Imaging of theCerebellopontine Angle

Magge Lakshmi, MD*, Christine M. Glastonbury, MBBS

KEYWORDS� Cerebellopontine angle � Internal auditory canal� Hearing loss � Vestibular schwannoma� Posterior fossa meningioma

There is a wide range of pathologic processeswithin the cerebellopontine angle (CPA). Theseprocesses may present because of mass effecton local structures, such as the fifth to 12th cranialnerves, or because of mass effect on the pons orcerebellum, which may result in fourth ventricularobstruction. MR imaging is vastly superior to CTfor the evaluation of the complete extent ofa CPA mass and for best characterization of itslikely nature. MR imaging is also superior to CTfor the detection of leptomeningeal processes.The authors’ approach to differential diagnosis ofCPA lesions starts by grossly dividing pathologicfindings into masses, vascular lesions, and moresubtle leptomeningeal disease. Although themost common CPA mass is the benign vestibularschwannoma, there are many different massesand disease processes that may need to beconsidered in the complete differential diagnosis.The authors present their approach to CPAimaging with the critical anatomy, imaging strate-gies, and a differential diagnosis algorithm.

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CEREBELLOPONTINE ANGLE CRITICAL ANATOMY

The CPA, by definition, is the cisternal spaceanterior to the cerebellar hemispheres and lateralto the pons. It can also be thought of as an invertedtriangular-shaped cerebrospinal fluid (CSF) spaceat the lateral aspect of the posterior fossa, withthe base of the triangle formed by the tentorium.The lateral margin of this triangle is formed by theposterior aspect of the petrous temporal bone.The cistern extends into the petrous bone as theinternal auditory canal (IAC), with its medial aper-ture being the porus acousticus. The fifth cranial

Department of Radiology and Biomedical Imaging, Univ* Corresponding author.E-mail address: magge.lakshmi@radiology.ucsf.edu (M. L

Neuroimag Clin N Am 19 (2009) 393–406doi:10.1016/j.nic.2009.06.0041052-5149/09/$ – see front matter ª 2009 Elsevier Inc. All

nerve traverses the superior aspect of the cisternfrom the lateral pons to Meckel’s cave. The seventh(facial) and eighth (vestibulocochlear) cranialnerves arise from the low CPA cistern at the lateralaspect of the inferior medulla, emerging at the levelof the foramen of Luschka. These nerves ascend tothe porus acousticus, with the facial nerve slightlyanterior and superior to the eighth cranial nerve.Both nerves are intimately related to the anteriorinferior cerebellar artery (AICA), and in two thirdsof patients, the AICA loops into the IAC on at leastone side. The ninth through 11th cranial nervesarise from the medulla and upper cervical cordand traverse the inferior lateral aspect of the CPAcistern to exit the skull base at the jugular foramen.The 12th cranial nerve exits anteriorly through thehypoglossal canal of the basiocciput.

IMAGING STRATEGIES

MR imaging and CT are complementary modali-ties, although MR imaging offers greater utility forthe detection and characterization of CPAprocesses. The authors favor the use of heavilyT2-weighted three-dimensional sequences, suchas fast imaging using steady-state acquisition orconstructive interference in the steady state,which provide high-resolution detail of cranialnerves and the blood vessels associated withthem. Such thin-slice heavily T2-weighted imagingalso allows good evaluation of the inner ear struc-tures, which is important in the complete evalua-tion of sensorineural hearing loss. The authorsalways administer gadolinium and perform pre-and postcontrast 2-mm T1-weighted sequencesin the axial and coronal planes. Fat saturation after

ersity of California, San Francisco, CA, USA

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Table1MR imaging protocol

Sequence/PlaneRepetitionTime/Echo Time

Fieldof View Slice/Skip

No.Excitations

T1-weighted axial 500/MF 17 2 mm/0 2

Three-dimensional, fast spin echo,T2-weighted

3000/MF 16 1 mm/0 1

T1-weighted axial, after administrationof gadolinium, fat saturated

500/MF(17) 17 2 mm/0 2

T1-weighted coronal, after administrationof gadolinium, fat saturated

500/MF 17 2 mm/0 2

Abbreviation: MF, minimum full.a Fast imaging using steady-state acquisition or constructive interference in the steady state, if available, is preferred in

lieu of three-dimensional, fast spin echo, T2-weighted MR imaging.

