Skull Base, Orbits,Temporal Bone, andCranial Nerves:Anatomy on MRImaging

ni

PROTOCOL

At the authors institution, conventional or fast spin-echo (FSE) T1-weighted (T1W) images in the axialand coronal planes, axial or coronal T2-weighted

lesions (Tables 13). The images are obtained

high-resolution images, parallel imaging tech-niques (if available) can be helpful.3 A shortinversion-time inversion-recovery (STIR) sequencemay be used as an alternative to fat-saturated T2Wimages in patients with extensive maxillofacialfacial hardware. STIR provides better fat suppres-

longer to acquire and is susceptiblew in adjacent vessels.8

ging of the cavernous sinuses, theimaging field should extend from the orbital apex

ive, Ann Arbor, MI 48109, USAcal Center Drive, Ann Arbor,

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.comMagn Reson Imaging Clin N Am 19 (2011) 439456 iThe authors have nothing to disclose.a Department of Radiology, University of Michigan, 1500 East Medical Center Drb Department of Internal Medicine, University of Michigan, 1500 East MediMI 48109, USAc Division of Neuroradiology, Department of Radiology, University of MichigaMedical Center Drive, Room B2A205, Ann Arbor, MI 48109, USA* Corresponding author.E-mail address: amorani@med.umich.edu(T2W) images, and postcontrast-enhancedimages (with and without fat suppression) are ob-tained in all patients with suspected skull base

sion but takesto pulsatile floFor MR imaarticle focuses on the radiologic anatomy of theskull base pertinent to MR imaging evaluation.

tion hardware causing susceptibility artifacts anddistortion.8 To reduce the acquisition time of theseAccurate delineation, diagnosis, and treatmentplanning of skull base lesions require knowledgeof the complex anatomy of the skull base. Becausethe skull base is not directly accessible for clinicalevaluation, imaging is critical for the diagnosis andmanagement of skull base diseases.13 AlthoughCT is excellent for outlining the bony detail, MRimaging provides better soft tissue detail1,4,5 andis helpful for evaluating the adjacent meninges,brain parenchyma, and bone marrow of the skullbase. Thus, CT and MR imaging complementeach other and are often used together forcomplete evaluation of skull base lesions.1,3 ThisAjaykumar C. Morani, MDa,*, Nisha S. RamaJeffrey R. Wesolowski, MDc

KEYWORDS

Skull base Orbits Temporal bone Cranial MR imaging Anatomydoi:10.1016/j.mric.2011.05.0061064-9689/11/$ see front matter 2011 Elsevier Inc. All, MBBSb,

with higher resolution, using a smaller field ofview with a slice thickness of 3 mm. Intravenousgadolinium is important to clearly delineate theextent of pathology and to detect intracranialextension, particularly meningeal involvement.3,6,7

Fat-suppressed T1W images can be obtained ifthe lesion is in the vicinity of fat-containing areas,such as the orbits. However, the skull base regionis extremely susceptible to artifacts from tissueinhomogeneity because of air in the adjacent para-nasal sinuses. Therefore, fat-suppression tech-nique is not always successful, particularly if thepatient also has dental or craniofacial reconstruc-

verights reserved. mr

Table 1MR imaging of cranial nerve II (orbit)

Slice Orientation SAG T1W HEAD DWI HEAD AX FLAIR HEAD COR T2 FS AX T1 FS W/WO COR T1 FS W/WO COR T1 AX T1 HEAD

Field of view (mm) 240 230 230 180 190 180 180 240

Matrix 256/512 128/256 320/512 256/512 256/512 256/512 256/512 256/512

No. of slices/location 23 24 20 40 28 40 40 24

Slice thickness/gap 4/0.5 mm 4/1 mm 6/1 mm 2/default 2/default 2/default 2/default 6/1 mm

Contrast Pre- Pre- Pre- Pre- Pre-/Post- Pre-/Post- Post- Post-

TE 10 59 125 90 10 10.6 10.6 10

TR Shortest Shortest 11,000 Shortest Shortest Shortest Shortest Shortest

Flip angle 90 90 90 90 90

Number of Excitations 1 1 1 3 2 2 2 1

Coronal coverage is from anterior globe through the optic chiasma. Axial coverage is centered over orbits. Dose of contrast media is determined according to patient weight.Contrast media is determined according to patient weight.Abbreviations: AX, axial; COR, coronal; DWI, diffusion weighted imaging; FLAIR, fluid attenuation inversion recovery; FS, fat saturated; SAG, sagittal; TE, echo time; TR, repetition

time; W/WO, with and without.

Table 2MR imaging of cranial nerves V and IX through XII (skull base)

Slice Orientation SAG T1 HEAD DWI HEAD AX FLAIR HEAD AX T2 AX T1 POST AX T1 AX T1 F/S COR T1 AX T1 HEAD

Field of view (mm) 240 230 230 160 160 160 160 160 240

Matrix 304/512 128/256 320/512 336/400 224/288 224/288 224/288 224/288 256/512

No. of slices/location 21 28 25 34 34 34 34 38 25

Slice thickness/gap 5/1 mm 4/1 mm 5/1 mm 4/1 mm 4/1 mm 4/1 mm 4/1 mm 3/default 5/1 mm

Contrast Pre- Pre- Pre- Pre- Pre- Post- Post- Post- Post-

TE 10 59 125 90 9.1 8 8 10.5 10

TR Shortest Shortest 11,000 Shortest 500 590 545 500 500

Flip angle 90 90 75 90 90 90

Number of excitations 1 1 1 1 1 3 2 4 1

Axial thin coverage is from the top of the frontal sinus to mid-C3, and from the mandible to the spinous process. Coronal coverage is from the frontal sinus to the posterior pons.Dose of contrast media is determined according to patient weight.

Moranietal

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Table 3MR imaging of cranial nerves VII and VIII

Slice Orientation SAG T1 HEAD DWI HEAD AX FLAIR HEAD VISTA AX T2 AX T1 W/WO COR T1 AX T1 HEAD

Field of view 240 230 230 180 190 190 200 240

Matrix 256/512 128/256 320/512 360/1024 256/512 256/512 256/512 256/512

No. of slices/location 19 28 20 74 18 18 18 25

Slice thickness/gap 6/1 mm 4/1 mm 6/1 mm 0.3 mm 2/default 2/default 2/default 5/1 mm

Contrast Pre- Pre- Pre- Pre- Pre- Pre-/Post- Post- Post-

TE 10 51 125 187 90 10 10.5 10

TR Shortest Shortest 11,000 1500 3000 500 500 500

Flip angle 90 90 90 90 90

Number of excitations 1 1 1 1 3 2 2 1

Coverage for thin axial and coronal slices includes internal auditory canals. VISTA images should be reformatted in both sagittal and coronal planes. Imaging of each side is per-formed separately.Dose of contrast media is determined according to patient weight.Abbreviation: VISTA, volumetric isotropic T2w aquisition.

