420 March-April 2001 RG f Volume 21 ● Number 2
Figure 1. Table (a) and drawing (axial view)
(b) show the segmental approach to diagnosis
of unusual lesions of the CPA based on their
site of origin. DNET dysembryoplastic neu-
Lesions of the cerebellopontine angle (CPA) are
frequent and represent 6%–10% of all intracranial
tumors (1,2). Acoustic neuromas, which are also
called vestibular schwannomas (3), and meningi-
omas are the two most frequent lesions and ac-
count for approximately 85%–90% of all CPA
tumors (1). The other 10%–15% encompass a
large variety of lesions that radiologists will en-
counter more and more frequently because of the
remarkable sensitivity and accuracy of magnetic
resonance (MR) imaging in evaluation of a CPA
In most cases, MR imaging and computed to-
mography (CT) show typical features of acoustic
neuromas or meningiomas and are sufﬁcient to tures of these rare CPA lesions. In addition, a seg-
establish the diagnosis. Acoustic neuromas are mental approach to diagnosis of these lesions
usually round or oval masses in the cerebellopon- based on their site of origin is proposed (Fig 1).
tine cistern that emerge from the internal auditory Although the lesions usually arise from the cer-
canal, widen the porus, and grow posteriorly be- ebellopontine cistern, they can also invade the
cause of the anterior limit represented by the cis- CPA from extension of a skull base tumor or even
ternal segment of the facial nerve (4). They can a brainstem tumor. The attenuation at CT, signal
be heterogeneous due to cystic components. Con- intensity at MR imaging, enhancement, shape
versely, meningiomas are usually hemispheric, and margins, extent, mass effect, and reaction of
semilunar masses with a broad petrous base to the adjacent bone structures of the skull base are
which they are attached and are usually asymmet- also helpful in making the correct preoperative
ric to the internal auditory canal. diagnosis.
After excluding acoustic neuromas and menin-
giomas, we retrospectively studied a large variety Tumors Originating in the CPA
of surgically proved unusual CPA lesions from The CPA is outlined by the meninges of the cer-
the collections of four institutions. In this article, ebellopontine cistern; thus, in addition to cere-
we discuss and illustrate the neuroimaging fea- brospinal ﬂuid (CSF), the CPA contains nerves
and arteries and possibly embryologic remnants.
Each of these structures can be the site of origin
of an unusual CPA lesion.
RG f Volume 21 ● Number 2 Bonneville et al 421
Figure 2. Epidermoid cyst in a 52-year-old
woman with epilepsy. (a) Axial T1-weighted MR
image shows an epidermoid cyst with characteristic
focal marbling in the left CPA (arrow). (b) Axial
T2-weighted MR image shows the lobulated mar-
gins of the cyst impinging on the pons (arrowhead).
(c) Axial heavily T2-weighted (constructive interfer-
ence in the steady state) MR image shows the extent
of the tumor.
nerves and arteries in the cisterns rather than dis-
placing them (6).
On CT scans, epidermoid cysts appear hypoat-
tenuating, almost isoattenuating to CSF, and
have characteristic irregular, lobulated margins.
As opposed to arachnoid cyst, which is the main
differential diagnosis, epidermoid cyst produces
no reaction of the adjacent bone structures. Slight
marginal calciﬁcations or enhancement after con-
trast material administration are rarely seen (7).
Masses originating in the CPA are extraaxial At MR imaging, epidermoid cysts have slightly
and thus widen the homolateral subarachnoid higher signal intensity than CSF on T1- and T2-
cisterns. They displace or encase neurovascular weighted images, often with heterogeneous and
structures. These lesions can be separated from marbled features (Fig 2). Sometimes, when the
the brainstem by a thin CSF layer, and there is signal intensity is very similar to that of CSF, the
usually no brainstem edema. ﬂuid-attenuated inversion-recovery sequence is
more sensitive than conventional sequences in
Epidermoid Cyst differentiation of epidermoid and arachnoid cysts
Epidermoid cysts or tumors, also known as pri- because it suppresses the signal of CSF (8). With
mary cholesteatomas, are the third most frequent this sequence, epidermoid cysts have high signal
tumor of the CPA (5). They arise from normal intensity, whereas the signal of arachnoid cysts is
epithelial cells included during neural tube clo- suppressed. Diffusion-weighted imaging is also
sure. Their growth is due to accumulation of well known to allow differentiation of epidermoid
keratin and cholesterol produced by desquama-
tion of the squamous epithelium lining the mass.