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contrast is always applied (Table 1). CT is largelyreserved for defining any bone changes that maybe associated with CPA masses, such as meningi-omas, but is not a first-line tool for evaluating orcharacterizing CPA masses.

APPROACH TO DIFFERENTIAL DIAGNOSIS

The authors’ approach to the differential diagnosisof CPA lesions starts by grossly dividingpathologic changes into masses, vascular lesions,and more subtle leptomeningeal disease (Fig. 1).Vestibular schwannoma is the most commonCPA mass, with meningioma, epidermoid,arachnoid cyst, metastasis, and aneurysmaccounting for the remainder of most masses.Masses can be further subdivided into those thatenhance, such as schwannoma, meningioma,aneurysm, and metastases, and nonenhancinglesions, such as arachnoid cyst and epidermoid.The latter two masses are readily distinguished

CPA Proce

Mass

Enhancing

Schwannoma

Meningioma

Aneurysm

Metastasis

Non-enhancing

EpidermoidDWI+

Arachnoidcyst DWI-

Vascularlesion

Aneurysm AV

Fig.1. Approach to differential diagnosis of CPA lesions.

by the presence of restricted diffusion in anepidermoid as compared with increased diffusionin an arachnoid cyst. An aneurysm mostcommonly arises from the AICA and is the mostcommon vascular lesion of the CPA. It may alsobe found in association with an arteriovenousmalformation (AVM). Aneurysms and AVMs arewell evaluated with CT angiography or magneticresonance angiography, although complete studymay require catheter angiography. Manyleptomeningeal processes present as cranialneuropathies, and most are evident because ofthe presence of enhancement. Leptomeningealprocesses are typically much more subtle imagingfindings, even with MR imaging. The 3-mm thin-slice imaging allows better detection of subtleabnormal enhancement as compared with routine5- to 7-mm thickness whole-brain imaging. Thepresence of leptomeningeal enhancement aloneresults in a wide differential diagnosis, whichmay require CSF sampling for definitive

sses

M

Leptomeningealdisease

Enhancement

Carcinomatosis

Sarcoidosis

Infection

GRE signalloss

Hemosiderosis

Imaging of the Cerebellopontine Angle 395

differentiation of processes, such as sarcoidosis,carcinomatosis, and infections. Hemosiderosisdoes not enhance but is evident by T2* signalloss of the leptomeninges.

CEREBELLOPONTINE ANGLEMASSESVestibular Schwannomas

Vestibular schwannomas account for 70% to 80%of all CPA lesions. Previously known as acousticneuromas, these lesions most often arise from theinferior vestibular nerve within the IAC and presentwith hearing loss or tinnitus.1 Schwannomas can beentirely intracanalicular or have intracanalicular andcisternal components, resulting in the descriptionof an ‘‘ice-cream cone’’ tumor (Fig. 2). Rarely,vestibular schwannomas are purely intracisternal.Intracisternal tumors tend to reach a larger sizebefore presenting with mass effect on thecerebellar hemispheres and fourth ventricle ratherthan hearing loss. CPA masses, such as cisternalschwannomas, may cause mass effect on the fifthcranial nerve also.

T2-weighted and, in particular, postcontrastimaging can reveal subtle imaging variations investibular schwannomas. Although small lesionstend to have homogeneous enhancement, largerlesions more frequently have cystic spaces anddemonstrate heterogeneous enhancement. Thosetumors with cystic areas tend to be faster growingand to have a less favorable surgical outcome with