AnatomyonMRIm

aging

441

Vascular time-of-flight MR angiography images

Morani et al442to the prepontine cistern. Comprehensive imagingshould also include routine T2W/fluid attenuatedinversion recovery (FLAIR), and precontrast andpostcontrast T1W sequences though the entirebrain. Specific sequences through the cavernoussinuses should include 3-mm-thick postcontrastT1W slices in coronal and axial planes, with thefat-saturation technique used in at least one ofthese planes (see Table 2). Individual cranialnerves in the sinuses and adjacent cisterns maybe visualized using thin-slice three-dimensionalheavily T2W images (such as fast imaging usingsteady-state acquisition [FIESTA] or, alternatively,constructive interference in steady state [CISS]).9

For orbital imaging, phased-array surface coilscan be used for detailed imaging of the anterioroptic pathway, but these coils do not have enoughpenetration to image the posterior optic pathway,including the brainstem (which generally requiresa standard head coil). Thus a dual-coil approachis often used for orbital MR imaging. Improve-ments in multichannel head coils and theincreasing use of 3T scanners, however, oftenprovide detailed imaging of both the anterior opticpathway and the posterior fossa structures, allow-ing for the use of a single coil.8 Most routine orbitalimaging protocols include mainly T1W and T2Wspin-echo or FSE sequences, all acquired withslice thickness of 3 mm and slice gap of 1 mm.Axial or coronal T2W FSE with fat saturation andaxial T1W images are acquired, followed by post-contrast axial and coronal T1W with fat saturation(see Table 1). To evaluate the lacrimal system, MRdacryocystography can be obtained through fillingthe lacrimal sac and nasolacrimal duct with a dilutemixture of gadolinium-containing contrast agentadministered typically via cannulation of thelacrimal canaliculi.8

MR imaging of the temporal bone always coversimaging of the internal auditory canal, cerebello-pontine angle, and the labyrinth (see Table 3).Three-millimeter-thick conventional spin-echo orattenuation recovery T1W images provide goodimaging of the labyrinth. However, currently 2-mm spin-echo or 1-mm gradient-echo T1Wimages are used to show different turns of thecochlea, vestibule, semicircular canals, and, inseveral cases, the endolymphatic sac. Neurovas-cular structures in the internal auditory canal(IAC) and cerebellopontine angle are well seen onthese images. Fat-suppressed coronal T1W spin-echo images can also be used while imaging thetemporal bone to eliminate the high T1 signalintensity of the fatty bone marrow in the walls ofthe internal auditory canal.For detailed evaluation of the labyrinth, 0.5-to 0.7-mm heavily T2W gradient echo or FSEshould also be used, particularly in patients withpulsatile tinnitus. On these images, arteries arehyperintense in appearance, whereas the nervesand veins remain hypointense. These images canalso be used to show neurovascular conflicts,vascular tumors, or vascular malformations in rela-tion to the temporal bone.10

Axial T2/FLAIR images of the brain are also usedto complete the study of temporal bone MRimaging, mainly to exclude intra-axial lesions. Ifa central lesion is suspected as the cause ofvertigo or sensorineural hearing loss, 4-mm-thickheavily T2W spin-echo images through the brain-stem are also acquired. The myelinated structurescan be easily seen on these heavily T2W images,from which the location of the nuclei and the ves-tibulocochlear pathways can be presumed.10 Thelocation of the nuclei of other cranial nerves canalso be presumed using the same principle.

ANATOMY

The anterior, middle, and posterior cranial fossaeare the three naturally contoured regions formingthe skull base when seen from above. However,no defined boundaries correspond to these fossaewhen seen from below. The anterior skull base isformed by the frontal bone and sinus, along withthe roof of the ethmoid sinuses, nasal cavity, andorbits. The central skull base is formed predomi-nantly by the sphenoid bone, and theposterior skullbase is formed by temporal and occipital bones.1

Bones forming the skull base contain normalfatty marrow, which appears hyperintense onT1W images without fat suppression, and losessignal on fat-saturated images. The marrow alsonormally appears dark on fat-saturated T2Wimages, which increases the conspicuity of thethree-dimensional images are very useful andprovide high contrast between the cerebrospinalfluid, intralabyrinthine fluid, nerves, and thebone. These sequences are very useful to evaluatethe facial nerve and the three branches of thevestibulocochlear nerves in the internal auditorycanal. Submillimeter images can also distinguishbetween the scala tympani and scala vestibuli/scala media. High-resolution images of both innerears can be acquired with a good signal-to-noiseratio using a small field of view (95 mm) and a ma-trix of 192 256. Multiplanar three-dimensionalreconstructions and virtual images of the fluid-containingmembranous labyrinth can be obtainedusing these small field of view images. Contiguousthree-dimensional intraluminal view can be dis-played with virtual otoscopy.skull base bony lesions.1 The cortical bones

may not have a medullary cavity and therefore

of the frontal bone, which form the orbital roof and

Anatomy on MR Imaging 443roof of the ethmoid sinuses. The orbital plate of thefrontal bone is actually the largest area of the ante-rior skull base. Although the cribriform plate formsthe weak part of anterior skull base and the site ofcommonbony injuries, erosions, and cerebrospinalfluid (CSF) leaks, the orbital roof is thick and sturdywith relative resistance to these changes.1,3

CENTRAL SKULL BASE

The sphenoid bone forms most of the central skullbase and floor of the middle cranial fossa. Its ante-rior border is formed by the greater wings of thesphenoid bone, and the posterior border is formedby the anterior surface of the petrous temporalbone. The medial part of the central skull base isformed by the body of the sphenoid bone, withmarrow fat. For example, the orbital roof andethmoid sinuses do not have marrow cavities,whereas the clivus has a marrow cavity normallycontaining abundant fat.11 However, in the pedi-atric age group, the marrow may still be hemato-poietic and not replaced by fat, appearingrelatively hypointense on T1W imaging.3,11

ANTERIOR SKULL BASE

The anterior skull base forms the floor of the ante-rior skull and includes the roof of the ethmoidsinuses, the nasal cavity, and the orbits (Figs. 1and 2). Anteriorly, it is bounded by the posteriorwall of the frontal sinus and its posterior marginis formed by the lesser wing of the sphenoidbone.1,3 Medially along the anterior skull base,the thin cribriform plate of the ethmoid bone andlateral fovea ethmoidalis constitute the roof of thenasal cavity and ethmoid sinuses. The cribriformplate possesses multiple small perforations trans-mitting olfactory nerves from the nasal mucosa tothe olfactory bulb (see Fig. 1). The middle nasalturbinate is delicately attached to inferior surfaceof the cribriform plate. The anterior ethmoid arteryenters the anterior cranial fossa from the ethmoidsinus through the lateral lamella of the cribriformplate before reentering the nose. Laterally, theanterior skull base is bounded by the orbital platescontain nonmobile protons similar to the air in theadjacent paranasal sinuses, hence these appearas signal voids on all of the MR pulse sequences.8

Although cortical erosion is more confidently diag-nosed on CT, infiltration of the marrow spaces isbetter delineated on MR imaging. CT often under-estimates the frequency and extent of skull baseinvolvement.1 However, the skull base bonesthe cavernous sinuses on either side; the lateralaspect of the central skull base is constituted bythe squamous temporal bone. The central largedepression in the sphenoid body is called the sellaturcica.1,12 The roof of the sella turcica is formedby a fold of dura called diaphragma sella, whichis perforated to allow passage of the pituitary stalkor infundibulum (see Fig. 1).The pituitary gland is composed of two lobes

that are distinct anatomically, embryologically,and physiologically. The anterior lobe is the largerpart, constituting 75% of the pituitary volume, andis also called the adenohypophysis. It appearshomogenous and isointense to gray matter onT1W and T2W images. The posterior lobe of thepituitary occupies the posterior third of the sellaturcica and is also called the neurohypophysis.The so-called posterior pituitary bright spot is thehyperintense signal of the neurohypophysis onT1W images because of the proteinaceous anti-diuretic hormone complex.12,13 The posterior pitu-itary enhances before the anterior pituitary duringdynamic imaging, because it has a direct bloodsupply via the meningohypophyseal artery.13 Thesphenoid sinus is located inferior to the sella turci-ca and its degree of pneumatization is variable.The internal carotid artery and cranial nerve V2(maxillary division) frequently groove the lateralwall of sphenoid sinus during their course throughthe cavernous sinus.1 The posterior slopingportion of the sphenoid bone is called the clivus,which forms the roof of the nasopharynx alongits inferior surface.1,12