These slow-growing tumors encase and surround
422 March-April 2001 RG f Volume 21 ● Number 2
MR Imaging Characteristics and Suggestive Features of Unusual Lesions Arising from the
Appearance by Image Type
Lesion T1-weighted T2-weighted Enhancement Suggestive Features
Epidermoid cyst Hypointense Hyperintense No Hyperintense on diffusion-weighted images
Dermoid cyst Hyperintense Hypointense No Fat and calciﬁcation, fat-ﬂuid levels
Arachnoid cyst Hypointense Hyperintense No Isointense to CSF, hypointense on diffu-
Miscellaneous Hypointense Hyperintense No Mimic arachnoid cyst except for lack of
cysts bone erosion
Lipoma Hyperintense Isointense to fat No Exactly matches signal intensity of subcuta-
neous fat with all sequences
Schwannoma Hypointense Hyperintense Yes Follows the anatomic course of the nerve
Aneurysm Hypointense Hypointense Possible Well-circumscribed hypointense lesion on
Melanoma Hyperintense Isointense or Yes Spontaneous hyperintense lesion on T1-
hypointense weighted images
Miscellaneous Hypointense Variable Yes Sarcoidosis is hypointense on T2-weighted
and arachnoid cysts. The apparent diffusion coef- neuroimaging, their attenuation and signal inten-
ﬁcient of an epidermoid cyst is signiﬁcantly lower sity match those of CSF almost exactly. These
than that of an arachnoid cyst; therefore, epider- masses have smooth and rounded edges, displace
moid cysts have high signal intensity on diffusion- neurovascular structures (Fig 3), and erode adja-
weighted images, whereas arachnoid cysts, like cent bone structures. There is no calciﬁcation or
CSF, have very low signal intensity (Table 1) (9). enhancement. On T1- and T2-weighted images
Recently, diffusion-weighted imaging has also they can appear similar to an epidermoid cyst. As
been reported to allow conﬁrmation of the pres- described, ﬂuid-attenuated inversion-recovery
ence of residual postoperative tumor (10). Con- and constructive interference in the steady state
structive interference in the steady state or three- sequences as well as diffusion-weighted imaging
dimensional fast spin-echo sequences, which are helpful in making the distinction (8,9).
produce heavily T2-weighted images and thus
excellent contrast between CSF and all other Miscellaneous Cysts
structures, are necessary to precisely assess the Three other rare cystic lesions can be encoun-
extent of the tumor (8). tered in the CPA: neurocysticercosis, neurenteric
cysts, and neuroepithelial cysts. Cysticercosis is a
Dermoid Cyst parasitosis due to encystment of Taenia solium
Like epidermoid cysts, dermoid cysts result from larvae in tissues. In the subarachnoid region, cys-
inclusion of ectodermal elements during neural tic lesions can be found in the CPA and usually
tube closure but may originate a bit earlier (11). occur in the racemose form. They appear as lobu-
Dermoid cysts are midline lesions that rarely in- lated cysts with no mural nodule or enhancement
vade the CPA laterally and contain elements from and have signal intensity similar to that of CSF
all layers of the skin. Thus, fat, hair, sebaceous with all sequences (12,13). They are therefore
glands, and sweat glands can be found in addition difﬁcult to diagnose, and cysticercosis should be
to squamous epithelium. Typically, dermoid cysts considered whenever a solitary enlarged cistern is
have negative attenuation values on CT scans and seen in a patient from an area where the disease is
high signal intensity on T1-weighted images due endemic. In addition, on CT scans, the lack of
to their fatty content, may have a very suggestive adjacent bone erosion can be useful in distin-
fat-ﬂuid level, and contain calciﬁcations (7). guishing neurocysticercosis from arachnoid cyst,
which is the main differential diagnosis.
