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Normal anatomy, clinical importance and
nerve specific pathologies
Best imaging modality for any simple or complex cranial
neuropathy is MRI.
The only exception to this is imaging of distal vagal
neuropathy where it is necessary to image the aorto-
pulmonary window on left with CECT.
If a lesion is located in bony area such as skull base,
sinuses or mandible, CT in bone window is
recommended to provide complimentary bone anatomy
& lesion related information.
Contrast CT is not necessary if T1, T2 and contrast T1
MR is available.
Cranial nerves do not stop at the skull base !!
CN 2, 3, 4 & 6 – include focused orbital sequences
CN 5 – include entire face to inferior mandible if V3 affected
CN 7 – include CP angle, temporal bone and parotid space
CN 8 – include CPA, IAC and inner ear
CN 9-12 – include basal cisterns, skull base and
nasopharyngeal carotid space
Why specific MRI sequences ??
Traditional magnetic resonance (MR) imaging sequences
provide excellent soft-tissue resolution, they may lack the
spatial resolution necessary to define smaller structures
such as cranial nerves
Steady-state free precession (SSFP) sequences allow
much higher spatial resolution and clearer depiction of tiny
An SSFP sequence is any gradient-echo sequence in
which a nonzero steady state develops between pulse
repetitions for both the longitudinal and transverse
relaxation values of the interrogated tissues. A small flip
angle and short relaxation time are required for this to
Advantages of SSFP -
ability to generate a strong signal in tissues that have a
high T2/T1 ratio, such as cerebrospinal fluid (CSF) and fat
particularly useful for visualizing the cisternal segments of
cranial nerves because they provide excellent contrast
resolution between CSF and nerves, as well as high
spatial resolution with submillimetric section thicknesses
Disadvantages of SSFP –
reduced contrast resolution between different soft tissues
global landmarks may be poorly depicted because of the
submillimetric section thicknesses
Thus, SSFP sequences play a supplemental role alongside
traditional sequences in MR imaging of the cranial nerves.
Usually referred to by their trade names or acronyms (eg,
constructive interference steady state, or CISS, and fast
imaging employing steady-state acquisition, or FIESTA)
Normal anatomy, clinical importance and
nerve specific pathologies
There are twelve cranial nerves and their defining
feature is that they exit the cranial cavity through
foramina or fissures.
All cranial nerves innervate structures in the head
In addition, the vagus nerve [X] descends through
the neck and into the thorax and abdomen where
it innervates viscera.
Cranial Nerve I: The Olfactory Nerve
Unlike most cranial nerves, the olfactory nerve
consists of white-matter tracts and is not
surrounded by Schwann cells.
The neurosensory cells for smell reside in the
olfactory epithelium along the roof of the nasal
The axons of these cells extend through the
cribriform plate of the ethmoid bone into the
olfactory bulb at the anterior end of the
Courses posteriorly through the anterior cranial fossa in
the olfactory groove.
Posterior to the olfactory groove, the cisternal segment of
the nerve runs below and between the gyrus rectus and
the medial orbital gyrus.
These secondary axons in the olfactory nerve eventually
terminate in the inferomedial temporal lobe, uncus, and
To avoid confusing the olfactory nerve with the gyrus
rectus on axial images, it is important to remember that
the olfactory nerve is situated deep in the olfactory
groove, inferior to the gyrus rectus.
Coronal images are easiest to interpret because the
nerves are seen in cross section.
SSFP MR images show the olfactory nerve (white arrow) within the CSF-filled
olfactory groove and the optic nerve
Coronal image shows the cisternal segment of the olfactory nerve (arrow), which is
located inferior to and between the gyrus rectus (r) and the medial orbital gyrus (o).
Imaging recommendations -
Coronal sinus CT is best for isolated anosmia
Identifies nasal vault and cribriform plate lesions
MRI of brain, anterior cranial fossa and sino-
nasal region best for complex anosmia cases
Identifies intracranial and dural lesions
CN1 dysfunction produces unilateral anosmia.
Esthesioneuroblastoma arises from olfactory
epithelium in the nasal vault
Head trauma may cause anosmia – cribriform
plate fracture or anterior temporal lobe injury
Seizure activity in the olfactory area may
produce „uncinate fits‟, imaginary odor,
oroglossal automatisms and impaired
also known as an esthesioneuroblastoma
is a tumor arising from the basal layer of the olfactory epithelium in
the superior recess of the nasal cavity
bimodal age distribution with one peak in young adult patients (~
2nd decade) and another peak in the 5th to 6th decades
no recognized gender predilection
usually secondary to nasal stuffiness and rhinorrhoea or epistaxis
Presentation is often delayed
Patients often present late with larger tumors which can extend
into the intracranial compartment (25 - 30% at diagnosis) and
usually result in anosmia
begin as masses in the superior olfactory recess and initially
involve the anterior and middle ethmoid air-cells on one side.
