The cerebellopontine angle (CPA) masscontributes to a vast amount of skull basepractice and radiological evaluation isinvaluable in diagnosis. Vestibular schwan-nomas are the most common lesions atthe CPA, accounting for 90%. Diagnosticscreening using T1 weighted MR withgadolinium contrast enhancement for the‘at risk’ group of asymmetric sensorineuralhearing loss patients remains the gold stan-dard. Tumours are isointense to brain onT1, and enhancement is shown withgadolinium contrast. This however is rela-tively costly compared to other MRI scan-ning protocols, due to the need for 30minutes of MR time and gadolinium. Insome institutions costs have been reducedby using other sequences such as fast spin-echo (FSE) techniques where the fluidcompartments of the inner ear and internalauditory canal are well visualised. Thisresults in a reduced total charge and a 98%accuracy of diagnosis compared to T1scanning with gadolinium.1 Carrier et al.have modified the T1 protocol using a T1sagittal localiser and performing 3mm axialslices through the internal auditory meatusthus reducing scanning time to 12minutes.2 Other modifications on the T2protocol, such as three-dimensional aquisi-tion fast spin echo T2 protocols (FIESTASequences), where the tumour appearshyperintense, can be used to reduce acqui-sition time whilst minimising any loss in

sensitivity (Figure 1).3 In some instanceswhere MR is contraindicated (for examplemetallic implants or claustrophobia) orunavailable, CT scanning may play a role indiagnosis and surveillance too; widening ofthe internal auditory canal (IAC) may beseen, as well as enhancement with contrast.

As well as vestibular schwannoma, thedifferential diagnosis for a CPA massincludes meningioma (3%), congenitalcholesteatoma (2.5%), facial nerve schwan-noma (1%) and more rarely, cholesterolgranulomas, epidermoids and arachnoidcysts. On MR meningiomas are broadbased and eccentric to the porusacousticus, with an obtuse angle at thebone-tumour interface. 60% have a duraltail, and if high grade they may invade brainparenchyma. On T2-weighted scans,meningiomas appear less bright thanvestibular schwannomas. They may beisointense because of cellularity or evenhypointense because of calcification. OnT1 signal is variable but usually appearsisointense to brain. In 25% calcificationmay manifest as small focal signal voids. Athin hypointense covering is usuallyobserved, which represents a thin layer ofCSF called the CSF cleft. They enhancebrightly with gadolinium.

Congenital petrous apex cholesteatoma isan expansile mass on CT scanning. It usuallyhas a smooth margin and doesn’t exhibitenhancement, other than sometimes a

Radiology in Skull Base ENT

despite

radiological

imaging

techniques

becoming ever

more sophisticated

there remains a

crucial role for the

neurotologist in

interpreting and

correlating

findings with

clinical signs and

symptoms

When considering radiology in skull base ENT there are essentially two

prime modalities: computerised tomography (CT) and magnetic reso-

nance (MR) scanning. High resolution CT (0.5-0.625mm) has trans-

formed the diagnostic imaging of the temporal bone. The anatomical clarity offered,

particularly of bony structures, has granted utility in imaging the external auditory canal,

middle ear, mastoid and inner ear and petrous bone, either electively or acutely following

trauma. MR scanning is more useful for soft tissue abnormalities, and lends itself to the

evaluation of skull base and petrous apex tumours and inflammation, particularly where

clarification of intracranial or perineural spread is required. Whilst CT scanning necessi-

tates a dose of ionising radiation MR scanning does not. Both modalities have their

particular strengths and limitations, and both are applied with various refinements to an

extensive range of pathologies and clinical entities afflicting the skull base. The aim of this

article will be to highlight some particular areas of current interest, rather than to detail

an exhaustive list of applications.