Arachnoid Cyst The origin of neurenteric cysts is not precisely
Arachnoid cysts are pouchlike intraarachnoid known, but they probably result from dysgenesis
masses of uncertain origin ﬁlled with CSF (1). At of the neurenteric canal during the third week of
embryogenesis (14). The cyst walls are lined by a
typical epithelium that resembles gastrointestinal
or respiratory tract mucosa, and mucinlike secre-
RG f Volume 21 ● Number 2 Bonneville et al 423
Figure 3. Arachnoid cyst in a 27-year-old woman with headaches. (a) Axial T1-weighted MR image shows
an arachnoid cyst with signal intensity similar to that of CSF stretching the left seventh and eighth cranial nerve
complex (arrow). (b) Axial T2-weighted MR image shows the cyst displacing the vascular structures of the
Figure 4. Lipoma in a 7-year-old boy with a polymalformation syndrome. (a) Axial CT scan shows a well-
deﬁned hypoattenuating lipoma of the left CPA. (b) Axial T1-weighted MR image shows that the lipoma has
signal intensity similar to that of subcutaneous fat.
tions ﬁll the cysts. They are generally located in cranial pressure (18). At neuroimaging, neuroepi-
the spine and only a few are intracranial, but thelial cysts appear as round or ovoid cystic le-
those usually occur in the CPA (15). The signal sions with possible septa but without calciﬁcation
intensity depends on the content of the cyst, but it or pathologic enhancement; the attenuation and
is usually isointense or slightly hyperintense to signal intensity are similar to those of CSF (19).
CSF on T1- and T2-weighted images (14). No
calciﬁcation or enhancement is typically seen. Lipoma
The origin of neuroepithelial cysts is still con- Lipomas in the CPA are maldevelopmental masses
troversial, but they are probably secondary to that arise from abnormal differentiation of the me-
simple folding of neuroepithelium out of the ninx primitiva (11). They are homogeneous fatty
primitive ventricular system (16). In fact, these lesions surrounding and encasing normal adjacent
well-deﬁned cysts are lined by a columnar epithe- neurovascular structures with very dense adhesions.
lium that resembles ependyma. They can arise Nevertheless, lipomas are rarely symptomatic, and
anywhere in the central nervous system and are conservative follow-up is often preferred to aggres-
frequent in the choroid plexus and ventricles but sive and potentially risky resection (20).
uncommon in the CPA (17). Neuroepithelial Lipomas appear as fat (Fig 4): homogeneously
cysts are mostly asymptomatic but can compress hypoattenuating with a negative attenuation value
the brainstem and cerebellum in the posterior
cranial fossa and produce signs of increased intra-
424 March-April 2001 RG f Volume 21 ● Number 2
Figures 5, 6. (5) Schwannoma in a 34-year-old man with right trigeminal neuralgia. Contrast material– enhanced
axial T1-weighted MR image shows a homogeneous, enhanced, dumbbell-shaped right trigeminal schwannoma
involving the cisternal part of the nerve and Meckel cave. (6) Schwannoma in a 52-year-old woman with left ear
pain. (a) Gadolinium-enhanced coronal T1-weighted MR image shows an enhancing schwannoma of the ninth,
10th, and 11th cranial nerves in the lowest part of the CPA. (b) Coronal T2-weighted MR image shows the extent
of the schwannoma along the course of the nerves (arrows) and beneath the normal left internal auditory canal (ar-
rowheads). (c) Axial CT scan shows smooth erosion of the skull base and especially the jugular foramen by the
on CT scans and as characteristic and suggestive Schwannoma
homogeneous high signal intensity on T1-weighted Although vestibular schwannomas account for
images, which decreases on fat-suppressed images. 95% of intracranial schwannomas, neural sheath
There is no enhancement after contrast material tumors can develop in all the other nerves of the
administration. Therefore, nonenhanced T1- CPA, especially the trigeminal nerve (Fig 5) and
weighted imaging should be performed when evalu- facial nerve but also cranial nerves IX to XII (Fig
ating a CPA syndrome to look for spontaneous hy- 6). Neuroimaging features are the same as for
perintense lesions, such as lipomas. vestibular schwannomas, including possible cystic
or hemorrhagic components (21). Keys to the
diagnosis are the neuroanatomic location of the
tumor, its extension following the courses of the
nerves, smooth enlargement of their foramina,
RG f Volume 21 ● Number 2 Bonneville et al 425
Figures 7, 8. (7) Asymptomatic aneurysm in a
68-year-old man with lymphoma and right trigemi-
nal neuralgia. Axial T2-weighted MR image shows
an aneurysm of the left posterior inferior cerebellar
artery with typical lack of signal (arrow). Note the
lymphoma in the right pterygopalatine fossa (arrow-
heads), which explains the neuralgia. (8) Aneurysm
in a 75-year-old man with hypoglossal nerve palsy.
