easily recognized.

CTA is roughly equivalent in sensitivity to MRA but depends upon equipment capabilities. Interpretation tends to be more straight forward. The advantage of CTA is that it directly reveals the rela­tionship between the vessels and bone. This is partic­ularly helpful in planning an endovascular or surgical approach to vascular lesions at the skull base.

Skull Base Pathology: Skull base pathology may be divided roughly

into developmental, traumatic, inflammatory, neo­plastic, and vascular lesions. The selection of appro­priate imaging techniques depends on the clinical sus­picion. The differential diagnosis may be further refined by including the patient's history in your imaging request. Other physical findings may also be a clue to the specifics skull base pathology.

Developmental abnormalities includes the presence of clefts, anomalous bone development, and abnormalities of facial growth. Hypoplasia of the greater wing of the sphenoid may be associated with neurofibromatosis type I. These patients may present with pulsatile exophthalmos. Hypertelorism as well as cleft lip and palate"5 may be a clue to the presence of central skull base anomalies including variations of encephalocele or myelomeningocele. This occurs in approximately 1 in 4000 cases77-l25. Transphenoidal and spheno-ethmoidal lesions involve the central skull base while transethmoidal defects involve the

anterior skull base. These lesions are often unrecog­nized or misdiagnosed as polyps.

Developmental anomalies of the three tissues that join together to make up the skull base and its contents may leave small rests of abnormal tissue. Hamartoma of the tuber cinereum often presents with precocious puberty. On MRI the lesion is homoge­neous, isointense on Tl and hyperintense on T2 l6-21. These may form choristomas which are often cystic in nature. These may be as simple as duplication of the arachnoid that forms an arachnoid cyst (containing CSF)73- '"•l22-l27, may involve abnormal inclusion of epithelium producing epidermoids'17, or may be asso­ciated with more advanced dermal appendages referred to as dermoids'1*5. Arachnoid cysts when intrasellar must be distinguished from a dehiscent diaphragma producing an empty sella syndrome''9. Most intracranial epidermoids involve the cerebellar pontine angle and thus the posterior skull base"14. Overall 18% of epidermoids occur in the parasellar region. Epidermoids are hypointense in Tl and hyperintense on T2 weighted images with rim enhancement*"'. These tumors can be distinguished from lipomas which do not enlarge and often contain normal neural structures'70. Teratomas are congenital rests that contain all three tissue types. These may have a variable course with malignant transformation in some. Often these involve the parasellar region or extend into the orbit.

Rathke's cleft cysts form from remnants of the terminal notochord. Most Rathke cleft cysts are asymptomatic but they are common enough that they make up approximately 1.5% of symptomatic parasellar lesions'2. While pituitary dysfunction is the most common feature, visual disturbances (56%) and headaches (49%) may be present when sympto­matic174. MR findings are extremely variable. The cyst is hyperintense in 47%, isointense in 11 % and hypointense in 43%5, l59. While the cyst itself does not enhance the wall often brightens with gadolinium"1*. Other congenital cysts include cholesterol granulo­mas4' "7 and neuroenteric cysts felt related to persist­ence of the neuroenteric canal '''. Cholesterol contain­ing cysts demonstrate high signal on both Tl and T2 weighted images51*''5. Diffusion weighted images as well as fluid attenuated inversion sequences (FLAIR) may play an important role in the differential diagno-

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sis of possible cholesteatomas55. One of the most common bony abnormalities

is that of fibrous dysplasia. In about 25% of cases, this involves the skull base, often involving the body of the sphenoid". Pathology does not respect suture lines and will often involve the sphenoid and sur­rounding bones. The pathology appears to be an abnormality in maturation of bone and failure of reab-sorption. This will produce varying amounts of bone enlargement, compromise of foramina, and resultant proptosis. MRI findings include low signal intensity on both Tl and T2 weighted images21-28. The Tl sig­nal is midway between brain and CSF. On T2 images approximately one third may be hyperintense'"'. These lesions often enhance because of their exten­sive vascularization.

Osteopetrosis involves diffuse enlargement of the bone itself". Paget's disease (osteitis deformans) is an osseous lesion of unknown etiology that is not uncommon in older patients (up to 3% of patients over the age of 40). Although it affects the pelvis most frequently it not uncommonly affects the skull81. Histologically there is evidence of resoiption of tra-beculae of the walls of the haversian canals. There may also be a late sclerotic phase. Secondary sarco­mas have been reported141'. Other forms of bony abnormalities include osteogenesis imperfecta, fibrous histocytomas, and bone abnormalities associ­ated with mucopolysacchacridoses.

Developmental anomalies may also include abnormalities in the vascular anatomy. The presence of a persistent trigeminal artery may have implica­tions especially during embolization. The carotid artery is often uncovered within the lateral wall of the sphenoid bone and thus may be damaged during any surgery at the skull base within the sphenoid sinus. In uncommon cases, the carotid arteries may actually come close to meeting in the midline, thus obscuring the approach to the sella. Another vascular anomaly is that of the foramen of the Vesalius61. This is locat­ed anterior medial to the foramen ovale and may con­nect the cavernous sinus to the pterygoid plexus below. This also has implications in the setting of a carotid cavernous fistula or attempts at embolization at the skull base.