Mr Sanjay Verma,MBBCh,FRCS(ORL-HNS),MA, PhD,ConsultantOtolaryngologist

CorrespondenceMr Sanjay Verma,ConsultantOtolaryngologist,Department of ENT,Leeds General Infirmary,Great George Street,Leeds, West Yorkshire,LS1 3EX, UK.E: drsanjverma@gmail.com

Declaration ofCompeting InterestsNone declared.

peripheral rim. Larger lesions may expandtowards the horizontal portion of theinternal carotid artery, the trigeminal nerveand the middle and posterior cranial fossae.On T1 weighted MR cholesteatomaexhibits a hypointense signal, whilst on T2it is hyperintense. While CT scanning alonewill often yield considerable diagnosticdetail, MR scanning can aid in assessingintracranial extension and complications,such as abscesses and lateral venous sinusthrombosis. Recently there has also beenconsiderable interest in the use of diffu-sion-weighted echo-planar MR imaging(DW-EPI) in the diagnosis of post-opera-tive residual or recurrent cholesteatoma.This can present a diagnostic dilemmasince it is often difficult to distinguish gran-ulation and scar tissue from cholesteatomaon conventional CT or MR scans.Essentially diffusion-weighted MR

produces images of tissues weighted withthe local microstructural characteristics ofwater diffusion, rather than the more tradi-tional T1 and T2 relaxation rates. The earlyindicators are promising, with studiesdemonstrating a sensitivity of 83% andspecificity of 82% for DW-EPI in diagnosingresidual cholesteatoma.4 Venail et al. havealso compared DW-EPI with conventionalpost-contrast T1 weighted MR (DPI) andconcluded that DW-EPI was more specificbut less sensitive than DPI, and thus thatthe concurrent use of both modalities mayhelp benefit patients by avoiding unduesurgery.5

Facial nerve schwannomas are relativelyrare and only 5% present with a facial nervepalsy. Imaging plays an important role sincemanagement is generally conservative withdecompressive surgery for high grade facialpalsies only. Since those afflicted are mostlyin their third decade unwitting surgicalresection may result in a disastrous facialnerve deficit to the patient. With a facialnerve schwannoma CT may demonstrate asmooth enlargement of the Fallopian canal,a mass in the middle ear or an effusionwithin the middle ear/ mastoid. On MRscanning the lesion enhances with ahypointense T1 signal and hyperintense T2.

Finally cholesterol granulomas are reac-tive masses occurring after haemorrhageinto petrous apex air cells. On CT in theearly stages there is non-specific soft tissue.

Later an isodense mass with rim enhance-ment following contrast may be evidentwith bony scalloping and opacification ofthe middle ear / mastoid. A cholesterolgranuloma may be distinguished fromcholesteatoma on MR since it is hyperin-tense on T1 and T2 images whereascholesteatoma is hypointense on T1 andhyperintense on T2.

Petrous apicitis is a rare but feared infec-tion, often pseudomonal, spreading fromthe middle ear or mastoid. Whilst the clas-sical clinical presentation is that ofGradenigo syndrome (middle ear infection,retro-orbital pain through involvement ofthe trigeminal ganglion and abducensnerve palsy) often times the diagnosis isnot immediately obvious. CT scanning candemonstrate opacification of petrous apexair cells, enhancement of the cavernoussinus and bony erosion. MR may showhypointensity on T1, hyperintensity on T2and ring enhancement. Both imagingmodalities are valuable in distinguishingother differential diagnoses such as skullbase lymphoma. Furthermore, since treat-ment generally comprises a protractedcourse of antibiotics over several weeks,MR scanning which does not deliver a doseof ionising radiation is particularly useful inmonitoring response to treatment.

Otitic infection may also spread to causelateral venous sinus thrombosis. This canalso result from trauma, coagulopathiesand systemic inflammatory diseases. MRandMR venography (MRV) are particularlyuseful. T1 weighted MR shows hyperin-tense signal in the sinus and with contrastshows the empty delta sign; enhancementof the dural leaves without signal in thesinus itself. MRV shows absent filling of thesinus.

