diagnostic imaging of cerebellopontine angle masses

C.N.S.Cerebellopontine Angle Masses

Mohamed Zaitoun

Assistant Lecturer-Diagnostic Radiology Department , Zagazig University Hospitals

EgyptFINR (Fellowship of Interventional

Neuroradiology)-Switzerlandzaitoun82@gmail.com

Knowing as much as possible about your enemy precedes successful battle

and learning about the disease process precedes successful management

Cerebellopontine Angle Masses1-Vestibular schwannoma (Acoustic Neuroma)2-CPA Meningioma3-Epidermoid Cyst4-Trigeminal Schwannoma5-Arachnoid Cyst6-Aneurysm7-Metastases8-Skull base / Temporal bone tumors9-Skull base infection10-CPA Lipoma

The CPA is region between the pons & cerebellum and the posterior aspect of the petrous temporal bone

Important structures of the CPA includes 5th , 7th & 8th cranial nerves & AICAMost lesions of the CPA are extra-axial & located in the CPA cistern itself

1-Vestibular schwannoma (Acoustic Neuroma)a) Incidenceb) Locationc) Radiographic Featuresd) Differential Diagnosis

a) Incidence :-Commonest CPA mass , 80%-More in females-Benign tumor of Schwann cell origin-90% are solitary , multiple schwannomas are

commonly associated with NF2

b) Location of Schwannoma :1-CPA (CN VIII , most commonly from superior

portion of vestibular nerve), 90%2-Trigeminal nerve (CN V)3-Other intracranial sites (rare) :-Intratemporal (CN VII)-Jugular foramen / bulb (CNs IX , X , XI)4-Spinal cord schwannoma5-Peripheral nerve schwannoma6-Intracerebral schwannoma (very rare)

c) Radiographic Features :1-Mass2-CT3-MRI

1-Mass :->2 mm difference between left and right IAC-Erosion and flaring of IAC-IAC > 8 mm-Extension into CPA ( path of least resistance ) : ice-cream

cone appearance-Marked brainstem compression may occur and produce

obstructive hydrocephalus-Significant signal heterogeneity with cystic or hemorrhagic

areas is more typical of vestibular schwannoma than meningioma

-Intracanalicular component representing “the cone”

-The CPA component representing the “ice cream”

2-CT :-Isodense by CT-Presents as a solid (small) or complex (large)

enhancing mass with an intracanalicular component that expands the porus acousticus and internal auditory canal

CT+C

CT , Bilateral acoustic neuroma , NF2

CT+C , Bilateral acoustic neuroma in NF2

3-MRI :*T1 : 70% hypointense, 30% isointense*T2 : Hyperintense*T1+C : Dense enhancement : -Homogeneous if small-Heterogeneous if large*May contain cystic degenerative areas +/-

hemorrhage within large lesions*Marginal arachnoid cysts

T1

T1

T1

T1

T2

T2

T2

T2

T1+C shows a rounded tumour in the right CPA with extension into the internal auditory meatus , the IAM is expanded , this is the ice-cream cone sign

T1+C

T1+C

T1+C

T1+C

T1+C

T1+C

T1+C

T1+C

d) Differential Diagnosis :1-From CPA masses2-From Neurofibroma

1-From CPA masses : Schwannoma Meningioma Epidermoid

1-Epicenter IAC Dural based CPA

2-CT Density Isodense Hyper/Isodense Hypodense

3-Calcification None Frequent Occasional

4-Porus acusticus/IAC

Widened Normal Normal

5-T2W signal intensity relative GM

Hyperintense 50% isodense Hyperintense

6-Enhancement Dense Dense None

Acoustic neuroma Meningioma Epidermoid

2-From Neurofibroma :-Plexiform neurofibromas are unique to

neurofibromatosis type 1 (NF1)-They do not occur primarily in the cranial cavity

but may extend into it from posterior ganglia or as an extension of peripheral tumors

Schwannoma Neurofibroma

1-Origin Schwann cells Schwann cells and fibroblasts

2-Association NF2 NF1

3-Incidence Common Uncommon

4-Location CN VIII > other CN Cutaneous and spinal nerves

5-Malignant degeneration

No 5-10%

6-Growth Focal Infiltrating

7-Enhancement +++ ++

8-T1W 70% hypointense, 30% isointense

Isointense with muscle

9-T2W Hyperintense Hyperintense

2-CPA Meningioma :a) Incidenceb) Radiographic Featuresc) Differential Diagnosis

a) Incidence :-10 % of CPA masses (2nd most common)-Age: 40 to 60 years-Three times more common in females

b) Radiographic Features :1-CT2-MRI3-Angiography

1-CT :a) Signal Intensity b) Morphologyc) Bony Abnormalities

a) Signal Intensity :-Hyperdense (75%) or isodense (25%) on

noncontrast CT-Strong homogeneous enhancement (90%)