The cavernous sinuses are located on eitherside of the sphenoid bone. They form the lateralwalls of the pituitary fossa or sella.1,9,12 Eachcavernous sinus contains the internal carotidarteries as the most medial structure called thecarotid trigone, and cranial nerves III, IV, and VIand the first (ophthalmic-V1) and second (maxil-lary-V2) divisions of cranial nerve V. Cranial nerveVI runs in the center of the cavernous sinus adja-cent to the internal carotid artery, whereas theother nerves run in the lateral wall of the cavernoussinus (oculomotor trigone). The cavernous sinusis actually a multiseptate space, which showsintense contrast enhancement of the slower-flowing venous blood. The internal carotid arteryappears as a signal void structure on the standardMR imaging sequences and appears hyperintenseon time-of-flight and other bright blood MRsequences.8 Because of intense backgroundenhancement, detection of intracavernous lesionsis challenging. T2W images without fat saturationoften provide better contrast resolution betweenthe cavernous sinus and intracavernous lesions,and hence should be included to evaluate the

cavernous sinus pathologies.8 Sometimes the

Morani et al444sinuses may contain fatty deposits, which arenormal and may be more prominent in obesepatients, those with Cushing syndrome, or pa-tients receiving steroids.9 Individual cranial nervesin the sinuses may be visualized using thin-slicethree-dimensional heavily T2W images (suchCISS or FIESTA).9

The optic canal, superior orbital fissure, foramenrotundum, foramen ovale, foramen spinosum, andvidian canal are found within the sphenoid bone

Fig. 1. Coronal fat-suppressed T2W images of the anteriocontrast image of the posterior skull base (C). (A) At the(Globe), the cranial nerves I are well seen (arrow) just infe(Ethm) are located slightly inferiorly and laterally, whereaslocated more inferiorly. (B) At the level of the sella turcicainfundibulum (black arrow) present just inferior. The interarrowhead (cavernous segment) and arrow (paraclinoid snoted to course inferior and lateral to the cavernous sin(FO). (C) At the level of the posterior skull base, the intern(arrowhead) and medial porus acusticus (arrow) noted. Mocondyles (OC) are seen. The posterior skull base is intimate(CP). Note the enhancing tympanic segment of the left crand form part of the central skull base.1,14 Theoptic canal is the only canal that passes throughthe lesser wing of sphenoid. It transmits cranialnerve II and the ophthalmic artery. The lesserwing is attached to the sphenoid body with tworoots, which form the roof, lateral wall, and floorof the optic canal. The sphenoid body forms themedial wall of the optic canal. The inferior root oflesser wing or the optic strut separates the opticcanal from the superior orbital fissure. The superior

r (A) and central (B) skull base, and coronal T1W post-level of the maxillary sinuses (Max) and optic globesrior to the gyrus rectus (asterisk). The ethmoid sinusesthe middle and inferior nasal turbinates (MNT, INT) are, the optic chiasm is noted (ellipse), with the pituitarynal carotid artery flow voids are denoted by the whiteegment). The third division of the trigeminal nerve isus (asterisks) as it heads toward the foramen ovaleal auditory canal (IAC) is seen with the lateral fundusre inferiorly, the hypoglossal canals (HC) and occipitally associated with the pons and the cerebral peduncleanial nerve VII (circled), which is a normal finding.

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Anatomy on MR Imaging 445Fig. 2. Orbital coronal T2W imaging with fat saturationThe conal muscles are well seen, namely the inferior, mlique (SO) and the superior rectus/levator palpebrae scomplex is noted, with the T2 hypointense (white mattspinal fluid (circled). The superior ophthalmic vein is nopostcontrast fat-suppressed imaging, the vitreous humthe recti muscles (MR, LR) enhance avidly. The optic nlacrimal glands enhance avidly (arrowheads). Moreenhancing cavernous sinus (arrow) just anterior to theorbital fissure is formed by the lesser wing of thesphenoid bone superiorly, the greater wing inferi-orly, and the sphenoid body medially. This fissuretransmits cranial nerves III, IV, VI, and V1. Theoptic canal and the superior orbital fissuretogether form the orbital apex, one of the impor-tant transition zones between intracranial andextracranial contents.1

The foramen rotundum is seen inferior to thesuperior orbital fissure and it transmits cranialnerve V2. This foramen connects the cavernoussinus in the middle cranial fossa to the pterygopa-latine fossa.1,15 The vidians canal is located at thejunction of the pterygoid process and the sphe-noid body. It connects the pterygopalatine fossaanteriorly and the foramen lacerum posteriorly.The vidian canal transmits the vidian artery, whichis a branch of the maxillary artery, and also trans-mits the vidian nerve, which is formed by thegreater superficial petrosal nerve and the deeppetrosal nerve.1 The fibrocartilage that plugs theforamen lacerum is one of the most resistanttissues to tumor infiltration.1

Fat is helpful in the evaluation of bones and theforamina of the central skull base, which are easilyseen on T1W images without fat saturation, partic-ularly in the coronal plane. The earliest sign ofinvolvement of these foramina or bones by any) and axial T1 postcontrast fat-saturated image (B). (A)al, and lateral recti (IR, MR, LR), as are the superior ob-erioris complex (SR/LPS). Intraconally, the optic nerveract) cranial nerve II surrounded by T2 bright cerebro-just inferior to the superior rectus (arrowhead). (B) Onof the optic globe remains hypointense (VH), wherease complex (ONC) remains hypointense. However, theteriorly, the pituitary stalk (circle) and the denselyntomedullary junction (PMJ) can be seen.malignant, infiltrative, or infective process is theobliteration of the normal fat content or normalfat planes, especially when compared from oppo-site side.1,16,17 Obliteration of the high fat signalintensity on T1W MR images is actually the keysign of impending perineural spread by the malig-nancies at the skull base.18

The foramen ovale transmits cranial nerve V3and is located in the posterolateral aspect of thegreater wing in the central skull base (see Fig. 1).It connects the middle cranial fossa and the masti-cator space. The foraminal size can be variable oneither side and also in different patients, butusually it should not differ by more than 4 mm onthe two sides in an individual. Foramen spinosumis another foramen of the central skull base,located posterolateral to the foramen ovale andusually less than 2 mm in diameter. It transmitsthe middle meningeal artery. If the diameter offoramen spinosum exceeds 5 mm, a middlemeningeal artery abnormality must be ruledout.19 Conversely, if absent, a persistent stapedialartery must be suspected.