Glomus tumours are chemodectomasor nonparaffin paragangliomas that mayarise throughout the temporal bone. Twoanatomic classifications exist to describethese tumours: Fisch (by extension) andGlasscock-Jackson (by category and byextension). Twenty per cent arise from thecochlear promontory, 25% from the hypo-tympanum (both termed glomus tympan-icum), 50% from the jugular foramen(glomus jugulare) and 5% below the skullbase (glomus vagale). Since 5-10% aremultiple, and up to 50% in familial cases, itis imperative that radiological imaging is asaccurate as possible. Moreover, the extentof the disease dictates the surgicalapproach necessary for removal. Theextent and anatomic location are initiallydefined using CT scanning. Glomustympanicum or tumour localised to the

feature

Figure 1: Axial T2 weighted FIESTA MR image demonstrating a large right sided vestibular schwannomawith brainstem compression.

Figure 2: Multiplanar oblique reformatted CTscan demonstrating dehiscent superiorsemicircular canal.

middle ear usually comprises small lesionsalong the promontory or hypotympanum.Glomus jugulare or jugulotympanicumenlarges the jugular foramen, erodes thejugular spine and destroys the marginbetween the jugular bulb and the carotidcanal. MR imaging is used to assessintracranial extension and anatomic rela-tionships with neural and vascular struc-tures. T1 weighted images with andwithout contrast in axial and coronalplanes show tumour extent effectively;small tumours are often hyperintense. Aclassic salt and pepper appearance onunenhanced T1 weighted scans may bedemonstrated, with hyperintensity repre-senting small haemorrhages within thetumour and signal voids representingfeeding vessels. Post contrast fat saturatedT1 weighted imaging is useful in differenti-ating tumour from surrounding marrowand fat. Fat saturation techniques usedwith contrast further aid in distinguishingrecurrence from post-surgical change.Glomus tumours are highly vascular andflow voids may often be visible within thetumour. Angiography can be a usefuladjunct to evaluate tumour blood supply,assess collateral circulation and embolise

preoperatively. MR venography is acomplementary examination to measurejugular vein invasion, occlusion, and collat-eral venous sinus drainage.

To conclude, a relatively recent exampleto illustrate how radiological advanceshave facilitated discovery of hithertounrecognised pathologies. More than 70years after Tullio and Hennenbertdescribed the phenomena of sound- andpressure-induced vestibular activation,Lloyd Minor in 2000 related these positivefindings directly to an anatomical defect ofthe superior semicircular canal (SSC) thatwas detected with high-resolution CT.6

Images require reformatting in obliqueplanes parallel and perpendicular to the

SSC and must be carefully interpreted sincethey may be complicated by partialvolume effect. As awareness of superiorsemicircular canal dehiscence (SSCD) hasincreased globally and more cases are iden-tified radiologically the challenge for theneurotologist is now to establish whichpatient’s symptoms are attributable to theapparent radiological anomaly and hencedetermine those who would benefit fromsurgical intervention. This serves to under-line that despite radiological imaging tech-niques becoming ever more sophisticatedthere remains a crucial role for the neuro-tologist in interpreting and correlating find-ings with clinical signs and symptoms. �

feature

References

1. Murphy MR, Selesnick SH. Cost-effective diagnosis of acoustic neuromas: a philosophical, macroeconomic, andtechnological decision. Otolaryngol Head Neck Surg 2002;127:253-9.

2. Carrier DA, Arriaga MA. Cost-effective evaluation of asymmetric sensorineural hearing loss with focused magnet-ic resonance imaging. Otolaryngol Head Neck Surg 1997;116:567-74.

3. Hermans R, Van der Goten A, De Foer B, Baert AL. MRI screening for acoustic neuroma without gadolinium:value of 3DFT-CISS sequence. Neuroradiology 1997;39:593-8.

4. Jindal M, Doshi J, Srivastav M, Wilcock D, Irving R, De R. Diffusion-weighted magnetic resonance imaging in themanagement of cholesteatoma. Eur Arch Otorhinolaryngol 2009.

5. Venail F, Bonafe A, Poirrier V, Mondain M, Uziel A. Comparison of echo-planar diffusion-weighted imaging anddelayed postcontrast T1-weighted MR imaging for the detection of residual cholesteatoma. AJNR Am JNeuroradiol 2008;29:1363-8.

6. Minor L. Superior canal dehiscence syndrome. Am J Otol 2000;21:9-19.