Fig. 2. (A) Axial T1-weighted, postcontrast, fat-saturatedclassic ‘‘ice-cream cone’’ vestibular schwannoma of the rigto the middle aspect of the right IAC and does not reachbe amenable to retrosigmoid resection. (B) Axial fast iweighted imaging of the same lesion (arrows) shows tfrom hyperintense CSF and with a ‘‘cap’’ of CSF between t

regard to integrity of facial nerve function.2 Theseheterogeneous lesions may also contain morehemosiderin deposition, which can alter the signalintensity further.3 Rarely, acute hemorrhagic expan-sion or degeneration of a schwannoma withenlargement can cause a patient to present withsudden onset of vertigo or emesis (Figs. 3 and4).3–5 Treatment options for vestibular schwanno-mas include observation, microsurgical removal,and stereotactic radiosurgery. Observation isreserved for elderly patients who do not experiencemass effect. Surgical outcome has improved withthe newer microsurgical techniques and instrumen-tation, including intraoperative facial nerve moni-toring and brainstem evoked potential recording.5

Treatment options depend, to a large extent, ontumor size, tumor extent into the IAC, and pres-ence of cystic components. Hence, whendescribing these masses, it is important todescribe the tumor dimensions in terms of thecisternal component and the component withinthe IAC. Lateral extent of the tumor within theIAC should be assessed, and particular mentionshould be made of whether or not the massextends up to or into the cochlear aperture. If themass is associated with a cyst, the size of thiscyst should be described.

Surgical approaches include the subtemporalmiddle cranial fossa approach, retrosigmoidapproach, and translabyrinthine approach. Withthe greater use of radiosurgery (‘‘gamma knife’’),

imaging demonstrates a heterogeneously enhancinght CPA and IAC (arrows). The ‘‘cone’’ portion extendsthe cochlea (curved arrow). This schwannoma would

maging using steady-state acquisition, thin-slice, T2-he contours of the schwannoma, clearly delineatedhe schwannoma and the cochlea.

Fig. 3. A 28-year-old patient presented with sudden-onset vertigo, hyperacusis, and left-sided headache. (A) AxialT1-weighted imaging demonstrates a heterogeneous mass in the left CPA with areas of marked T1 shortening(arrow), indicating the presence of methemoglobin from hemorrhage. (B) Axial T1-weighted postcontrast fastspin MR imaging demonstrates heterogeneous enhancement of a CPA-IAC mass. The CPA component (arrow)appears predominantly cystic, whereas the IAC component (curved arrow) is more heterogeneously enhancing.

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the middle fossa approach is used less often in theauthors’ institution because this is largely reservedfor smaller predominantly IAC masses. The retro-sigmoid approach is preferred for those

Fig. 4. A 38-year-old patient with known neurofibroma-tosis type 2 and known left vestibular schwannoma pre-sented with the acute onset of headache, nausea, andvertigo. An axial contrast-enhanced CT scan demon-strates a heterogeneously enhancing left CPA mass. Theareas with lack of enhancement (arrows) are new, sug-gesting degeneration of the known schwannoma. Theacute change in the size of the schwannoma was associ-ated with edema (curved arrow), mass effect, andobstruction of the fourth ventricle with hydrocephalus.

schwannomas with a large CPA componentbecause it allows good access to the entire CPA.The translabyrinthine approach results in completehearing loss and is generally reserved for thosetumors that extend far laterally within the IAC,because it is difficult to reach these lesions froma retrosigmoid approach. A combination of the ret-rosigmoid and translabyrinthine approach is usedfor large complicated lesions involving the IACand CPA. Immediate postoperative complicationsdepend on the surgical approach used and includecontusions of the cerebellum or AICA ischemia.Dural venous sinus thrombosis is also a well-described but uncommon complication.6 Whenevaluating postoperative scans after removal ofvestibular schwannomas, dural linear enhance-ment is expected in the IAC and along the CPAmargins. Attention should be paid to developingnodular enhancement, which might suggest recur-rent growth (Fig. 5).

Radiosurgical techniques include single-sessiongamma knife radiosurgery (GKS) or linear acceler-ator or fractionated therapy, such as Cyberknifetherapy. Of these, the best-studied and mostwidely used technique is GKS. GKS has emergedas the treatment of choice in many centers becauseof the potential advantages it offers in terms offaster recovery, reduced cost, and minimizedmorbidity and mortality.7 GKS is generally usedfor tumors when the cisternal component is lessthan 3 cm.8 In the first 6 to 12 months after treat-ment, tumors may depict more heterogeneousenhancement with areas of central necrosis.