POSTERIOR SKULL BASE

The posterior surface of the clivus forms the ante-rior portion of the posterior skull base and posterior

cranial nerve V2 also traverses within the orbital

Morani et al446cranial fossa. The clivus is formed from fusion ofthe basisphenoid and basiocciput. It extendsfrom the foramen magnum inferodorsally to thedorsum sellae superoventrally. The lateral portionof posterior skull base is formed by the posteriorsurface of the petrous temporal bone superiorlyand the condylar part of the occipital bone inferi-orly. The posterior portion of the posterior cranialfossa and posterior skull base is constituted bythe mastoid temporal bone and the squamousoccipital bone. The foramen magnum is entirelyformed within the occipital bone.1 The junctionbetween the petrous temporal bone anterolaterallyand the occipital bone posteromedially is called thepetro-occipital suture.The jugular foramen is seen at the posterior end

of petro-occipital suture.1,20 Its appearance variesdepending on the level of the imaging sections,because it courses anteriorly, then laterally, andfinally inferiorly through the skull base1 into thecarotid space. The right jugular foramen is largerthan the left in 75% of the population.21 Anteriorly,the caroticojugular spine, a bony ridge, separatesthe jugular foramen from the inferior carotidopening. Medially, an osseous bar called thejugular tubercle is an important landmark sepa-rating the jugular foramen from the hypoglossalcanal.1,21 Pars nervosa forms the anteromedialcompartment, and the pars vascularis forms theposterolateral compartment of the jugular fora-men. These compartments are separated by adividing fibrous or bony septum. Pars nervosa issmaller and more consistent in size, and transmitscranial nerve IX (glossopharyngeal nerve) with itstympanic branch (Jacobson nerve) and the inferiorpetrosal sinus. The inferior petrosal sinus formsa multichannel confluence with the sigmoid sinusin the pars nervosa and empties into the jugularbulb.21 The pars vascularis is larger and more vari-able in size, transmitting the internal jugular vein,cranial nerve X (vagus nerve) with its auricularbranch (Arnold nerve), cranial nerve XI (accessorynerve), and the posterior meningeal artery,1,2123

a branch of ascending pharyngeal artery supplyingthe posterior fossa meninges.21

The appearance of the jugular foramen isanatomically variable, and sometimes both cranialnerves IX and X traverse through the parsnervosa.21 Cranial nerves in the jugular foramencannot be seen on conventional MR imagingsequences, but may be well seen on the contrast-enhanced three-dimensional fast imaging usingsteady-state acquisition, which provides highcontrast and spatial resolution.21 The mostcommon pseudolesion of the jugular foramen onMR imaging results from the complex flow pattern

within a normal jugular bulb, which may befloor and divides into the zygomaticofacial and zy-gomaticotemporal nerve, which emerge throughthe respective foramina in the face. Theseforamina are sometimes seen on high-resolutionT1W MR imaging. Similarly, the anterior andposterior ethmoidal foramina transmitting the cor-responding ethmoidal vessel and nerve may beseen medially between the frontal bone and thelamina papyracea or within the frontal bone.24

The optic canal transmits cranial nerve II and theophthalmic artery. The inferior root of lesser wingor the optic strut separates the optic canal fromthe superior orbital fissure. The superior orbitalfissure is formed by the lesser wing of the sphe-noid bone superiorly, the greater wing inferiorly,and the sphenoid body medially. This fissuremisinterpreted as intraluminal thrombus or a glo-mus tumor. This pseudolesion can produce inter-mediate signal or high signal on postcontrastT1W images. T2W images in these cases shouldshow lack of flow artifact. If still unclear, MR venog-raphy can help resolve the issue. Another potentialpitfall is a large or high jugular fossa caused bynormal variation in size and symmetry, which maybemistaken as a sign of a space-occupying lesion.When the roof of the jugular bulb is seen above thelevel of inferior margin of the cochlear basal turn, itis called a high-riding jugular bulb, which is morecommon on the right side. It compromises theexposure during translabyrinthine surgery andduring surgery for cerebellopontine angle lesions.21

ORBITS

Contents of the orbit are located within a bonypyramid. Its roof is formed by the orbital plate ofthe frontal bone. The lacrimal gland lies in thelacrimal fossa, a recess of the frontal bone antero-laterally in the orbit. The lateral orbital wall isformed by the orbital surface of zygomatic boneand the greater wing of sphenoid bone. From ante-rior to posterior, the medial orbital wall is formedby the maxillary bone, lacrimal bone, lamina papy-racea of the ethmoid bone, and lesser wing of thesphenoid. The lacrimal sac lies in the fossa alongthe anteromedial orbital wall.The orbital floor is formed by the zygomatic,

maxilla, and the palatine bones. The infraorbitalgroove containing the infraorbital nerve traversesthe orbital floor, ending in the infraorbital canaland foramen. If the distal portion of the infraorbitalcanal is not formed, the infraorbital nerve maytraverse through the underlying maxillary sinus.In these cases, it is vulnerable to any sinus surgeryor sinus pathology.24 The zygomatic branch oftransmits the cranial nerves III, IV, VI, and V1.1,24

Anatomy on MR Imaging 447High-signal marrow of the optic strut may be seenseparating the optic nerve within the optic canalfrom cranial nerve III and other cranial nerves inthe superior orbital fissure on high-resolutionT1W MR imaging.24 The inferior orbital fissurelies between the orbital floor and the greaterwing of sphenoid, and communicates with thepterygopalatine fossa and masticator space.24

The optic canal and the superior orbital fissuretogether form the orbital apex, one of the impor-tant transition zones between intracranial andextracranial contents.1

Fat-saturated pulse sequences allow betterassessment of the lacrimal glands, optic nerves,and fatty reticulum of the orbit. However, this tech-nique may be suboptimal if the patient has hard-ware as previously described. STIR images, ifused as an alternative to fat-saturated T2W imagesbecause of dental or craniofacial hardware, mayshowbetter fat suppression, butwill be susceptibleto eye movements as they take a longer time toacquire.8 The lacrimal glands appear hypointensecompared with the surrounding orbital fat on T1Wimages, whereas they are slightly hyperintense tomuscle on T2W images and show homogenouscontrast enhancement (see Fig. 2). MR imaging it-self is not always sufficient for reliable assessmentof the lacrimal canaliculi, lacrimal sac, and nasola-crimal duct, for which MR dacryocystography maybe useful.8 The ophthalmic artery accompaniescranial nerve II in the optic canal. Its retinal arterybranch traverses within cranial nerve II, and otherbranches accompany the corresponding nervesin the orbit. Intraorbital arteries are generallybeyond the resolution of conventional MR imaging,although larger vascular lesions may show flowvoids and partly may be seen on time-of-flightMR angiography images.8 The superior ophthalmicvein is larger and more consistently visualized onboth coronal and axial imaging. It lies lateral tothe superior oblique muscle anteriorly, and passesposteriorly beneath the superior muscle complexwhere it can be seen on coronal imaging. It issupplied by the facial vein and drains via the supe-rior orbital fissures into the cavernous sinus,providing an important route for the spread ofthrombosis from the face in cases of orbital cellu-litis or facial infection.24

The extraocular muscles consist of four rectusmuscles, two oblique muscles, and one levatorpalpebrae superioris, in each orbit. These musclesare particularly well seen on coronal MR imaging(see Fig. 2), and appear hypointense to orbital faton T1W and T2W images.8 These musclesenhance intensely after contrast because of theabsence of a bloodtissue barrier.24 The medial,

lateral, superior, and inferior rectus muscles andsuperior oblique muscle originate at the annulusof Zinn at the optic foramen. Rectus muscles insertdirectly on the globe behind the limbus, whereasthe superior oblique passes through the tendinoussling (trochlea) posterior to superior orbital marginbefore it inserts into the sclera in the middle of theglobe. The inferior oblique muscle arises from theorbital floor posterior to the lacrimal sac and thentraverses beneath the inferior rectus, medial tothe lateral rectus, and inserts into the sclera adja-cent to superior oblique. Because the superiorrectus and levator palpebrae are not seendiscretely from one another, these are oftenreferred to together as the superior musclecomplex on imaging.24