(a) Axial T2-weighted MR image shows a throm-
bosed aneurysm of the right posterior inferior cer-
ebellar artery with focal calciﬁcation (arrowhead).
Note the normal right hypoglossal canal (arrow), a
ﬁnding inconsistent with a schwannoma. (b) Con-
trast-enhanced coronal T1-weighted MR image
shows homogeneous enhancement of the organized
thrombus, which completely ﬁlls the aneurysm.
High-ﬂow aneurysms appear as oval or round
masses that have no signal (ﬂow void) with all
spin-echo sequences (Fig 7). Extreme low signal
intensity on T2-weighted images is very sugges-
tive of such lesions and indicates that normal ar-
and fatty amyotrophy of the corresponding ener- teries need to be evaluated. When an aneurysm is
vated muscles (22,23). thrombosed, high signal intensity on T1-weighted
images due to methemoglobin could be sugges-
Aneurysms of tive, but the signal intensity is variable; radiolo-
Posterior Fossa Arteries gists should be aware that enhancement of the
The vertebral and basilar arteries and some of mass can be observed due to organization of the
their branches pass through the cerebellopontine thrombus (Fig 8), thus mimicking a schwannoma
cistern, where a tortuous segment or ectasia or (26).
even an aneurysm can develop. Although not
neoplasms, such lesions can cause mass effect on
the neural structures of the CPA and thus pro-
duce neurologic symptoms (24,25).
426 March-April 2001 RG f Volume 21 ● Number 2
Figure 9. Melanoma in a 58-year-old woman with
a left cerebellar syndrome. (a) Axial CT scan shows
a hyperattenuating melanoma of the left CPA.
(b) Axial T1-weighted MR image shows a well-de-
ﬁned extraaxial mass at the posterior edge of the
petrous bone. The high signal intensity is suggestive
of melanin. (c) Gadolinium-enhanced axial T1-
weighted MR image shows a normal left internal
auditory canal (arrow) and lack of dural tail en-
Melanocytes are normally present in the intracra-
nial leptomeninges, usually in the posterior cere-
bral fossa (27), and can give rise to benign (me-
ningeal melanocytoma) or malignant (malignant
melanoma) tumors. However, metastatic malig-
nant melanomas greatly outnumber primary
melanocytic neoplasms (28). At CT, primary
melanocytic lesions appear as well-circumscribed,
isoattenuating to hyperattenuating extraaxial tu- plasms can mimic benign tumors of the CPA (Fig
mors (Fig 9) with homogeneous enhancement. 10) (32). Therefore, when an unusual aggressive
Even if there is no hyperostosis, they can mimic a schwannomalike mass is encountered and no pri-
meningioma (29). MR imaging demonstrates mary cancer is known to be present, radiologists
variable signal intensity on T1- and T2-weighted should be aware of the possibility of metastases
images in proportion to the amount of melanin and check the lungs and breast carefully. How-
within the tumor (30). Therefore, melanocytic ever, in most cases, there are numerous other in-
neoplasms are isointense or hyperintense to adja- tra- and extraaxial masses, which are very sugges-
cent normal brain tissue on T1-weighted images tive of metastases when correlated with a clinical
and isointense or hypointense on T2-weighted history of carcinoma.
images (31). They enhance after intravenous ad- Tuberculosis is one of the main causes of focal
ministration of contrast material. pachymeningeal thickening in the posterior cra-
nial fossa, but this ﬁnding is not speciﬁc (33).