Fig. 5. (A) Patient with prior combined translabyrinthine and retrosigmoid approach resection of a large CPA-IACvestibular schwannoma. Axial T1-weighted, postcontrast, fat-saturated MR imaging demonstrates marked rightcerebellar volume loss (open arrow) with enlargement of the fourth ventricle from operative vascular injury.There is subtle linear enhancement in the IAC (arrows), which is an expected postoperative finding. (B) AxialT1-weighted, postcontrast, fat-saturated MR imaging performed 6 years later shows interval enlargement ofthis nodular enhancement (arrow), indicating recurrent schwannoma.

Imaging of the Cerebellopontine Angle 397

Overall tumor size may actually increase for the first2 years, which is why tumors larger than 3 cm areexcluded from this treatment modality. After thistime, gradual regression in size can be expected.Hydrocephalus from mass effect attributable toincreased tumor size can be treated with shuntingor steroids. Development of cranial neuropathieswarrants surgical intervention.9–12

Meningiomas

Meningiomas are the second most common tumorto arise in the CPA, representing up to 10% to 15%of tumors. These slow-growing masses arise fromarachnoid meningoepithelial cells and, as withmeningiomas elsewhere, occur more frequentlyin individuals older than the age of 40 years. Inthe CPA, these tumors tend to arise from thedura of the dorsal aspect of the petrous temporalbone. They typically present with hearing loss,tinnitus, and headache, whereas larger tumorsmay present with cerebellar signs and trigeminalneuropathy.13 On MR imaging, the masses are iso-intense to cerebral cortex on T1- and T2-weightedsequences and depict avid postcontrast enhance-ment. Because meningiomas may involve the po-rus acousticus and extend into the IAC, thedetection of a dural tail is a key finding to differen-tiate meningiomas from vestibular schwannomas(Figs. 6 and 7). Other imaging features to facilitatethis imaging differentiation are identified morereadily on CT: the presence of tumoral

calcifications and hyperostosis of adjacent bone.Advanced MR imaging techniques have attemp-ted to distinguish these tumors further. Althoughsubtle differences have been reported using diffu-sion-weighted imaging (DWI), perfusion-weightedimaging, and magnetic resonance spectroscopy(MRS), there is considerable overlap of findings.

UTILITYOFADVANCEDMR IMAGINGTECHNIQUES IN DIFFERENTIATING VESTIBULARSCHWANNOMAS ANDMENINGIOMAS

High apparent diffusion coefficient (ADC) values investibular schwannomas are probably related tothe presence of increased amounts of extracellularwater in the tumor matrix.14 Another study15 thatlooked into ADC values of different extra-axialtumors demonstrated higher ADC values inschwannomas than in meningiomas. Some over-lap did exist, however; hence, this cannot be abso-lutely or independently relied on.

On perfusion MR imaging, the mean regionalcerebral blood volume ratio of meningiomas hasbeen reported to be higher than that of schwanno-mas and has proved to be statistically signifi-cant.16 A prominent choline peak, absence orlow amount of N-acetylaspartate and creatine,and presence of alanine are common characteris-tics of the spectral pattern of atypical and typicalmeningiomas on MRS. Schwannomas typicallydepict the presence of increased myoinositol.17

Fig. 6. Axial postcontrast T1-weighted MR imagingperformed before surgery in a patient with headachesand hearing loss demonstrates an intensely enhancingmass, subtended on the posterior margin of thepetrous temporal bone and located posterior to theporus acousticus (open arrow). A subtle small duraltail is evident (white arrow), which also suggestsmeningioma as the most likely diagnosis.

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At the authors’ institution, MRS is not routinelyperformed, although perfusion MR imaging isoften obtained for CPA masses.

Fig. 7. Patient with right sensorineural hearing loss and aand coronal (B) postcontrast, T1-weighted, fat-saturated Mmass (open arrows in A) subtended on the posterior marshow extension of the mass into the IAC (white arrow),attachment, however, favors meningioma.