Divisions of cranial nerve III (oculomotor nerve)supply the superior, medial, and inferior rectusmuscles and the inferior oblique along with amotorroot to ciliary ganglion. The abducens nerve,cranial nerve VI, supplies the lateral rectus andthe trochlear nerve supplies the superior oblique.Cranial nerve V1 (ophthalmic nerve) divides intothree branches in the distal cavernous sinusbefore entry into the orbit. These branches arethe frontal, lacrimal, and nasociliary nerves. Supra-orbital and supratrochlear (medial) branches of thefrontal nerve traverse just above the superiormuscle complex with the accompanying artery.The lacrimal nerves lie in the lateral portion of theorbit, superior to the lateral rectus muscle. The na-sociliary nerve crosses from lateral to medial sideabove the optic nerve to reach the superior surfaceof medial rectus, where it may be seen on high-resolution T1W images.24

The aqueous and vitreous humor in the ocularglobe appear isointense to CSF on all the pulsesequences. In the globe, T2W images are usedto evaluate lesions in the vitreous and aqueouschambers, whereas precontrast and postcontrastT1W images are used to evaluate the uveoretinalstructures. The optic nerve-sheath complex, asthe name suggests, includes the central cranialnerve II and surrounding sheath (dura and arach-noid), which contains CSF communicating withthe subarachnoid space. MR imaging distin-guishes the nerve, the dura, and the subarachnoidspace on T2W and contrast-enhanced T1W MRimaging (see Fig. 2).24

Cranial nerve II is generally isointense to cere-bral white matter and the surrounding extraocularmuscles on T1W and T2W images.8 MR imagingis not the preferred modality for assessing orbitalfractures, calcifications, and wooden foreignbodies, for which CT is very useful. Finally, imagingof orbits is incomplete without evaluation of thecranial nerves III through VI and the cavernous

sinus through which they traverse.

TEMPORAL BONE

The petrous temporal bone is located between theposterior and central skull base. MR imaging ofthe temporal bone always includes imaging ofthe internal auditory canal, cerebellopontine angle,and the labyrinth.11 The IAC traverses the petrousbone anterolaterally in an approximately horizontalplane (Fig. 3). Its lateral end is called the porusacusticus and the medial end is called the fundus.Cranial nerve VII and the nervus intermedius travel

in the anterosuperior aspect of the canal, whereasthe cochlear nerve (from cranial nerve VIII) travelsin the anteroinferior aspect of the canal. The supe-rior and inferior vestibular nerves (from cranialnerve VIII) travel in the posterior half of the canal.These nerves enter the labyrinth through thin,perforated bone at the fundus of the IAC. Vascularstructures seen in the IAC include the intracanalic-ular loop of the anterior inferior cerebellar arteryand the internal auditory artery. The labyrinthcan be divided by its layer or region. The bony

therucpeithrmrioSSestiiore sy, w

Morani et al448Fig. 3. Axial T2, T1 postcontrast, and CISS images ofinternal auditory canal (IAC) region (E) sagittal reconstlow signal intensity of the pons and middle cerebellar(circled) are well seen, as are a few linear structures wartery (IC) flow voids. (B) Postcontrast images show noVII tympanic segment enhancement (arrow). The antelooping into the IAC. (C) The sharp definition of the CIVII anteriorly (arrow) and the superior division of the vformats show the cranial nerve VII travelling just anterpontine angle cistern. Laterally, the four divisions can bcochlear division of VIII located inferiorly and anteriorl

VIII are present superiorly and inferiorly within the postertemporal bone (AC), along with cisternal (D) andtions of the CISS images. (A) T2 image shows intrinsicduncle (MCP). Fluid-filled cochlear (Co) and labyrinthin the IAC. Note the basilar (Ba) and internal carotidal geniculate ganglion (arrowhead) and cranial nerver inferior cerebellar artery (AICA) can be faintly seenimages allows for easy demonstration of cranial nervebular nerve posteriorly (arrowhead). (D, E) Sagittal re-to VIII (arrow), which is twice its size, in the cerebello-een, with VII located superiorly and anteriorly and thehereas the superior and inferior vestibular divisions of

ior aspect of the IAC (circled).

labyrinth, with an intervening layer of perilymph

Anatomy on MR Imaging 449fluid between the bony and membranous laby-rinth. The three parts of the bony labyrinth arethe vestibule of the ear, the semicircular canals,and the cochlea. The cochlea is spiral shaped andis pointed anterolaterally. It consists of two anda half turns. The membranous cochlea is dividedalong its length into two roughly equal chamberscontaining perilymph, which are scala vestibulianteriorly and the scala tympani posteriorly. Thescala media, also called the cochlear duct, isa small endolymphatic chamber anterior to thespiral lamina that contains the organ of Corti. Themodiolus is the signal void of the tissue corelocated at the central axis of the cochlear spiral.It is composed of the nervous tissue of the spiralganglia and the supporting bone and soft tissueof the spiral lamina. The vestibule is located post-erosuperiorly relative to the cochlea. The threesemicircular canals named the lateral, anterior,and posterior semicircular canal, are oriented atright angles to each other. Each canal is contig-uous at both ends with the vestibule. The endo-lymphatic duct is a small duct within the bonyvestibular aqueduct. It arises from the sacculeand utricle. Its dilated posterior portion is calledthe endolymphatic sac. These structures are reli-ably seen on MR imaging.25 Very-thin-slicegradient-echo T1W images are useful for showingdifferent turns of the cochlea, vestibule, semi-circular canals, and, in several cases, the endo-lymphatic sac. Fat-suppressed coronal T1Wspin-echo images is commonly used while im-aging the temporal bone, to eliminate the highsignal intensity of the fatty bone marrow in thewalls of the internal auditory canal. Submillimeterheavily T2W gradient-echo or FSE three-dimen-sional images are also useful for detailed evalua-tion of the labyrinth, providing high contrastamong the CSF, intralabyrinthine fluid, nerves,and the bone. Submillimeter images can alsodistinguish between the scala tympani and scalavestibuli/scala media. Neurovascular structuresin the internal auditory canal and cerebellopontineangle are also well seen on these images. Time-of-flight MR angiography images are also used toevaluate pulsatile tinnitus, neurovascular conflicts,vascular tumors, and vascular malformations.10,25

