-
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,
n Medical Center, 1500 East
ner
.thec
linics
.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: [email protected](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
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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
440
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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
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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
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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.
-
(Aediuper ttedorervpospo
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
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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