(hallmark)-Calcifications , 20%-Cystic areas , 15%

CT

CT

CT

CT

CT

CT+C , left CPA homogeneously enhancing meningioma with a broad base against the petrous bone

b) Morphology :-Round unilobulated sharp margin (most

common)-Dural tail : extension of tumor or dural reaction

along a dural surface-Edema is absent in 40% because of the slow

growth

c) Bony Abnormalities : 20 %-No changes (common)-Hyperostosis (common)-Bone erosion ( rare , if present may indicate

malignant meningioma )

2-MRI :*T1&T2 :-Tumors are typically isointense with GM*T1+C :-Strong gadolinium enhancement*Dural tail (60%) is suggestive but not specific for

meningioma*Increased vascular flow voids

T1

T2

T1+C

T1+C

T1+C

T1+C

T1+C

T1+C shows the typical appearance of a meningioma with the flat surface against the petrous bone and the dural tails , this tumor is arising anterior to the left IAM , it may extend into the IAM as seen here

3-Angiography :-Spokewheel appearance-Dense venous filling-Persistent tumor blush ( comes early and stays

late ) = Mother’s in law sign-Well-demarcated margins-Dural vascular supply

f) Differential Diagnosis : (see before)-CPA meningiomas can be differentiated from vestibular

schwannomas by virtue of their broad-based attachment to the petrous bone and more homogeneous signal

-They are typically less bright on T2 and enhance uniformly

-A small tongue of tissue may extend into the internal auditory canal but there is usually no expansion

-Peripheral (dural tail) and hyperostosis suggests meningioma

3-Epidermoid Cyst :a) Incidenceb) Locationc) Radiographic Featuresd) Differential Diagnosis

a) Incidence :-5 % of CPA masses-Congenital lesions which account for about 1%

of all intracranial tumours-They result from inclusion of ectodermal

elements during neural tube closure

-Although predominantly congenital, epidermoid cysts are usually very slow growing and as such take many years to present, typically patients are between 20 and 40 years of age

-An uncommon association exists with anorectal anomalies, sacral anomalies and pre-sacral mass and is known as the Currarino triad

b) Location :*Intradural (90 %) >>1-CPA, 40 %2-Suprasellar region3-4th ventricle4-Middle cranial fossa*Extradural ( 10 % ) >>-Most within the skull

c) Radiographic Features :1-CT :-Lobulated lesion with CSF density-No enhancement

2-MRI : CSF-like signal*T1 :-Usually isointense to CSF-Higher signal compared to CSF around the periphery of the lesion

is frequently seen-Rarely shows high T1 signal (white epidermoids)*T2 :-Usually isointense to CSF (65%)-Slightly hyperintense (35%)*T1+C :-No enhancement-Thin enhancement around the periphery may sometimes be seen

*FLAIR :-Often heterogeneous /dirty signal ; higher than

CSF

*DWI :-Useful for differentiation from arachnoid cysts

due to increased signal (due to a combination to true restricted diffusion and T2 shine through) which isn’t seen with arachnoid cyst

T1

T1

T2

T2

T1+C

T1+C

FLAIR

FLAIR

DWI

DWI

ADC

ADC

**N.B. : T2 shine through-Refers to high signal on DWI images that is not due

to restricted diffusion but rather to high T2 signal which (shines through) to the DWI image, T2 shine through occurs because of long T2 decay time in some normal tissue

-This is most often seen with subacute infarctions due to vasogenic edema but can be seen in epidermoid cyst

-To confirm true restricted diffusion one should always compare the DWI image to the ADC

-In cases of true restricted diffusion the region of increased DWI signal will demonstrate low signal on ADC

-ADC is a value that measures the effect of diffusion independent of the influence of T2 shine-through, ADC maps thus portray restricted diffusion such as in ischemic injury , as hypointense lesions relative to normal brain

-In contrast , in cases of T2 shine-through , the ADC will be normal or high signal

d) Differential Diagnosis :1-From Dermoid cyst :-See table

Epidermoid Dermoid

Content Squamous epithelium, keratin, cholesterol

Also has dermal appendages (hair, sebaceous fat, sweat glands)

Location Off midlineCPA most commonParasellar, middle fossaIntraventricular, diploic space (rare)

MidlineSpinal canal most commonParasellar, posterior fossa

Rupture Rare Common (chemical meningitis)

Age Mean 40 years Younger adults

CT density CSF density May have fat

Calcification Uncommon Common

Enhancement Occasional peripherally None

MRI CSF-like signal Proteinaceous fluid

Incidence 5-10 times more common than dermoid

Less common

2-From Arachnoid cyst : (See table)-DWI allows differentiation of epidermoid and

arachnoid cysts :The ADC of an epidermoid cyst is significantly

lower than that of an arachnoid cyst , therefore , epidermoid cysts have high signal intensity on DWI , whereas arachnoid cysts , like CSF , have very low signal intensity

Arachnoid Epidermoid

1-Signal intensity Isointense to CSF on T1Isointense to CSF on PDIsointense to CSF on T2