As they grow, they tend to destroy surrounding bone, and can
extend in any direction
This invasion may be superiorly into the anterior cranial fossa,
laterally into the orbits and across the midline into the
contralateral nasal cavity.
Particular attention should be paid to the presence of cervical
and retropharyngeal nodal metastases which are present in 10
- 44% of cases at diagnosis
particularly useful in assessing bony destruction, although it cannot
distinguish olfactory neuroblastomas from other tumours that arise in
the same region
soft tissue attenuation, with relatively homogeneous enhancement
Focal calcifications are occasionally present
relatively slow growing and thus, the bony margins are often
remodelled and resorbed, rather than being aggressively destroyed
T1 : heterogeneous intermediate signal
T2 : heterogeneous intermediate signal
T1 C+ (Gd) : variable enhancement (usually moderate to intense)
When intracranial extension is present, peritumoural cysts between it
and the overlying brain are often present. This may be helpful in
distinguishing it from other entities. The margins of these cysts
Angiography / DSA
Angiography demonstrates a prominent tumour
blush with arteriovenous shunting, and
As with other neuroblastomas, olfactory
neuroblastomas are MIBG avid.
This potentially helps to differentiate them from
other tumors that arise in the region
Treatment and prognosis
usually involves combined chemotherapy and / or
radiotherapy with surgical excision
Prognosis is significantly affected by presence of
no distant metastases : 60% 5-year survival
distant metastases : 0% 5-year survival
Small localised tumours have a high cure rate, up to 85
Unfortunately, imaging alone often struggles to distinguish between olfactory
neuroblastomas and other aggressive malignancies in the region.
rare and indistinguishable on imaging
olfactory groove meningioma / haemangiopericytoma
especially if inferior extension
may appear identical
usually older patients
lack peritumoural cysts
epicentre more posteriorly located
usually older patients
juvenile nasopharyngeal angiofibroma
epicentre more posteroinferiorly located
almost exclusively in males
often somewhat younger
Like the olfactory nerve, the optic nerve is a white-matter
tract without surrounding Schwann cells.
It includes four anatomic segments:
Cranial Nerve II: The Optic Nerve
The intra-ocular segment leaves the ocular
globe through the lamina cribrosa sclerae
(the optic foramen of the sclera).
It is 1mm in length.
20-30 mm in length
Extends postero-medially from back of globe to orbital
apex with in the intraconal space.
Covered by 3 meningeal layers of brain & the
subarachnoid space with CSF is continuous with the
suprasellar cistern. So fluctuations in IC pressure are
transmitted via SAS of optic nerve-sheath complex.
Central retinal artery(1st br of ophthalmic artery) enters the
optic nerve 1cm posterior to globe with accompanying vein
to run to retina.
It is a 4-9 mm segment within the bony optic
Ophthalmic artery lies inferior to CN2.
Dura of CN2 fuses with the periosteum of the
This segment of the nerve is frequently
overlooked on radiologic images, so it should be
specifically sought when imaging for vision loss.
Intra-cranial /cisternal segment
About 10mm length from optic canal to chiasm.
Covered by pia & surrounded by CSF within the
Ophthalmic artery runs infero-lateral to nerve.
The anterior cerebral artery passes over the
superolateral aspect of the cisternal segment
of the nerve.
Optic nerve. the retinal (black arrow), orbital (black arrowheads), and canalicular
(white arrowhead) segments.
The cisternal segment of the optic nerve (white arrow) leads to the chiasm, which
resembles the Greek letter X in this plane.
The optic tract (white arrowheads) leads backward from the chiasm to the thalamus.
Important anatomic landmarks include the mamillary bodies (black arrowhead) and
the anterior cerebral artery (black arrow).
Key anatomic landmarks - in the suprasellar cistern
include the infundibulum (stalk) of the pituitary gland, the
anterior cerebral artery, and, posterior to the chiasm, the
The optic nerve terminates at the optic chiasm, where the
two nerves meet, decussate, and form the optic tracts.
The optic tracts travel around the cerebral peduncles, after
which most axons enter the lateral geniculate body of the
thalamus, loop around the inferior horns of the lateral
ventricles (Meyer loop), and enter the visual cortex in the
Because a single image obtained with an SSFP
sequence usually depicts only a short segment of the
optic nerve, thick-section reconstruction of SSFP
acquisitions may be needed to allow examination of the
entire length of the nerve on a single image.
Standard T2-weighted images also are useful for this
Optic nerve pathology: Monocular visual loss
Optic chiasm pathology: Bitemporal heteronymous hemianopia
(loss of bilateral temporal visual fields)
Retrochiasmal pathology: Homonymous hemianopsia
Increased intracranial pressure transmitted
along SAS of optic nerve-sheath complex
Manifests clinically as papilledema
Imaging shows flattening of posterior sclera, tortuosity and
elongation of intraorbital optic nerves and dilatation of perioptic
Optic nerve sheath meningioma
benign tumour arising from the arachnoid cap sells of optic nerve
represents ~ 10 - 30% of all orbital meningiomas
majority are direct extensions from intracranial meningiomas
approximately 1/3rd of all optic nerve neoplasms (gliomas MC)
are usually seen in adults (mean age at presentation 40 years)
However up to 25% present in children, in which case they tend to be
The vast majority of cases are sporadic, although patients
with neurofibromatosis type II (NF2) are at increased risk.