EPIDERMOIDS

Epidermoid tumors represent around 5% of CPAmasses and are the third most common mass tooccur in this location. They arise during earlyembryogenesis from inclusion of ectodermalepithelial tissue during neural tube closure.Because these cysts are lined by stratified kerati-nized epithelium, they grow from desquamationand the accumulation of keratin and cholesterolwithin the cysts. These lobulated malleablemasses have a tendency to insinuate betweencranial nerves and vessels. Because of this char-acteristic, they tend to present only when large.Clinically, epidermoids present with symptomssimilar to schwannomas and meningiomas oftinnitus, hearing loss, or even hemifacial spasm.Epidermoid tumors are, however, readily distin-guished from schwannomas and meningiomasby their imaging appearance.

Epidermoid tumors have the appearance of non-enhancing cysts on CT and MR imaging, followingthe density and intensity of CSF on mostsequences. The feature that distinguishes theselesions from arachnoid cysts is a relative hyperin-tensity to CSF on fluid-attenuated inversionrecovery imaging and reduced diffusion signalintensity on DWI sequences (Fig. 8).18 This charac-teristic of epidermoid tumors also serves as a usefulfeature for the detection of any residual tumor onpostoperative follow-up examinations (Fig. 9).

large right CPA mass found on MR imaging. Axial (A)R imaging demonstrates a markedly enhancing CPA

gin of the right petrous temporal bone. Both planesmimicking a vestibular schwannoma. The long dural

Imaging of the Cerebellopontine Angle 399

Malignant transformation of an epidermoid tosquamous cell carcinoma is an extremely rareevent. These are characterized by areas ofcontrast enhancement within the lesion.19 Unusualappearances of epidermoids have been reportedand include so-called ‘‘white epidermoids.’’ Theseepidermoids demonstrate reversed signal intensi-ties, that is, hyperintensity on T1-weightedsequences and hypointensity on T2-weightedsequences. This has been attributed to the highproteinaceous content of these cysts.20 Whenthe appearance is atypical, MRS may aid in

Fig. 8. Patient with sensorineural hearing loss and a largedemonstrates marked deformity of the CPA with displacem(open arrow) and cerebellar hemisphere. The mass is isrecovery (FLAIR) reveals heterogeneous signal in the prepCPA is fraught with flow artifacts on many FLAIR sequencebut predominantly restricted diffusion (arrows), a charactenoid cyst.

diagnosis, because epidermoids have been re-ported to depict elevated lactate peaks.21

ARACHNOID CYSTS

Arachnoid cysts are the major differential diag-nosis for a nonenhancing cystic CPA mass. DWIhas proved to be exquisitely able to distinguishthese lesions and other nonenhancing CPAmasses. Because arachnoid cysts are truly thin-walled masses containing only CSF, they have noenhancement and demonstrate increased diffusion

CPA epidermoid. (A) Axial T2-weighted MR imagingent of the pons and deformity of the brachium pontis

ointense to CSF. (B) Axial fluid-attenuated inversionontine signal, suggesting a solid lesion; however, thes. (C) Axial DWI through the CPA shows heterogeneousristic that differentiates an epidermoid from an arach-

Fig. 9. Postoperative evaluation of an epidermoid tumor. This is the same patient as in Fig. 8, after attemptedresection. (A) Axial DWI demonstrates residual increased signal in the right CPA (arrow). (B) ADC map confirmsthat there is true restricted diffusion, indicating residual tumor (arrow).

Lakshmi & Glastonbury400

or reduced signal intensity on DWI (Fig. 10). Arach-noid cysts can be found in many cerebral locations,but only 5% to 10% are reported to arise in theposterior fossa.22 Most are asymptomatic anddiscovered incidentally; less commonly, they maypresent with mass effect on lower cranial nerves.

