Radiology in the Imaging of Acoustic Neuroma and Skull Base ENT; article in ENT News

1. The cerebellopontine angle (CPA) mass
contributes to a vast amount of skull base
practice and radiological evaluation is
invaluable in diagnosis. Vestibular schwan-
nomas are the most common lesions at
the CPA, accounting for 90%. Diagnostic
screening using T1 weighted MR with
gadolinium contrast enhancement for the
‘at risk’ group of asymmetric sensorineural
hearing loss patients remains the gold stan-
dard. Tumours are isointense to brain on
T1, and enhancement is shown with
gadolinium contrast. This however is rela-
tively costly compared to other MRI scan-
ning protocols, due to the need for 30
minutes of MR time and gadolinium. In
some institutions costs have been reduced
by using other sequences such as fast spin-
echo (FSE) techniques where the fluid
compartments of the inner ear and internal
auditory canal are well visualised. This
results in a reduced total charge and a 98%
accuracy of diagnosis compared to T1
scanning with gadolinium.1
Carrier et al.
have modified the T1 protocol using a T1
sagittal localiser and performing 3mm axial
slices through the internal auditory meatus
thus reducing scanning time to 12
minutes.2
Other modifications on the T2
protocol, such as three-dimensional aquisi-
tion fast spin echo T2 protocols (FIESTA
Sequences), where the tumour appears
hyperintense, can be used to reduce acqui-
sition time whilst minimising any loss in
sensitivity (Figure 1).3
In some instances
where MR is contraindicated (for example
metallic implants or claustrophobia) or
unavailable, CT scanning may play a role in
diagnosis and surveillance too; widening of
the internal auditory canal (IAC) may be
seen, as well as enhancement with contrast.
As well as vestibular schwannoma, the
differential diagnosis for a CPA mass
includes meningioma (3%), congenital
cholesteatoma (2.5%), facial nerve schwan-
noma (1%) and more rarely, cholesterol
granulomas, epidermoids and arachnoid
cysts. On MR meningiomas are broad
based and eccentric to the porus
acousticus, with an obtuse angle at the
bone-tumour interface. 60% have a dural
tail, and if high grade they may invade brain
parenchyma. On T2-weighted scans,
meningiomas appear less bright than
vestibular schwannomas. They may be
isointense because of cellularity or even
hypointense because of calcification. On
T1 signal is variable but usually appears
isointense to brain. In 25% calcification
may manifest as small focal signal voids. A
thin hypointense covering is usually
observed, which represents a thin layer of
CSF called the CSF cleft. They enhance
brightly with gadolinium.
Congenital petrous apex cholesteatoma is
an expansile mass on CT scanning. It usually
has a smooth margin and doesn’t exhibit
enhancement, 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
W
hen 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,
Consultant
Otolaryngologist
Correspondence
Mr Sanjay Verma,
Consultant
Otolaryngologist,
Department of ENT,
Leeds General Infirmary,
Great George Street,
Leeds, West Yorkshire,
LS1 3EX, UK.
E: drsanjverma@
gmail.com
Declaration of
Competing Interests
None declared.