Clinical presentation –
95% of cases
painless and progressive
more pronounced with lesions that affect the orbital
exacerbated during pregnancy
60-90% of cases
more pronounced with anterior lesions near the globe
occurs later, as the tumour enlarges
may be exacerbated by associated hyperostosis
same imaging characteristics as meningiomas elsewhere
tubular : 65%
exophytic : 25%
fusiform : 10%
Perioptic cysts- occasionally cysts filled with arachnoid
fluid between the globe and the anterior margin of the
tumour as a result of impaired CSF flow backwards
On axial or oblique sagittal imaging the enhancing
tumour surrounding the non-enhancing optic
nerve results in the so-called tram-track sign.
On coronal imaging the tumour appears as a cuff of
enhancing tumour around a central non-enhancing dot
Tumour extending into the optic canal may lead to canal
widening, or alternatively hyperostosis
greater ability to delineate posterior extension
T1 : isointense to somewhat hypointense compared to
the optic nerve
T2 : isointense to somewhat hyperintense compared to
the optic nerve
T1 C+ (GAD) : homogeneous enhancement
Careful examination of the orbital apex and optic nerve
canals is essential if small intracanalicular tumours are
to be identified
Cranial Nerve III: The Oculomotor
It is divided in to
Thin sections in Axial & Coronal section in the region of
periaqueductal white matter.
The oculomotor nerve originates from nuclei deep to the
superior colliculus, ventral to the cerebral aqueduct, and
inferior to the pineal gland.
The nerve then travels across the midbrain from posterior
The oculomotor nerve root emerges into the
interpeduncular cistern, and this root entry zone in the
cistern is a good way to identify the oculomotor nerve on
axial FIESTA images.
Courses anterolaterally through interpeduncular & pre-
pontine cisterns, passes between the posterior cerebral
artery & superior cerebellar artery which makes it easy to
identify on coronal FIESTA images.
Crosses the petro-clinoid ligament & penetrates the dura
to enter roof of cavernous sinus.
The cavernous segment of the oculomotor nerve runs
along the lateral wall of the cavernous sinus and is the
most superior of the nerves in this sinus.
It is surrounded by narrow oculomotor CSF cistern.
The oculomotor nerve then enters the orbit through the
superior orbital fissure, before splitting into superior and
inferior divisions lateral to the optic nerve.
Superior branch- LPS & superior rectus
Inferior branch – inferior rectus, medial rectus & inferior
The inferior branch also gives fibers to ciliary ganglion.
FIESTA image shows the nerve (small arrows) where it emerges from the inter-
peduncular cistern (large arrow)
SSFP MR image shows the oculomotor nerve (white arrow) in cross section between
the posterior cerebral artery (white arrowhead) and the superior cerebellar artery
Oculomotor nerve compression in an 82-year-old woman with ptosis of
the right eye. Axial 0.8-mm-thick FIESTA MR image shows displacement and
compression of the right oculomotor nerve in the root entry zone (long arrow) by the
distal basilar artery (short arrow). The left oculomotor nerve (arrowhead), in
comparison, appears normal.
Uncal herniation pushes CN3 on petro-clinoid ligament.
During trauma downward shift of brainstem upon impact can
stretch CN3 over petroclinoid ligament.
CN3 is susceptible to compression by PCA aneurysms.
Strabismus, ptosis, pupillary dilatation, downward abducted
globe and paralysis of accommodation
Cranial Nerve IV: The Trochlear Nerve
The trochlear nerve is the only nerve with a root entry zone
arising from the dorsal (posterior) brainstem.
After exiting the pons, the trochlear nerve curves forward
over the superior cerebellar peduncle, then runs alongside
the oculomotor nerve between the posterior cerebral and
superior cerebellar arteries.
The trochlear nerve then pierces the dura to enter the
cisterna basalis between the free and attached borders of
the cerebellar tentorium.
After completing its cisternal course, the trochlear nerve
runs through the lateral cavernous sinus just below the
oculomotor nerve and enters the orbit through the superior
orbital fissure to innervate the superior oblique muscle.
The nerve is named for the trochlea, the fibrous pulley
through which the tendon of the superior oblique muscle
The cisternal segment of this tiny nerve is most easily
identifiable posterolateral to the brainstem.
Along part of its intracranial course, the trochlear nerve
lies between dural layers, where it is difficult to visualize
on radiologic images.