LIPOMAS

Lipomas are benign hamartomatous lesions thatare much more commonly found in the supratento-rial brain. In the CPA, they typically encase, and aredensely adherent to, surrounding neurovascularstructures.23 Unlike their supratentorial counter-parts, posterior fossa lipomas are not usually asso-ciated with cerebral malformations. They can bediscovered incidentally or present secondary tomass effect on adjacent cranial nerves. Hearingloss, tinnitus, and vertigo are common presentingsymptoms. Trigeminal neuralgia and hemifacialspasm have also been reported.

Because lipomas are composed of matureadipose tissue, their CT and MR imaging charac-teristics are identical to those of subcutaneousfat, including signal suppression on chemicalselective fat-suppressed sequences. Infrequently,calcifications are reported, and they do notenhance on contrast administration. Neurovascu-lar structures seem to pass through the lipomaand are usually not displaced.24

METASTASES

Metastases are uncommon CPA masses. They canpresent with mass effect, mimicking other CPA

masses, such as schwannomas or meningiomas,or they may present with cranial neuropathies asleptomeningeal carcinomatosis. Clinically, metas-tases may present with more acute symptom onsetand progression than the tumors they mimic.25

The most common primary tumors to metasta-size to the CPA are lung, breast, melanoma, andlymphoma.26 Any of these tumors may presentwith leptomeningeal carcinomatosis or as a CPAmass. Occasionally, leptomeningeal seeding maybe confined to the IAC. This pattern is morecommon with lung or breast primaries or withnon-Hodgkin’s lymphoma.27 In the absence ofa history of primary malignancy, the careful searchfor and detection of multiple lesions can suggestthe diagnosis of metastasis and direct the clinicianto CSF sampling. The presence of bilateral IAClesions suggests metastatic disease or neurofibro-matosis type 2 (NF2), for which the presence ofbilateral vestibular schwannomas is diagnostic. Itis key to remember that this syndrome presentsin children or young adults and that the new diag-nosis of NF2 in an older patient is extremelyunusual (Fig. 11).

OTHER CEREBELLOPONTINE ANGLESCHWANNOMAS

Schwannomas in the posterior fossa can arisefrom any of the cranial nerves from the fifth(trigeminal) to the 12th (hypoglossal) cranialnerves. These nonvestibular schwannomas sharethe same CT and MR imaging features as vestib-ular schwannomas but are often distinguished bytheir presenting symptoms and their precise

Fig. 10. Incidental CPA arachnoid cyst. (A) Axial T2-weighted MR imaging demonstrates a subtle lesion (openarrow) in the low right CPA, with mass effect on the glossopharyngeal nerve as it exits the medulla (white arrow).The lesion is isointense to CSF. (B) Axial fluid-attenuated inversion recovery MR imaging shows reduced signal inthe right CPA (open arrow), although the signal does not ‘‘disappear’’ completely. (C) Axial DWI shows increaseddiffusion with low signal in the right CPA, isointense to CSF, at the site of the mass (open arrow). This confirms thediagnosis of arachnoid cyst.

Imaging of the Cerebellopontine Angle 401

relation to cranial nerves and skull base foramina.A trigeminal schwannoma may mimic a cisternalvestibular schwannoma but has more of an ante-roposterior orientation and extends toward orinto Meckel’s cave.28 When a trigeminal schwan-noma extends into Meckel’s cave, it may be diffi-cult to distinguish this tumor from a meningioma(Figs 12 and 13).

Facial nerve schwannomas can sometimes bedifficult to distinguish from small vestibular schwan-nomas. On contrast-enhanced MR imaging,abnormal enhancement can be seen to extend into

the labyrinthine segment toward the geniculateganglion.29 Whena facial schwannoma issuspected,CT should be recommended to demonstrateenlargement of the facial nerve canal (Fig. 14).30

OTHER CEREBELLOPONTINE ANGLEMASSES

Other masses that can project into the CPAcistern include intra-axial lesions, such as brainstem gliomas, or cerebellar pilocytic astrocy-tomas. The intra-axial tumors do not havea CSF cleft between the mass and brain

Fig. 11. Axial T1-weighted postcontrast MR imagingdemonstrates bilateral enhancing IAC masses (whitearrows) in a middle-aged woman. The presence ofbilateral IAC masses suggests NF2, which is unlikelyto present in an adult, or metastatic disease. Thisproved to be breast carcinoma. Leptomeningealtumor seeding is also seen on the surface of the leftbrachium pontis (curved black arrow).