Trauma to the skull often results in pathology affecting not just the skull base itself but also its con­

tained tissues. Trauma is usually divided into that due to blunt force and that due to penetration. In the case of blunt force trauma, fractures may occur anywhere at the skull base. The finding of fluid within the sphe­noid sinus may be an indication of otherwise unrecog­nized fractures within the body of the sphenoid. These patients are at risk for tears within the carotid artery, often leading to a carotid cavernous fistula or, in rare cases, a pseudo-aneurysm extending into the sphenoid sinus. These fistula or secondary aneurysms may be evaluated by MRA or CTA or eventually by angiography if necessary. Any fracture of the skull base will usually be associated with a tear in the dura. This potentially can lead to CSF leaks when the dura involves bone opposite from air-containing sinuses. The ethmoid, sphenoid and less commonly the frontal sinuses may be involved. CSF leaks may result in meningitis, intracranial hypotension, and pneumo-cephalus. CT scan tends to be particularly sensitive but may be combined with CT cisternography with metrizamide if necessary47-15s, MRI scans may iden­tify the area of a CSF leak by abnormal signal within the mucosal surrounding structures49-71145

The appropriate imaging of penetrating trau­ma depends on the history and particulars of the fly­ing object. If there is any concern regarding the pres­ence of a metallic foreign body, the patient should be evaluated with CT or even plain films prior to order­ing an MRI. Unfortunately bullet or other metallic fragments produce artifact on CT.

Obstruction of normal mucosal outflow from the paranasal sinuses may produce a mucocele"-l72. This is particular true following trauma or surgery. These may grow to extraordinary size and, when involving the sphenoid or posterior ethmoid sinus, may be present in the central skull base, producing cranial nerve palsies including optic neuropathy and ophthalmoplegia. CT scanning is particularly sensi­tive to distinguish bony abnormalities but MR is far more specific for identifying the lesion and its con­tent l «•'*•'».

Inflammatory lesions may be divided into those that are related to a specific bacterial, viral, or protozoan etiology [infectious] and those where pre­sumably the immune reaction is triggered by some­thing else. Unexplained autoimmune inflammatory conditions include sarcoidosis and Wegener's granu-

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lomatosis. These lesions frequently involve the skull base. Overall granulomatous disease makes up approximately 3% of juxtasellar lesions'2. About 5% of patients with sarcoidosis have evidence of CNS involvement with approximately 1/3 involving the sella region70,92-144. Similar to germinomas there is a high incidence of diabetes inspidus. Tuberculosis rarely involves the central nervous system but when it does there is a high incidence of parasellar involve-ment'"i,!il. Non specific granulomatous inflammation of the cavernous sinus (often referred to as Tolosa Hunt syndrome7'' "'9) may produce nontumoral painful ophthalmoplegia*6, l90.

Bacterial infectious processes are the most common inflammatory lesions with intracranial and skull base involvement. The most common pathogen is staphylococcus aureus although streptococcus pneumoniae, and other aerobic and anaerobic bacteria have been reported. These often start in the area of the sinuses, particularly the posterior ethmoid and sphe­noid sinus. One additional source of infection involv­ing the central skull base follows inflammation and infection involving the inner ear, which may extend from otitis media to petrositis most commonly due to pseudomonas25 K. Inflammation of the petrous apex may produce cranial nerve palsies (particularly a VI nerve palsy), increased intracranial pressure due to venous outflow obstruction, and variations in hearing, facial weakness, and numbness. Gradenigo's syn­drome has become much less common in this antibi­otic era but still may occur. Pseudo-Gradenigo's syn­drome (CN VI, VII, VIII palsy with facial pain) may be seen with extension of nasopharyngeal cancer into the skull base and cavernous sinus121.

Infection may also spread to involve the epidural space, producing an epidural abscess. Infectious involvement of the subdural space pro­duces a subdural empyema or even occlusion of the cavernous sinus as septic cavernous sinus thrombo­sis41,50.

Fungal infections include mucormycosis in patients with diabetic ketosis or immune suppres­sion. Aspergillomas also occur also in the setting of immune compromise but can actually be seen in an otherwise immune competent patient. These lesions often produce bone destruction at the orbital apex and central skull base but it actually may show little

in the way of imaging characteristics. CT scan is particularly sensitive to the bone changes, while MR will better demonstrate soft tissue abnormalities. Characteristically MRI demonstrates dark signal on both Tl and T2 weighted images'89. An allergic response to fungus (allergic fungal sinusitis) may cause expansion of the sinuses much like a muco­cele, producing mass effect.

The fourth and most common cause of pathol­ogy of the central skull base is neoplasia. These are best detected by imaging''1. By absolute number, the most common lesion affecting the central skull base is a pituitary tumor10. Overall pituitary tumors make up 9.6% of primary intracranial tumors'2 and occur more frequently in women1"2. While the majority of these are microadenomas"'4 and therefore produce endocrine findings rather than evidence of mass effect, when the tumor extends superiorly, optic neu­ropathy or chiasmal syndrome may be expected1". Older imaging studies reveal the enlargement of the sella on plain films, often with erosion of the dorsum sella. This is similarly obvious on CT scanning. CT scanning will pick up the majority of macroadenomas demonstrating variable enhancement'04. MRI, microadenomas may be seen as less enhancing than the surrounding pituitary tissue. Microadenoma loca­tion within the sella itself may be an indication of its tumor type with prolactinomas tending to present more laterally104. Gadolinium permits visualization of microadenomas as small as 3-4mml5S with a detection rate of 85-90%51. Gadolinium adds about a 10% increase in detection sensitivity'"and an additional 10% may be seen with dynamic imaging52. Detection rates for ACTH secreting tumors are lower than for prolactinomas41'. The position of the normal gland within the sella may be helpful in the differential diagnosis. Craniopharyngiomas tend to displace the gland inferiorly and germinomas push the gland ante­riorly while pituitary tumors tend to displace the nor­mal gland superiorly or posterosuperiorly160.

Macroadenomas are usually isointense on Tl images with a variable signal on T255- l2°. Most macroadenomas will enhance with gadolinium"5. In between 15 and 43% of cases, pituitary tumors may extend laterally to involve the area of the cavernous sinus79. Cavernous sinus extension is more common­ly seen with prolactin and ACTH secreting tumors.

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