Particular attention should be given to the anterior
aspect of the tentorium in patients in whom the presence
of isolated trochlear nerve palsy is suspected.
CN 4 is the smallest cranial nerve & has the
longest intra-cranial course (~7.5 cm)
Optimally visualized if thin sections are taken
Axial section margins : orbital roof -
diencephalon to maxillary sinus roof- medulla
Coronal section margins: 4th ventricle to
Trochlear nerve. FIESTA MR image shows both trochlear nerves (arrows) where
they emerge from the dorsal midbrain to cross the ambient cisterns.
The characteristic course of the trochlear nerves allows their differentiation from
the nearby superior cerebellar artery (arrowheads).
CN4 neuropathy divided into simple and complex
Simple CN4 neuropathy (isolated)
Most common form; usually secondary to trauma
Cisternal segment injury by free edge of tentorium cerebelli or
from posterior cerebral or superior cerebellar artery aneurysm
Contusion of superior medullary velum
Complex CN4 neuropathy (associated with other CN injury,
CN3 ± CN6)
Brainstem stoke or tumor
Cavernous sinus thrombosis, tumor
Paralysis of superior oblique muscle results in extorsion
(outward rotation) of affected eye. Extorsion is secondary
to unopposed action of inferior oblique muscle
Patient complaints: Diplopia, weakness of downward
gaze, neck pain from head tilting.
Cranial Nerve V: The Trigeminal
largest (thickest) cranial nerve
composed of a large sensory root that runs medial to a
smaller motor root
The roots emerge from the lateral mid-pons and travel
anteriorly through the pre-pontine cistern.
Enters middle cranial fossa by passing beneath tentorium
at the apex of the petrous temporal bone & passes through
an opening in the dura called the porus trigeminus to enter
the Meckel’s (trigeminal) cave.
Meckel (trigeminal) cave is a CSF-containing pouch in the
middle cranial fossa which is continuous with the pre-
pontine sub-archnoid space.
Pia covers the CN 5 in Meckel‟s cave.
Because the trigeminal nerve is large and its course
proceeds straight forward from the lateral pons, it is easy to
recognize on most MR images.
FIESTA MR image shows the sensory (arrowhead) and motor (large arrow)
roots of the trigeminal nerve where they cross the prepontine cistern and
enter the Meckel cave (small arrows).
In the Meckel cave, the nerve forms a mesh-like web that
can be visualized only with high-resolution imaging.
Along the anterior aspect of the cavity, the trigeminal
nerve forms the trigeminal (Gasserian / Semilunar)
ganglion before splitting into three subdivisions.
Imaging pitfall –
Trigeminal ganglion lacks blood-nerve barrier, so
normally enhances in post contrast images.
The ophthalmic (V1) and maxillary (V2) divisions of the
nerve move medially into the cavernous sinus and exit
the skull through the superior orbital fissure and foramen
The mandibular division (V3), which includes the motor
branches, exits the skull inferiorly through the foramen
FIESTA image at the level of the Meckel cave shows the complex web of
trigeminal nerve branches (arrows), which coalesce anteriorly to form the
Sagittal T2 MR along line of proximal trigeminal nerve shows the
preganglionic segment between the root entry zone in the lateral
pons and the trigeminal ganglion in the anteroinferior Meckel cave.
The cerebrospinal fluid within Meckel cave communicates with
prepontine cistern through the porus trigeminus.
Trigeminal neuralgia / tic douloureux
abrupt unilateral shock like facial pain lasting seconds to
Neurovascular contact by an artery or a vein at the root entry
zone of the trigeminal nerve is linked to trigeminal neuralgia.
The average diameter of the unaffected trigeminal nerve has
been estimated on transverse MR images to be 4 mm, with the
range being 2–6 mm .
In the majority of cases, there is atrophy of the nerve tissue
which is secondary to chronic compression of the nerve by
aging and tortuous vessels along the course of the nerve after
its point of exit from the brainstem.
Up to 42% of symptomatic nerves have gross atrophy.
•Images in a 63-year-old man with trigeminal neuralgia, with NVC
caused by the superior cerebellar artery.
•Two adjacent transverse 3D CISS MR images show that the
superior cerebellar artery (short arrow) has compressed the REZ of
the right trigeminal nerve (long arrow) at the medial site.
Male patient with left facial pain. Axial FIESTA image (A) and sagittal
reconstruction (B) show that the root entry zone of the left trigeminal nerve is
thinned and displaced by an adjacent vessel (thick white arrow points to the
nerve and thin white arrow points to the vessel)
Cranial Nerve VI: The Abducens
Emerges from nuclei anterior to the fourth ventricle
Courses anteriorly through the pons to the
pontomedullary junction and into the prepontine cistern
After crossing the prepontine cistern in a posterior-to-
anterior direction, the abducens nerve runs vertically
along the posterior aspect of the clivus, within a fibrous
sheath called the ‘Dorello canal’.