Lakshmi & Glastonbury402

parenchyma and also tend to narrow the CPAcistern as opposed to extra-axial tumors, whichwiden the corresponding cistern and have

Fig.12. Large left CPA meningioma mimics a trigeminal sca heterogeneous mass in the left CPA. The mass (arrows) da vestibular schwannoma. (B) Postcontrast T1-weighted Mmass (arrow) and demonstrates anterior extension of theappearance suggests a schwannoma or a meningioma, awas performed, showing vascular supply arising from the martery; the pattern of enhancement favored a meningiom

extrinsic mass effect on the adjacent cerebellumand brain stem.

Intraventricular tumors, such as papilloma orependymoma, can extend into the CPA throughthe foramen of Luschka. Skull base tumors, suchas glomus jugulare, or bony tumors, such as chon-drosarcoma or chordoma, can also project into theCPA cistern (Fig. 15). At CT, the skull base tumorsshow evidence of widening of skull base foraminaor bony sclerosis and erosion.

LEPTOMENINGEAL PROCESSES

Any inflammatory or infectious process affectingthe dura or leptomeninges can involve the CPAcistern. Differentiating among these entities andbetween these entities and metastatic disease(leptomeningeal carcinomatosis) by imagingfeatures alone is difficult. A clear clinical historyis extremely important, but the final diagnosisoften requires CSF analysis.

Neurosarcoidosis

Sarcoidosis is commonly referred to as the greatmimicker because of its multitude of differentmanifestations in the central nervous system(CNS). At imaging, leptomeningeal enhancementis the most common presentation, seen in up to40% of cases. The enhancement may be diffuseor nodular and spreads along perivascular spaces,

hwannoma. (A) Axial T2-weighted MR imaging showsoes not extend into the IAC, suggesting that it is not

R imaging shows heterogeneous enhancement of themass into Meckel’s cave (curved arrow). The imagingnd no dural tail was evident. Catheter angiographyeningohypophyseal artery of the left petrous carotid

a, which was found at surgery.

Fig.13. Trigeminal schwannoma. Pathologically proved trigeminal schwannoma with a similar appearance to theprevious meningioma case. Axial T2-weighted (A) and axial T1-weighted (B) postcontrast MR imaging showsa heterogeneous mass in the right CPA (arrows), which does not extend into the IAC but does extend anteriorlyto Meckel’s cave (curved arrow in B). No dural tail is seen, which favors a diagnosis of schwannoma.

Imaging of the Cerebellopontine Angle 403

causing the appearance of intraparenchymallesions. There is a predilection for basilarmeninges, and the appearance is indistinguishablefrom tuberculosis or lymphoma. Intraparenchymallesions are usually T2 hypointense and demon-strate avid enhancement. Central necrosis isuncommon.31 Cranial nerve involvement can beisolated or part of a more diffuse leptomeningealprocess (Fig. 16). It is to be noted that there is

Fig. 14. A 46-year-old woman with a large CPA mass. (Aa ‘‘classic’’ ice-cream cone–like CPA-IAC mass (arrow), suweighted postcontrast MR imaging more superiorly showsegment of the facial nerve to the geniculate ganglion (arr

poor correlation between clinical symptomatologyand imaging appearance.32

CNS involvement at postmortem examination isreported in 25% of patients who have provedsystemic sarcoidosis. Symptomatic CNS involve-ment that is appreciable on imaging is lesscommon and is seen in 5% to 10% of all cases.Isolated CNS involvement in the absence ofsystemic disease is much less common and is

) Axial T1-weighted postcontrast MR imaging showsggestive of a vestibular schwannoma. (B) Axial T1-

s this mass to extend anteriorly along the labyrinthineow), indicating that this is a facial nerve schwannoma.