CRANIAL NERVES

MR imaging is the preferred method to evaluatethe cranial nerves. Although the skull base for-labyrinth, or osseous labyrinth, is the network ofpassages with bony walls lined with periosteum.The membranous labyrinth runs inside the bonyamina can be seen on CT, the nerves themselvescan only be seen on MR. At 1.5T, synergy coilsshould be used along with standard head coils toevaluate the entire course of cranial nerves,including the brainstem nuclei. On 3T systems,synergy coils are not needed because of the highersignal-to-noise ratio. Microscopic coils can beused if very small superficial nerve branches areto be evaluated. The coronal plane is best suitedto study the cranial nerves I to VI, because theyhave a dominant posteroanterior course. The axialplane is the best for evaluating the remainingcranial nerves, which have a dominant mediolater-al course. Although the cranial nerve nuclei andfascicular segments cannot be seen in the brain-stem, their location can be predicted if thesurrounding myelinated structures are identified.These are best seen on T2W, proton-density,and especially multiecho fast-field echo (m-FFE)or T2W two-dimensional spoiled gradient-echomultiecho sequence images.Heavily T2W three-dimensional sequences are

used if the cisternal segments of the cranial nervesare to be examined. Heavily T2W sequences areusually 0.5 mm thick or less. Parallel imaging andthe asymmetric k-space filling technique can beused to reduce the time for these longersequences. For imaging around the brainstem inthe center of the image, the sequences based onsteady state (eg, CISS, FIESTA) are used.However, these produce artifacts in the peripheryof the image and thus are not suitable for superfi-cially located nerves. For high-resolution imagingof more superficial nerves (eg, nerves I, VII, VIII),other types of three-dimensional heavily T2Wsequences such as DRIVen equilibrium (DRIVE)or three-dimensional turbo spin-echo are usedwith a slice thickness of 0.5 mm. The nerves arebest seen on high-resolution contrast-enhancedtime-of-flight MR angiography images or high-resolution two-dimensional (spin-echo or turbospin-echo) T1W images, when they are sur-rounded by a venous plexus (III to VI in thecavernous sinus, VI behind the clivus in the basilarplexus, IX to XI in the jugular foramen, XII in thehypoglossal canal). On these images, the cranialnerves are seen as signal voids surrounded bythe hyperintense enhancing venous structures.Because the peripheral segments and branches

of the cranial nerves at the skull base and in theneck are surrounded by fat and soft tissue, high-resolution T1W spin-echo and turbo spin-echosequences are best to visualize these portions ofthe nerves. The use of fat suppression will makethe fat nearly black in appearance, making visual-ization of normal nerves difficult or impossible. Fatsuppression is useful only when an abnormal

enhancement of the nerve is expected or must

is located in the periaqueductal gray matter. The

Morani et al450be excluded. Brainstem and cisternal segmentsare evaluated using 4-mm slices. Contrast-enhanced 0.625-mm T1W fast-field echo slicesare obtained when evaluating through the cerebel-lopontine angle, internal auditory canal, and thejugular foramina.

Cranial Nerve I

Olfactory epithelium is present in the upper one-fifth of the nasal cavity and covers the septal andlateral surface of this cavity. Dendrites of thebipolar olfactory neurons reach the epithelialsurface, and its unmyelinated axons, which aregrouped in bundles called filia, pass through theopenings in the cribriform plate to reach the olfac-tory bulb. Some of these filia, which togetherconstitute cranial nerve I (olfactory), are some-times seen on high-resolution T2W images. Theolfactory bulb and tract are located in the olfactorysulcus between the gyrus rectus andmedial orbitalgyrus and are seen on coronal T2W or T1W images(see Fig. 1). These are actually the extensions ofthe brain and not truly cranial nerves. The olfactorytract divides posteriorly into the lateral, interme-diate, and medial stria in front of the anterior perfo-rated substance on high-resolution T2W images.The lateral stria terminates in the piriform lobeand connects to the orbital frontal cortex (highestcenter for olfactory discrimination) via the thal-amus. The intermediate stria reach the interme-diate cortical olfactory area, which is a smallfocus of gray matter at the level of the anteriorperforated substance. Some axons in the medialstria reach the septal area via the diagonal band,whereas others reach the contralateral olfactorytract via the anterior commissure across themidline.

Cranial Nerve II

Cranial nerve II (optic nerve) is also an extension ofthe brain and not a true cranial nerve. It can bedivided into several segments: intraocular, intraor-bital, intracanalicular, and intracranial. The opticpathway then continues in the optic chiasm andoptic tracts, which further extend to the optic radi-ation and visual cortex, which are discussed else-where in this issue. The axons of the retinalganglion cells form the intraocular optic nerve,which is difficult to visualize. The intraorbitalsegment runs from the ocular globe to the orbitalapex in the intraconal orbit. The subarachnoidCSF space surrounding the intraorbital nerve iscontiguous with the suprasellar cistern. The nerveand surrounding CSF are best visualized onheavily T2W or STIR images (see Fig. 2). The

central retinal artery, with its accompanying vein,fascicular segment of the nerve courses throughthe midbrain anterolaterally to emerge medial tothe cerebral peduncle (Fig. 4). The cisternalsegment starts in the interpeduncular cistern andthen courses below the posterior cerebral andabove the superior cerebellar artery. Further ante-riorly, it continues below the posterior communi-cating artery to pierce the dural roof of thecavernous sinus. This segment is best seen onhigh-resolution heavily T2W images and is alsolarge enough to be seen on T1W images. Thecavernous segment of the nerve runs in the lateralwall of the cavernous sinus and is highest in posi-tion superolateral to the cavernous internal carotidartery. It lies medial to cranial nerve IV in theanterior-most portion of the cavernous sinus butbecomes inferomedial to it in the superior orbitalfissure. Cranial nerve III courses through thecavernous sinus and is best seen on coronalcontrast-enhanced high-resolution T1W imaging,and reportedly also on contrast-enhanced heavilyT2W imaging. The nerve divides into the superiorand inferior divisions within the superior orbitalfissure. The superior division innervates the supe-rior rectus and levator palpebrae. The inferior divi-sion supplies the inferior rectus, medial rectus, andruns within the distal 1 cm of the intraorbitalsegment just behind the globe. The intracanalicu-lar segment, as the name suggests, is located inthe optic canal along with the ophthalmic artery(inferior to the nerve) and is best seen on MRimages. The intracranial segment (covered byonly pia matter) is approximately 1 cm long andextends from the optic canal to the optic chiasm.The optic chiasm is X-shaped and located anteriorto the pituitary stalk, and is best seen on reformat-ted three-dimensional T1W images, such asthree-dimensional fast-field echo, magnetizationprepared rapid gradient echo (MPRAGE), or T2Wimages like DRIVE and balanced fast-field echo.In the chiasm, fibers from the temporal hemiretinacontinue uncrossed into the ipsilateral optic tract,whereas fibers from the nasal hemiretina continueinto the contralateral optic tract after crossing themidline. Each optic tract divides into a smallermedial root carrying only 10% of its fibers anda larger root carrying 90% of fibers. The medialroot terminates in the medial geniculate body,and the lateral root terminates in the lateral genic-ulate body. The optic tracts are better seen onhigh-resolution T2W or FLAIR images.

Cranial Nerve III

The cranial nerve III (oculomotor nuclear) complexinferior oblique muscles. These branches can

rpe(bl(w

Anatomy on MR Imaging 451again be well seen on high-resolution coronal T1Wimages. Parasympathetic fibers in the nervecontinue via the branch to the inferior obliquemuscle to reach the ciliary ganglion, which givesrise to postganglionic parasympathetic fibers inthe short ciliary nerves.

Cranial Nerve IV

Cranial nerve IV (trochlear) nucleus is situatedinferior to the cranial nerve III complex at the levelof the inferior colliculus, ventral to the aqueductand posterior to the medial longitudinal fasciculus.Its fascicular segments cross themidline at the levelof superior medullary velum before exiting themidbrain along the dorsal surface just caudal tothe inferior colliculus. After exiting the brainstem,its cisternal segment runs in a nearly horizontalmediolateral direction to reach the free edgeof the tentorium and then courses anteriorly around

Fig. 4. T1W thin-section coronal images along the intecles, cranial nerves III can be seen exiting the midbrainnoted to travel between the posterior cerebral arteryarrowhead).the brainstem. It passes through the gap betweenthe superior cerebral artery and superior cerebellarartery, lateral to the cranial nerve III. Its course prox-imal to the cavernous sinus is usually only seen onhigh-field-strength (3T) FIESTA- or CISS-styleimaging.26 The cavernous segment of the nerve isalso seen in lateral wall of the cavernous sinus adja-cent to cranial nerve III, as described previously. Itenters the orbit through the superior orbital fissureand supplies the superior oblique muscle.