Continues over the medial petrous apex and through the
medial cavernous sinus, entering the orbit through the
superior orbital fissure to innervate the lateral rectus
• The ponto-medullary junction
• Cerebellopontine angle (CPA) and
• Basilar artery (arrowhead)
are important anatomic landmarks.
FIESTA image shows the abducens nerve where it enters the Dorello canal (arrow)
along the posterior aspect of the clivus
Vascular landmarks include the basilar artery (black arrowhead) and the anterior
inferior cerebellar artery (white arrowhead).
Axial and coronal MR sequences should include
brainstem, fourth ventricle, cavernous sinus and orbit
Cisternal segment routinely visualized on high-resolution
CN6 entrance into Dorello canal is visualized due to
invagination of cerebrospinal fluid into proximal canal
Abducens nerve is only cranial nerve to lie within
It is important to note that the abducens nerve runs
almost the entire length of the clivus.
Should be vigilant for clivus and petrous apex
abnormalities in the setting of abducens nerve palsy.
Although the abducens nerve lies near the anterior
inferior cerebellar artery and has a similar caliber, the two
structures course in orthogonal directions and are thus
In abducens neuropathy, affected eye will not abduct
Image of brainstem and prepontine cisterns shows proximal cisternal CN6
closely associated with the belly of the pons. CN3 is seen passing between
posterior cerebral and superior cerebellar arteries.
III (long black arrow),
IV (black arrowhead),
V1 (long white arrow),
V2 (white arrowhead), and
VI (short black arrow) are clearly demonstrated in the normal
Cranial Nerves VII and VIII: The Facial and
The facial and vestibulocochlear nerves have similar
cisternal and canalicular courses .
They both emerge from the lateral aspect of the lower
border of the pons and traverse the cerebellopontine
angle cistern at an oblique angle.
There, they may be in close proximity to the anterior
inferior cerebellar artery.
The nerves cross the porus acousticus (an opening
between the cerebellopontine angle cistern and the internal
auditory canal; also known as the internal acoustic meatus)
and traverse the length of the internal auditory canal.
Radiologic images that precisely depict the relationship of
the nerves to masses in the cerebellopontine angle can
help in surgical planning.
FIESTA image shows the parallel courses of the facial (black arrowheads) and
superior vestibular (white arrowheads) nerves as they cross the cerebellopontine
angle to enter the internal auditory canal through the porus acusticus (double
Facial & vestibulocochlear nerves
Facial nerve is anterior & superior to vestibulocochlear nerve within CPA& lAC.
The anteroinferior cerebellar artery loop is a constant fixture in the normal anatomy of
the CPA & lAC area.
meningioma in a 52-year-old
woman with left sensorineural
(a) Axial 0.8-mm-thick FIESTA
image shows a tumor that fills
the internal auditory canal
(arrow) and extends into the CP
(b) Coronal oblique 0.8-mm-
thick FIESTA image shows
direct involvement of the facial
nerve (arrowhead), a
against surgical resection.
The tumor was treated instead
with stereotactic radiosurgery.
Within the internal auditory canal, the vestibulocochlear
nerve splits into three parts (cochlear, superior vestibular,
and inferior vestibular).
These three vestibulocochlear nerve branches, along with
the facial nerve, have a characteristic appearance on
sagittal oblique FIESTA cross-sectional images.
Images in that plane are most frequently used for the
detection of cochlear nerve aplasia.
Cochlear nerve aplasia in a 4-year-old girl with congenital hearing loss who was under
consideration for cochlear implantation.
Sagittal oblique FIESTA images, obtained in planes perpendicular to the left (a) and
right (b) internal auditory canals -
facial (white arrow),
superior vestibular (white arrowhead),
inferior vestibular (black arrowhead)
However, the cochlear nerve (black arrow in a) is absent in b, and that finding is a
contraindication against cochlear implantation for the right ear. Incomplete separation of
the superior and inferior vestibular nerves, also shown in b, is a normal variant.
On any single axial FIESTA image, only two of the four nerves
within the internal auditory canal typically are visible.
If one of the nerves is seen to enter the modiolus of the cochlea,
then the two visible nerves are the cochlear and inferior vestibular
If the central modiolus is not depicted on the image, the visible
nerves are the facial and superior vestibular nerves.
A filling defect within the membranous labyrinth on FIESTA images
may signal a nerve abnormality in a branch of the facial or
The facial nerve exits the internal auditory canal and enters the
After a complex course within the petrous bone, the facial nerve
exits the skull base through the stylomastoid foramen and enters
the substance of the parotid gland.
Cochlear schwannoma –
Patient presented with right-sided sensorineural hearing loss
Thin T1W fat saturated contrast-enhanced image (A) shows a well-defined enhancing
mass lesion (white arrow) of 2-mm size, confined to the right internal auditory canal.