Mildly hyperintense to CSFHyperintense to CSF on PDIsointense to CSF on T2

2-Enhancement No No

3-Margin of lesion Smooth Irregular

4-Effect on adjacent structures

Displaces Engulfs , insinuates

5-Pulsation artifact Present Absent

6-Diffusion imaging Follows CSF Hyperintense to CSF

7-FLAIR imaging Suppresses like CSF Hyperintense to CSF

8-Calcification No May occur

4-Trigeminal Schwannoma :-Similar in appearance to vestibular

schwannoma but arising from the V cranial nerve

Trigeminal schwannoma of right gasserian ganglion with smooth margins , relatively low signal in T1 ( A) and high homogenous signal intensity on T2 ( B )

Trigeminal Schwannoma of right gasserian ganglion , relatively low signal on T1 ( A ) and high heterogeneous signal intensity on T2 ( B ) , note normal Meckel’s cave ( curved arrow ) on ( C )

T1+C shows normal non-enhancing Meckel’s cave on the right side (arrow) , in the left Meckel’s cave , a heterogenous enhancing mass (arrow head) is seen extending in the cavernous sinus , trigeminal schwannoma

A homogeneous , enhanced , dumbbell-shaped right trigeminal schwannoma involving the cisternal part of the nerve and Meckel cave

5-Arachnoid Cyst :-Benign CSF-filled lesion that is usually

congenital-Although most arachnoid cysts are

supratentorial , the CPA is the most common infratentorial location

-Arachnoid cyst will follow CSF signal on all sequences including complete suppression on FLAIR , unlike epidermoid cyst , arachnoid cyst doesn’t have restricted diffusion

Arachnoid cyst (a) T1 shows an arachnoid cyst with signal intensity similar to that of CSF stretching the left seventh and eighth cranial nerve complex (arrow) , (b) T2 shows the cyst displacing the vascular structures of the CPA (arrowheads)

6-Aneurysm :-Large aneurysms arising from the vertebrobasilar

system (PICA , AICA , vertebral artery or basilar artery) may appear as well-defined avidly enhancing CPA lesion and may be initially mistaken for a schwannoma or meningioma on CT+C

-On MRI , clues to a vascular etiology would be flow void and pulsation artifacts , MRA or CTA are diagnostic

Aneurysm in a 75-year-old man with hypoglossal nerve palsy , (a) T2 shows a thrombosed aneurysm of the right PICA with focal calcification (arrowhead) , note the normal right hypoglossal canal (arrow) , a finding inconsistent with a schwannoma , (b) T1+C shows homogeneous enhancement of the organized thrombus which completely fills the aneurysm

7-Metastases :-See Brain Tumors 8-Skull Base / Temporal Bone Tumors :-e.g. Glomus tumors & cholesterol granuloma 9-Skull Base Infection :-Gradenigo's syndrome (osteomyelitis of the petrous

apex) and malignant otitis externa10-CPA Lipoma :-See brain tumors

Melanoma in a 58-year-old woman with a left cerebellar syndrome , (a) CT shows a hyperattenuating melanoma of the left CPA , (b) T1 shows a well-defined extraaxial mass at the posterior edge of the petrous bone , the high signal intensity is suggestive of melanin , (c) T1+C shows a normal left internal auditory canal (arrow) and lack of dural tail enhancement

Metastases in a 67-year-old man with lung cancer, (a) T2 shows a metastasis of the right CPA that mimics a vestibular schwannoma but with unusual associated middle ear retention ( ) , ∗ (b) T1+C shows intense enhancement of the lesion which extends into the cochlea (arrow) , note the presence of another enhancing lesion at the tip of the right petrous bone (arrowhead)

Paraganglioma (a) T2 shows a huge paraganglioma destroying the petrous bone and invading the right CPA , massive flow voids (arrowheads) reflect the hypervascularity of the lesion , note the thin layer of trapped CSF (arrow) between the mass and the brainstem which indicates an extraaxial origin , (b) T1 shows the suggestive salt-and-pepper appearance of the paraganglioma , (c) T1+C shows intense enhancement of the lesion along with unusual dural tail enhancement of the meninges (arrows)

Cholesterol granuloma , (a) T1 shows a cholesterol granuloma at the apex of the right petrous bone with typical high signal intensity , an additional suggestive feature is the thin hypointense rim (arrowheads) which represents expanded cortical bone of the petrous apex , (b) T2 shows that the granuloma has heterogeneous signal intensity surrounded by a hypointense rim (arrowheads) , (c) T1+C shows the normal right trigeminal nerve (arrow) at the top of the mass

Apex petrositis in a 50-year-old woman with Gradenigo syndrome at clinical evaluation , (a) T1 shows an irregular lesion at the tip of the petrous apex (arrow) , (b) T1+C shows right-sided apex petrositis as an enhancing lesion along the courses of cranial nerves V and VI (arrow)

CPA lipoma (a) Axial CT scan shows a well-defined hypoattenuating lipoma of the left CPA , (b) T1 shows that the lipoma has signal intensity similar to that of subcutaneous fat

T1 shows CPA lipoma