Cranial Nerve V

The nuclei of cranial nerve V (trigeminal) arenumerous and a full discussion of them is beyondthe extent of this article. The cisternal or pregangli-onic segment of the nerve leaves from the mid-pons, also called the root entry zone. It iscomposed of sensory and motor roots. It coursesanterosuperiorly through the prepontine cistern,over the tip of the petrous apex, and then entersthe CSF-filled Meckel cave. It is best seen onheavily T2W images but can also be seen onhigh-resolution T1W images (Fig. 5). The pregan-glionic segment of the nerve ends in the gasserianganglion in Meckel cave; the postganglionic fibersexit through the three divisions of trigeminal nerve.The motor root passes under the gasserianganglion and exits through the foramen ovale.

Ophthalmic: first division (V1)V1 is seen in the lateral wall of the cavernous sinus,inferior to the fourth nerve and lateral to the sixthnerve. It is larger than these cranial nerves and isbetter seen on coronal contrast-enhanced high-resolution T1W images through the cavernoussinus. It then enters the superior orbital fissure,where it divides into frontal, lacrimal, and nasocili-ary nerves with sensory nerve supply from theglobe, nose, forehead, and scalp.

duncular cistern (A, B). (A) At the level of the pedun-ack arrowheads). (B) More anteriorly, the left nerve ishite arrow) and the superior cerebellar artery (whiteMaxillary: second division (V2)V2 courses in the wall of the floor of thecavernous sinus and exits the skull through theforamen rotundum, and is best seen on coronalimages. The nerve continues through the upperpart of the pterygopalatine fossa and then rea-ches the orbit through the inferior orbital fissureto terminate in the infraorbital nerve. In the ptery-gopalatine fossa, it gives off several side bran-ches: the posterior superior alveolar nerve, thezygomatic nerve, and two nerves to the pterygo-palatine ganglion. The infraorbital nerve exits theinfraorbital foramen after giving off the anteriorsuperior alveolar nerve, which runs in the lateralnasal wall.

Mandibular: third division (V3)V3 immediately exits the skull inferiorly throughthe foramen ovale without coursing through the

Morani et al452cavernous sinus. The motor root joins it in theforamen and then both continue to the masti-cator space. Its further detailed course belowthe skull base is beyond the scope of this article.It has a few salient features. The enhancingvenous plexus around the nerve just under theskull base allows the area of the buccal andanterior deep temporal nerve, major mandibularbranch, and the posterior extension of the nervecorresponding to the area of the otic ganglion,auriculotemporal nerve origin, and meningealbranch to be distinguished. On sagittal postcon-trast high-resolution T1W images, V3 can beseen in the oval foramen dividing into the lingualand inferior alveolar branches at the level of theinternal maxillary artery. The middle meningealartery, which passes through an opening in theauriculotemporal nerve just below the skullbase, is seen as a contrast-enhanced structure

Fig. 5. Coronal T1W postcontrast images of cranial nerve Vthe exiting cranial nerve V roots are well seen (circled). (noted within the Meckel caves (arrows). (C) The cavernouscarotid flow void (IC) and the descending and exiting maheads). SS, sphenoid sinus. (D) At the orbital apex, the conarrow). The foramen rotundum and vidians canal are denAC, anterior clinoid process.surrounded by a nerve with a low signal intensity.The inferior alveolar nerve can be seen in itscanal within the mandible, and the lingual nervecan always be seen in the pterygomandibularfat pad, located just behind and medial to theposterior free edge of the mylohyoid muscle onT1W images.27

Cranial Nerve VI

Cranial nerve VI (abducens) is a pure motor nerveand innervates only the lateral rectus muscle,which abducts the eye. Its nucleus is located inthe middle of the pons. The fascicular segmentof the nerve travels through the pontine teg-mentum to leave anteriorly at the lower border ofthe pons. Its cisternal segment crosses the pre-pontine cistern and follows an anterolateral supe-rior course to reach posterior aspect of the clivus

from posterior to anterior. (A) At the level of the pons,B) More anteriorly, the trigeminal nerves/ganglia aresinuses are now visualized (circled). Note the internalndibular division (V3) of the trigeminal nerve (arrow-tents of the superior orbital fissure can be seen (blackoted by the white arrowhead and arrow, respectively.

(Fig. 6). This segment is best seen in the axialplane on heavily T2W images, and also on coronalSTIR and T1W images. The nerve then pierces thedura to enter the Dorello canal, a channel betweentwo dural layers through the basilar venous plexus.Contrast-enhanced time-of-flight MR angiographyimages or three-dimensional fast-field echo im-ages are useful for seeing the signal void of thenerve within the enhancing venous plexus at thelevel of the Dorello canal. The nerve then runsover the petrous apex and enters the cavernoussinus just above the Meckel cave. It continueswithin the cavernous sinus itself, in contrast toother cranial nerves that run in the cavernous sinuswalls. It then enters the orbit through the superiororbital fissure to supply the lateral rectus. Thecavernous and extracranial segments are bestseen on gadolinium-enhanced high-resolutionT1W images.27 segment of cranial nerve VII begins after it leaves

Anatomy on MR Imaging 453Cranial Nerve VII

The cranial nerve VII loops around the nucleus ofcranial nerve VI in the pons, creating the facial col-liculus in the floor of the fourth ventricle. It thencontinues anterolaterally and exits the brainstemtogether with the intermediate nerve at the lowerborder of the pons. The cisternal segment ofboth nerves traverse through the cerebellopontineangle. These nerves are better seen on heavilyT2W images. The sensory and parasympatheticfibers are carried in the nervus intermedius, whichis located just posterior to the nerve proper(carrying motor fibers). The intracanalicular portionof cranial nerve VII is seen in the anterosuperiorpart of the IAC. Cisternal and the intracanalicular

Fig. 6. Axial CISS image shows cranial nerve VIascending within the prepontine cistern (arrows), B,

basilar artery.the stylomastoid foramen and enters the posteriorparotid. It may be seen along the proximal-mostextracranial segment on high-resolution T1W im-ages, but is no longer visible beyond this in theparotid. Its position may be assumed, because itnormally courses just lateral to the retromandibularvein. If needed, microscopic coils and strong gradi-ents may be used to visualize the intraparotidcourse of nerve. Finally, the nerve divides intomotorend branches supplying the muscles of facialexpression; the platysma, buccinator, stylohyoid,and occipitalis muscles; and the posterior belly ofthe digastric muscle.27 The temporal bone portionof the facial nerve and the greater superficialpetrosal nerve can show normal but mild enhance-ment throughout, except in the cisternal and cana-licular segments.28