2. peripheral rim. Larger lesions may expand
towards the horizontal portion of the
internal carotid artery, the trigeminal nerve
and the middle and posterior cranial fossae.
On T1 weighted MR cholesteatoma
exhibits a hypointense signal, whilst on T2
it is hyperintense. While CT scanning alone
will often yield considerable diagnostic
detail, MR scanning can aid in assessing
intracranial extension and complications,
such as abscesses and lateral venous sinus
thrombosis. Recently there has also been
considerable 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 dilemma
since it is often difficult to distinguish gran-
ulation and scar tissue from cholesteatoma
on conventional CT or MR scans.
Essentially diffusion-weighted MR
produces images of tissues weighted with
the local microstructural characteristics of
water diffusion, rather than the more tradi-
tional T1 and T2 relaxation rates. The early
indicators are promising, with studies
demonstrating a sensitivity of 83% and
specificity of 82% for DW-EPI in diagnosing
residual cholesteatoma.4
Venail et al. have
also compared DW-EPI with conventional
post-contrast T1 weighted MR (DPI) and
concluded that DW-EPI was more specific
but less sensitive than DPI, and thus that
the concurrent use of both modalities may
help benefit patients by avoiding undue
surgery.5
Facial nerve schwannomas are relatively
rare and only 5% present with a facial nerve
palsy. Imaging plays an important role since
management is generally conservative with
decompressive surgery for high grade facial
palsies only. Since those afflicted are mostly
in their third decade unwitting surgical
resection may result in a disastrous facial
nerve deficit to the patient. With a facial
nerve schwannoma CT may demonstrate a
smooth enlargement of the Fallopian canal,
a mass in the middle ear or an effusion
within the middle ear/ mastoid. On MR
scanning the lesion enhances with a
hypointense T1 signal and hyperintense T2.
Finally cholesterol granulomas are reac-
tive masses occurring after haemorrhage
into petrous apex air cells. On CT in the
early stages there is non-specific soft tissue.
Later an isodense mass with rim enhance-
ment following contrast may be evident
with bony scalloping and opacification of
the middle ear / mastoid. A cholesterol
granuloma may be distinguished from
cholesteatoma on MR since it is hyperin-
tense on T1 and T2 images whereas
cholesteatoma is hypointense on T1 and
hyperintense on T2.
Petrous apicitis is a rare but feared infec-
tion, often pseudomonal, spreading from
the middle ear or mastoid. Whilst the clas-
sical clinical presentation is that of
Gradenigo syndrome (middle ear infection,
retro-orbital pain through involvement of
the trigeminal ganglion and abducens
nerve palsy) often times the diagnosis is
not immediately obvious. CT scanning can
demonstrate opacification of petrous apex
air cells, enhancement of the cavernous
sinus and bony erosion. MR may show
hypointensity on T1, hyperintensity on T2
and ring enhancement. Both imaging
modalities are valuable in distinguishing
other differential diagnoses such as skull
base lymphoma. Furthermore, since treat-
ment generally comprises a protracted
course of antibiotics over several weeks,
MR scanning which does not deliver a dose
of ionising radiation is particularly useful in
monitoring response to treatment.
Otitic infection may also spread to cause
lateral venous sinus thrombosis. This can
also result from trauma, coagulopathies
and systemic inflammatory diseases. MR
and MR venography (MRV) are particularly
useful. T1 weighted MR shows hyperin-
tense signal in the sinus and with contrast
shows the empty delta sign; enhancement
of the dural leaves without signal in the
sinus itself. MRV shows absent filling of the
sinus.
Glomus tumours are chemodectomas
or nonparaffin paragangliomas that may
arise throughout the temporal bone. Two
anatomic classifications exist to describe
these tumours: Fisch (by extension) and
Glasscock-Jackson (by category and by
extension). Twenty per cent arise from the
cochlear promontory, 25% from the hypo-
tympanum (both termed glomus tympan-
icum), 50% from the jugular foramen
(glomus jugulare) and 5% below the skull
base (glomus vagale). Since 5-10% are
multiple, and up to 50% in familial cases, it
is imperative that radiological imaging is as
accurate as possible. Moreover, the extent
of the disease dictates the surgical
approach necessary for removal. The
extent and anatomic location are initially
defined using CT scanning. Glomus
tympanicum or tumour localised to the
feature
Figure 1: Axial T2 weighted FIESTA MR image demonstrating a large right sided vestibular schwannoma
with brainstem compression.
Figure 2: Multiplanar oblique reformatted CT
scan demonstrating dehiscent superior
semicircular canal.

3. middle ear usually comprises small lesions
along the promontory or hypotympanum.
Glomus jugulare or jugulotympanicum
enlarges the jugular foramen, erodes the
jugular spine and destroys the margin
between the jugular bulb and the carotid
canal. MR imaging is used to assess
intracranial extension and anatomic rela-
tionships with neural and vascular struc-
tures. T1 weighted images with and
without contrast in axial and coronal
planes show tumour extent effectively;
small tumours are often hyperintense. A
classic salt and pepper appearance on
unenhanced T1 weighted scans may be
demonstrated, with hyperintensity repre-
senting small haemorrhages within the
tumour and signal voids representing
feeding vessels. Post contrast fat saturated
T1 weighted imaging is useful in differenti-
ating tumour from surrounding marrow
and fat. Fat saturation techniques used
with contrast further aid in distinguishing
recurrence from post-surgical change.
Glomus tumours are highly vascular and
flow voids may often be visible within the
tumour. Angiography can be a useful
adjunct to evaluate tumour blood supply,
assess collateral circulation and embolise
preoperatively. MR venography is a
complementary examination to measure
jugular vein invasion, occlusion, and collat-
eral venous sinus drainage.
To conclude, a relatively recent example
to illustrate how radiological advances
have facilitated discovery of hitherto
unrecognised pathologies. More than 70
years after Tullio and Hennenbert
described the phenomena of sound- and
pressure-induced vestibular activation,
Lloyd Minor in 2000 related these positive
findings directly to an anatomical defect of
the superior semicircular canal (SSC) that
was detected with high-resolution CT.6
Images require reformatting in oblique
planes parallel and perpendicular to the
SSC and must be carefully interpreted since
they may be complicated by partial
volume effect. As awareness of superior
semicircular canal dehiscence (SSCD) has
increased globally and more cases are iden-
tified radiologically the challenge for the
neurotologist is now to establish which
patient’s symptoms are attributable to the
apparent radiological anomaly and hence
determine those who would benefit from
surgical intervention. This serves to under-
line that despite radiological imaging tech-
niques becoming ever more sophisticated
there remains a crucial role for the neuro-
tologist in interpreting and correlating find-
ings with clinical signs and symptoms.
feature
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