Oblique axial FIESTA image (B) reveals that the lesion is confined to the cochlear
nerve (antero-inferior) (white arrow)
Case of NF2 -
Bilateral acoustic schwannomas
Thickened left trigeminal nerve , possibility of schwannoma
Cranial Nerve IX: The Glossopharyngeal
The glossopharyngeal nerve emerges from the lateral
medulla into the lateral cerebellomedullary cistern, above
the vagus nerve and at the level of the facial nerve.
In the lateral cerebellomedullary cistern, the
glossopharyngeal nerve is closely associated with the
flocculus of the cerebellum.
The flocculus is a lobule of cerebellar tissue that is
directly adjacent to the glossopharyngeal nerve, and it
should not be mistaken for an abnormality.
From the lateral cerebellomedullary cistern, the nerve plunges
into the jugular fossa and exits the skull through the jugular
In the jugular foramen, the glossopharyngeal nerve is anterior
to the vagus and accessory nerves and is surrounded by its
own dural sheath (the glossopharyngeal canal).
Exits jugular foramen into anterior nasopharyngeal carotid
Passes lateral to internal carotid artery & stylopharyngeus
Terminates in posterior sublingual space in floor of mouth
(posterior 1/3 taste function)
Coronal oblique FIESTA image through the cerebellopontine angle shows the
glossopharyngeal nerve (arrow) just beneath the flocculus (f) of the cerebellum.
The two roots of the vagus nerve (arrowheads) are visible in the same plane, and
the superior and inferior vestibular nerves can be seen above the flocculus.
The glossopharyngeal nerve (CN9), vagus nerve (CNl0) and bulbar accessory nerve (CNll)
all exit the medulla laterally
CN9 is the most cephalad of these. With routine MR imaging it is not possible to see these
three cranial nerves individually.
In the upper medulla the vagus nerve is well seen leaving the brainstem via the postolivary
sulcus. The glossopharyngeal nerve is seen more laterally as it has already exited the
brainstem above the vagus nerve.
Cisternal segments is not always visualized on routine MR
High-resolution thin-section T2 / FIESTA sequences usually
demonstrate CN9, 10, 11 nerve complex passing through
Cranial Nerve X: The Vagus Nerve
The vagus nerve comprises two roots that emerge from
the side of the medulla, from a groove called the
Leaving the medulla, the nerve roots enter the lateral
cerebellomedullary cistern in a position inferior to the
glossopharyngeal nerve and run parallel to it through the
Because of their parallel course, it may be difficult to
distinguish between the glossopharyngeal and vagus
nerves on axial FIESTA images; coronal or oblique
coronal views along the course of the nerves are best for
After obliquely traversing the lateral cerebellomedullary
cistern, the vagus nerve enters the jugular fossa and
exits the skull through the jugular foramen, between the
glossopharyngeal and accessory nerves.
In the neck, the vagus nerve lies within the carotid
sheath, behind and between the internal jugular vein and
common carotid artery.
Coronal oblique SSFP MR image through the cerebellopontine angle shows the
glossopharyngeal nerve (arrow) just beneath the flocculus (f) of the cerebellum.
The two roots of the vagus nerve (arrowheads) are visible in the same plane, and
the superior and inferior vestibular nerves can be seen above the flocculus.
The vagus and glossopharyngeal nerves, which are difficult to distinguish
in this plane, are clearly distinguishable in the coronal oblique plane.
Cranial Nerve XI: The Accessory
The accessory nerve is composed of multiple cranial
and spinal rootlets.
The cranial rootlets emerge into the lateral
cerebellomedullary cistern below the vagus nerve.
The spinal rootlets emerge from upper cervical
segments of the spinal cord.
SSFP MR image at the level of the cervicomedullary junction (CMJ) shows the
cranial rootlets (arrowheads) of the accessory nerve.
Coronal oblique 0.8-mm-thick SSFP MR image shows the spinal
rootlets (arrows) of the accessory nerve arising from the upper spinal
cord to cross the foramen magnum and join the cranial rootlets.
After leaving the spinal cord, the spinal rootlets pass
superiorly through the foramen magnum into the cisterna
magna (ie, the posterior cerebellomedullary cistern), in a
position posterior to the vertebral artery, and join the
cranial rootlets in the lateral cerebellomedullary cistern.
The conjoined nerve fibers then leave the skull through the
jugular foramen, posterior to the glossopharyngeal and
Segmental spinal nerve roots at the C1 and C2 levels are
distinguishable from accessory nerve rootlets at these
levels because the spinal nerve roots are larger and extend
to the neural foramina instead of continuing superiorly.
CN 11 supply muscles of pharynx & larynx
Innervates sternomastoid muscle continues across floor
of posterior cervical space in cervical neck & terminate
in & innervate trapezius muscle.
Cranial Nerve XII: The Hypoglossal
The hypoglossal nerve arises from nuclei in front of the
fourth ventricle, within the medulla, and emerges as a
series of rootlets extending from the ventrolateral sulcus
of the medulla into the lateral cerebellomedullary cistern .