Fig.15. A 78-year-old patient with pulsatile tinnitus and a right glomus jugulare tumor. (A) Axial postcontrast, T1-weighted, fast spin image demonstrates a rounded heterogeneously enhancing mass (arrow) in the right CPA atthe level of the cochlea (curved arrow). No dural tail is evident. (B) On an inferior image, the mass is seen toextend into the right jugular foramen (arrow).

Lakshmi & Glastonbury404

reported in approximately 1% of cases.33

Common presenting symptoms include facialnerve paralysis, vision loss, headache, and othersigns of meningeal irritation.

Tuberculosis

CNS infection with Mycobacterium tuberculosiscan be in the form of a diffuse exudative leptome-ningitis, which is the most common presentation,

Fig.16. A 40-year-old woman with headaches, cranial neuT1-weighted MR imaging demonstrates multiple focal areaSome of the foci are subtle, whereas the thickening of theto be neurosarcoidosis after CSF analysis and pulmonary e

or can present less commonly as a more localizedform, such as tuberculoma or cerebritis. CNSinvolvement occurs in 2% of all patients whohave tuberculosis and in up to 10% of patientswho have AIDS. It often affects young patients,who commonly present with fever, change inmentation, neck stiffness, and other signs ofmeningitis. Cranial neuropathies and focalischemic symptoms as a result of vascular irrita-tion and spasm can also occur.

ropathies, and lung nodules. (A, B) Axial postcontrasts of abnormal leptomeningeal enhancement (arrows).choroid plexus is more floridly abnormal. This provedvaluation.

Imaging of the Cerebellopontine Angle 405

CT may demonstrate high attenuation of theCSF in the basilar cisterns (pseudo-subarachnoidblood appearance). On contrast-enhanced MRimaging, thick, sometimes nodular, diffuseenhancement of the basilar meninges can beseen. A triad of basilar meningitis, hydrocephalus,and brain infarctions has been reported. Occa-sionally, there may be surprisingly minimalenhancement in patients who have AIDS, whichhas been attributed to impaired immuneresponse.34 DWI characteristics of tuberculomasvary according to the stage of disease.35,36 Theconcomitant presence of lesions with varied DWIpatterns may point toward tuberculosis asa primary etiology. When they are extra-axial, tu-berculomas may mimic meningiomas. In thiscircumstance, MRS may be helpful, because tu-berculomas depict elevated lipid peaks at 0.9and 1.3 ppm.37,38 Imaging findings alone are notdistinctive enough, and CSF analysis, culture,and polymerase chain reaction analysis are ofparamount importance.

Vascular Lesions

Vascular masses make up only 3.4% of all CPAlesions, with most being aneurysms of the poste-rior inferior cerebellar artery or vertebral artery.39

It is not uncommon for a loop of the AICA to enterthe porus acousticus, without symptoms. Aneu-rysms associated with the AICA are extremelyrare. Strictly intracanalicular aneurysms are evenrarer, with few cases reported in the literature.These can sometimes be misinterpreted as intra-canalicular schwannomas. Initial presentationusually is with subarachnoid hemorrhage or cranialnerve VII and VIII neuropathies. At CT and MRimaging, strong uniform postcontrast enhance-ment similar to that of intracerebral blood vesselsis noted. Mixed signal intensities can be seen onMR imaging if the aneurysm is thrombosed.

Other vascular lesions, such as cavernomas andAVMs, can also be seen rarely in the CPA. Caver-nomas can be superficial intra-axial or extra-axialin their location.40 They have a heterogeneousappearance on T1- and T2-weighted sequencesand demonstrate a peripheral rim of low signalintensity hemosiderin. The truly extra-axial caver-nomas are associated with cranial nerves. Variableenhancement is present on administration ofgadolinium.41,42

A wide variety of pathologic processes can existin the CPA. Arriving at a short list of differentialdiagnoses requires a methodical approach,keeping in mind the location, signal, diffusion,and enhancement characteristics of the lesions.CT can be complimentary in some cases, such

as meningiomas or paragangliomas or schwanno-mas arising from nerves other than the eighthcranial nerve. Advanced MR imaging techniques,such as MRS, are not usually needed routinelybut may aid in the differential diagnosis when pre-sented with atypical masses.

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