Cranial Nerve VIII

Cranial nerve VIII is composed of a cochlear anda vestibular nerve. Both are sensory nerves andare formed by the bipolar neurons. The bipolarneurons of the cochlear nerve are located in thespiral ganglion within the modiolus of the cochlea.Peripheral fibers of these neurons are connectedto the organ of Corti in the scala media of thecochlea, and the central fibers join to form thecochlear nerve proper. The cochlear nerve entersthe IAC through an opening in the anteroinferiorpart of the fundus of the IAC and remains in the an-teroinferior quadrant of the IAC. It is joined by thesuperior and inferior vestibular nerves near the po-rus acusticus to form the vestibulocochlear nerveor cranial nerve VIII, which crosses the cerebello-pontine angle posterior to cranial nerve VII to reachthe lateral pontomedullary junction ending inportions of the nerve and nervus intermedius canbe distinguished on high-resolution T2W images,especially at 3T. The intratemporal segment ofthe nerve begins at the fundus of the IAC, whereit enters the labyrinthine part of the facial nervecanal. It runs anterior to reach geniculate ganglion,which gives off the greater superficial petrosalnerve carrying the parasympathetic fibers antero-medially for lacrimation.Fromthegeniculateganglion, thenervecontinues

posteriorly in the tympanic segment canal under thelateral semicircular canal to reach the posteriorgenu, where it turns inferiorly as the mastoidsegment (see Fig. 3). It supplies the stapediusmuscle andalsocarries taste fibers from theanteriortongue received from the lingual nerve through thechorda tympani nerve. These branches of the nerveare not seen on MR imaging. The extracranialcochlear nuclei.

Bipolar neurons of the vestibular nerve are lo-cated in the Scarpa ganglion. Its peripheral fibersconnect the maculae in the utricle and saccule,and the three cristae in the three ampullae of thesemicircular canals with the four vestibular nucleiin the lower pons. Its multiple fibers pass thoughthe foramina in the fundus of the IAC to form thesuperior and inferior vestibular nerves. The supe-rior vestibular nerve courses in the posterosuperiorquadrant and the inferior vestibular nerve in theposteroinferior quadrant of the IAC, respectively.These join to form a single vestibular nerve in porusacusticus and, further medially, join with thecochlear division to form the eighth cranial nerve.

Cranial Nerve X

Cranial nerve X (vagus) is a parasympathetic nervesupplying the head, neck, thoracic region, andabdominal viscera, and has motor function to thesoft palate, pharyngeal constrictor muscle, larynx,and palatoglossus muscles. It also carries sensoryinformation from the viscera, external ear, andtympanic membrane, and taste from the epiglottis.The nerve exits the brainstem just below cranialnerve IX and courses with this nerve to reach thepars vasculosa of the jugular foramen (seeFig. 7).26 The superior vagal ganglion is locatedin the jugular foramen, and the inferior vagal

heavily T2W images. The foraminal and extracra-

sk).

erve

Morani et al454Generally, within the cerebellopontine angle,cranial nerve VII is approximately half the size ofVIII (see Fig. 3). A subtle thickening can often beseen on the vestibular nerves in the IAC wherethe common vestibular branch splits into a superiorand inferior branch. This thickening correspondswith the Scarpa ganglion. Sometimes connectingfibers are seen between cranial nerve VII and thevestibular nerves on high-resolution T2W images.

Cranial Nerve IX

The nuclei for cranial nerve IX (glossopharyngealnerve) are located in the upper and middlemedulla. The nerve leaves the brainstem in thepostolivary sulcus and courses anterolaterallytogether with cranial nerves X and XI, which arelocated just caudal to cranial nerve IX (Fig. 7).26

It then enters the pars nervosa of the jugularforamen, where its superior and inferior gangliaare also located. Cranial nerve IX can be seenand distinguished from remaining structures inthe foramen on high-resolution gadolinium-enhanced fast-field echo or time-of-flight images.The nerve then enters the carotid space andcourses lateral to the carotid artery, stylopharyng-eus, and the palatine tonsil to reach the posteriorpart of sublingual space as the lingual nerve.

Fig. 7. Axial and coronal reformatted CISS images at theit heads from the medulla to the jugular foramen (arrowwhere the nerve (arrow) travels between the cranial n

inferiorly (black arrowhead). The cranial nerve VII/VIII comnial segments can be well seen on high-resolution T1W fast-field echo or time-of-flightimages. Cranial nerve X appears relatively thickerthan cranial nerves IX and XI.

Cranial Nerve XI

Cranial nerve XI is a pure motor nerve, innervatingthe sternocleidomastoid and trapezius muscles. Itis formed from the bulbar and spinal motor fibers.The spinal fibers arise from the spinal motornucleus lateral to the anterior horns of the cervicalspinal cord from the C1 to C5 vertebral levels. Thespinal fibers exit the cord from its lateral surfacebetween the anterior and posterior nerve roots.These fibers form an ascending nerve, which

ull base (A, B). (A) Note the course of cranial nerve X as(B) This is well demonstrated on the sagittal reformats,IX superiorly (white arrowhead) and cranial nerve XIganglion is located just below the skull base. TheArnold nerve branches off from the superiorcervical ganglion and carries sensory informationfrom the external ear. The other branches ofcranial nerve X include the pharyngeal branches,the superior laryngeal nerve, and the recurrentlaryngeal nerve, which ascends in the tracheoeso-phageal groove after looping around the subcla-vian artery on the right, or passes through theaortopulmonary window on the left side. Thecisternal segment is well seen on high-resolutionplex is circled.

Anatomy on MR Imaging 455reaches the jugular foramen after passing throughthe foramen magnum. The bulbar cisternalsegment is located just below cranial nerve X(see Fig. 7).26 The bulbar and spinal fibers join inthe lateral part of basal cistern. The nerve thenpasses through the pars vasculosa of the jugularforamen and then enters the carotid space belowthe skull base.

Cranial Nerve XII

Cranial nerve XII is a motor nerve, innervating theintrinsic and extrinsic tongue musculature. Itsnucleus is located in the lower medulla, producinga slight bulge into the fourth ventricle called thehypoglossal eminence. Its fascicular segmenttraverses anterolaterally and exits from the brain-stem from the preolivary sulcus. It emerges asa series of rootlets, which converge to form oneor two root nerves. This (cisternal) segment iswell seen on thin high-resolution T2W images. Itthen enters the skull base at the hypoglossal canal(see Fig. 1). On contrast-enhanced T1W three-dimensional fast-field echo images through thehypoglossal canal, this nerve can be seen asa gray arch from its entrance in the hypoglossalcanal down to the upper carotid space, in thebackground of surrounding hyperintensity fromthe enhancing veins. The nerve leaves the carotidspace at the inferior margin of the posterior bellyof the digastric muscle, coursing lateral to thecarotid bifurcation and the hypoglossus muscleto reach the tongue.27

SUMMARY

The skull base is a complex region with multiplecompartments and components, susceptible to amultitude of disease processes. Cross-sectionalimaging, particularly MR imaging, is vital in interro-gating these spaces, because they are not easilyevaluated clinically. Therefore, knowledge of thenormal appearance of this area on MR imaging isa prerequisite for evaluating pathologic processeswithin it.

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Morani et al456

Skull Base, Orbits, Temporal Bone, and Cranial Nerves: Anatomy on MR ImagingProtocolAnatomyAnterior skull baseCentral skull basePosterior skull baseOrbitsTemporal boneCranial nervesCranial Nerve ICranial Nerve IICranial Nerve IIICranial Nerve IVCranial Nerve VOphthalmic: first division (V1)Maxillary: second division (V2)Mandibular: third division (V3)

Cranial Nerve VICranial Nerve VIICranial Nerve VIIICranial Nerve IXCranial Nerve XCranial Nerve XICranial Nerve XII

SummaryReferences