The combined rootlets then cross the lateral
cerebellomedullary cistern, where the nerve is surrounded
anteriorly by the vertebral artery and posteriorly by the
posterior inferior cerebellar artery.
The hypoglossal nerve then exits the skull via the
hypoglossal canal, which runs obliquely in the axial plane,
at an angle of approximately 45° between the coronal and
After exiting the skull, the hypoglossal nerve runs medial to
the glossopharyngeal, vagus, and accessory nerves and
deep to the digastric muscle, looping over the hyoid bone
to innervate a large part of the tongue.
It is the motor cranial nerve to intrinsic & extrinsic muscles
of tongue(except Palatoglossus).
Coronal oblique 0.8-mm-thick FIESTA image shows multiple hypoglossal
nerve roots (arrows) converging toward the hypoglossal foramen (arrowhead).
The nerve roots are immediately posterior to the vertebral artery (V).
Axial 0.8-mm-thick FIESTA image shows the oblique course of the hypoglossal
nerve (black arrowhead) as it crosses the lateral cerebellomedullary cistern
toward the hypoglossal canal (white arrowheads).
The vertebral arteries (white arrows) are anterior to the nerve, and the posterior
inferior cerebellar artery (black arrow) is posterior to the nerve.
The cisternal part of the nerve might be affected in a
vertebrobasilar aneurysm or dolichoectasia, skull base
neoplasms, basal skull fractures or fractures of the
occipital condyle, basal meningitis or subarachnoid
Primary tumors of the hypoglossal nerve are very rare
but could include schwannomas.
Malignancies within the nasopharynx, oropharynx and
sublingual spaces might also invade the hypoglossal
canal and the peripheral aspect of the hypoglossal nerve.
Normal anatomy, clinical importance and
nerve specific pathologies
Benign encapsulated nerve sheath tumor composed of differentiated
neoplastic Schwann cells.
All cranial nerves (exceptions: Olfactory, optic nerves) have myelinated
schwann cell sheaths and are sites for intracranial schwannomas
98% of intracerebral schwannomas arise from cranial nerves.
Cranial nerve schwannoma:
Slow-growing extra-axial mass. Displaces ("buckles") cortex
A CSF-vascular "cleft" between tumor, brain may be seen.
NECT - Cranial nerve schwannoma –
Noncalcified extra-axial mass
Iso / slightly hyperdense compared to brain
May enlarge bony foramina ( lAC, foramen
ovale, facial nerve canal)
CECT: Strong, uniform enhancement
MR Findings -
Usually iso-, sometimes mixed iso/hypointense
Surrounding edema common
Solid portion of schwannomas shows no restriction (isointense to
normal brain parenchyma)
Elevated ADC values (reflect increased amount of extracellular
water in tumor matrix)
2/3 solid; 1/3 ring or inhomogeneous
Head & neck neurofibromas are usually plexiform tumors
Plexiform NF‟s are a unique feature of Neurofibromatosis1
They are not native to intracranial cavity but occur as central
extensions of the peripheral tumors
Commonly occur along the orbital division of the CN5
They are poorly delineated, diffusely infiltrating masses that
can expand & erode bone.
NCCT – Isodense
CECT - Moderate/strong enhancement
T1WI: Isointense infiltrating mass
TI post contrast: Enhances strongly, somewhat
Contrast-enhanced T1-weighted fat-
saturated axial MR image shows a poorly
demarcated enhancing mass lesion that
extends from the subcutaneous soft tissue
into the orbit (arrow), middle cranial fossa
(*), and cavernous sinus (arrowhead).
Optic nerve gliomas are the most common CNS tumor in NF1
Can involve one or both the optic nerves & commonly extend in
to the chiasma
Posterior extension in to optic tracts, LGB & optic radiation can
Most of these are histologically benign, low grade astrocytomas.
Most of them are appear
Variable contrast enhancement
MR images of a 5-year-old girl with NF-1.
(a) a large chiasmal glioma (black arrow) and a probable glioma in the medulla (white arrow)
(b) bilateral extension of the chiasmal glioma along the optic tracts. The high-signal tumor
involves the lateral geniculate bodies and continues to the basal ganglia bilaterally.
(c) Axial image at the level of the suprasellar cistern shows the chiasmal glioma (arrow), tumor
extension into the left temporal lobe with a large cystic lesion (probably a trapped left temporal
horn), and foci of increased intensity (“hamartomas”) in the bilateral cerebellar peduncles,
cerebellar white matter, and pons.
Associated with tumors of Schwann cells.
Bilateral acoustic schwannomas are diagnostic
Trigeminal nerve is the next most frequently
involved nerve after CN8
Involvement of more than one cranial nerve by
schwannomas favors the diagnosis of NF2.
MR images of a 17-year-old girl with NF-2.
(a) Axial image at the level of the internal auditory canals shows isointense
bilateral acoustic schwannomas.
(b) Axial image at a slightly more rostral level reveals focal thickening of the left
fifth nerve (probable fifth nerve schwannoma) (arrow).
Infectious meningitis results from viral, bacterial, fungal, or
Leptomeningitis is the most common form of intracranial
tuberculosis, particularly in the pediatric population.
Nerve impairment has been attributed to ischemia of the
nerve or entrapment of the nerve in basal exudates.
Cryptococcus neoformans is the most common fungus
to involve the CNS.
Optic neuropathy is a rare complication of cryptococcal
meningitis and usually occurs in non-AIDS patients.
Necrosis of the optic nerves and infiltration of the
meninges around the optic tracts, nerves, and chiasm
by cryptococcal organisms have been observed.
Lyme disease -
Multisystem inflammatory disorder caused by a spirochete
Cranial neuritis in Lyme disease may involve any of cranial
nerves III through VII, with the facial nerve most frequently
affected and often associated with cochleovestibular nerve
The affected segments appear thickened and enhance.
Enhancement of cranial nerves including root entry zones is
Lyme disease –
Manifestation of palsy of left seventh cranial nerve in 10-year-old male patient with
recent history of camping.
(a) Transverse T1-weighted postcontrast MR image shows enhancement of left
seventh cranial nerve (arrow).
(b) Coronal T1-weighted postcontrast MR image demonstrates left trigeminal
nerve enhancement (arrow)
Viral infections –
related to herpes simplex virus type 1, cytomegalovirus,
and Varicella zoster organisms
manifest with cranial nerve involvement and show
abnormal enhancement on MRI.
Bell‟s palsy -
A syndrome characterized by the acute onset of unilateral
facial paralysis which progresses over a 2-5 day period.
There is pathologic enhancement of the intracanalicular–
labyrinthine portion of CN 7.
The three criteria for pathologic enhancement of
the facial nerve:
Enhancement outside the facial canal,
Extension of enhancement to cranial nerve VIII,
Intense enhancement of the labyrinthine and mastoid
Ophthalmoplegic migraine –
is a rare condition characterized by headache and
oculomotor nerve palsy lasting days to weeks.
include reversible enhancement of the cisternal segment
of the oculomotor nerve
focal thickening at the exit of the nerve in the
57-year-old man with ophthalmoplegic migraine
Unenhanced axial (A) and enhanced axial (B) T1-weighted images reveal
smooth enlargement and homogeneous enhancement of cisternal
segment of left oculomotor nerve (arrows).
Granulomatous diseases -
Intracranial neurosarcoidosis has a predilection for the
basal leptomeninges, and involvement of every cranial
nerve has been described.
MRI shows a spectrum of CNS abnormalities including
diffuse or nodular thickening and abnormal
enhancement of the leptomeninges in the basal cisterns
and hypothalamic regions.
Facial nerve and optic nerve are most commonly affected
Perineural spread has also been reported in sarcoidosis.
•Enhancing lesion causing thickening
and enhancement of cisternal
segment of third cranial nerve
•Note enhancing suprasellar mass
(m) and diffuse leptomeningeal
•Enhancement of entire visible portions of
both optic nerves in orbit and optic canals
•Enhancement of extraocular muscles is
Idiopathic hypertrophic cranial pachymeningitis -
characterized by inflammation and fibrosis of the dura
It remains a diagnosis of exclusion but may be the
presenting manifestation of granulomatous diseases such
as sarcoidosis, Wegener‟s granulomatosis, or tuberculosis.
MRI - shows focal or diffuse thickening and enhancement
of the dura that encase cranial nerves causing recurrent
The oculomotor, abducens, and facial nerves are more
Postradiation Neuritis -
uncommon, usually delayed, complication of radiation
therapy or radiosurgery.
Cranial nerve deficits may be permanent or resolve
Pathology - Loss of the nerve–blood barrier due to
demyelination and ischemia, coagulation necrosis, or
peripheral fibrosis results in cranial nerve enhancement.
Radiation induced optic neuropathy occurs months to years
after exposure of the anterior visual pathways to ionizing
MRI shows smooth enlargement and enhancement of the
optic nerve and chiasm.
Recent advances in Cranial nerve imaging
The role of diffusion-tensor imaging (DTI) and
tractography of the cranial and peripheral nerves
MRI is the imaging modality of choice in almost all
cranial nerve pathologies
CT preferred to know the bony involvement
Precise knowledge about the origin and course of the
cranial nerves is essential
Various vascular and bony landmarks on MRI and CT
help in identification of cranial nerves and their
Relevant clinical details are necessary to image
specific anatomical locations
Diagnostic and surgical imaging anatomy – head, neck &
brain – Harnsberger, Osborn et al.
Radiology review manual – Dahnert
Appearance of Normal Cranial Nerves on Steady-State
Free Precession MR Images Sujay Sheth, BA, et al .
Internet – radiopaedia.org