Miscellaneous Meningeal Lesions Neurosarcoidosis can also appear as a dura-based
In addition to carcinomatous meningitis, many mass, like meningioma. The lesions of neurosar-
infectious or inﬂammatory diseases can have focal coidosis are hyperattenuating on CT scans and
nodular meningeal involvement and manifest as a isointense to gray matter on T1-weighted images
CPA mass. Leptomeningeal metastases from lung with homogeneous enhancement; however, the
cancer, breast cancer, melanoma, or other neo- possible low signal intensity on T2-weighted im-
ages may suggest the diagnosis (34).
RG f Volume 21 ● Number 2 Bonneville et al 427
Figure 10. Metastases in a 67-year-old man with lung cancer and right-sided hypoacusia, vertigo, and ear
pain. (a) Axial T2-weighted MR image shows a metastasis of the right CPA that mimics a vestibular schwan-
noma but with unusual associated middle ear retention ( ). (b) Contrast-enhanced axial T1-weighted MR im-
age shows intense enhancement of the lesion, which extends into the cochlea (arrow). Note the presence of an-
other enhancing lesion at the tip of the right petrous bone (arrowhead).
MR Imaging Characteristics and Suggestive Features of Unusual CPA Lesions Arising from the
Appearance by Image Type
Lesion T1-weighted T2-weighted Enhancement Suggestive Features
Cholesterol granuloma Hyperintense Hyperintense No Hypointense rim on T1- and
Paraganglioma Hypointense Hyperintense Yes Salt-and-pepper appearance
Chondroma Hypointense Hyperintense Variable Origin from a synchondrosis
Chordoma Hypointense Hyperintense Yes Intratumoral septa
Endolymphatic sac tumor Variable Hyperintense Yes Hyperintense cysts on T1- and
Pituitary adenoma Hypointense Hyperintense Yes Clinical history of pituitary
Apex petrositis Hypointense Hyperintense Yes History of otitis media
Tumors Originating from the expand in the posterior cranial fossa and produce
Skull Base with CPA Impingement nerve disturbances (35). At CT, cholesterol
Lesions originating from the various structures of granulomas appear as sharply and smoothly mar-
the skull base surrounding the posterior cranial ginated expansile lesions in the temporal bone,
fossa, especially the temporal bone, can spread isoattenuating with brain tissue and nonenhanc-
into or even mainly develop in the CPA. Usually, ing (36). At MR imaging, they characteristically
smooth bone erosion or even extensive destruc- have a large central region of increased signal in-
tion is seen around the site of origin of the lesion. tensity and a thin peripheral rim of decreased sig-
Such extradural masses, such as extraaxial le- nal intensity on both T1- and T2-weighted im-
sions, narrow the cisterns but displace neurovas- ages (Table 2, Fig 11) (37). The latter ﬁnding
cular structures without invading them. corresponds to expanded cortical bone and he-
mosiderin deposits. Spontaneous and homoge-
Cholesterol Granuloma neous central high signal intensity on T1-
Cholesterol granulomas can occur in any ob- weighted images is very suggestive of a choles-
structed air cells. They often arise from the apex terol granuloma.
of the petrous bone and may enlarge enough to
428 March-April 2001 RG f Volume 21 ● Number 2
Figure 11. Cholesterol granuloma in a 32-year-
old man with right trigeminal neuralgia. (a) Axial
T1-weighted MR image shows a cholesterol granu-
loma at the apex of the right petrous bone with typi-
cal high signal intensity. An additional suggestive
feature is the thin hypointense rim (arrowheads),
which represents expanded cortical bone of the pe-
trous apex. (b) Axial T2-weighted MR image shows
that the granuloma has heterogeneous signal inten-
sity surrounded by a hypointense rim (arrowheads).
(c) Contrast-enhanced coronal T1-weighted MR
image shows the normal right trigeminal nerve (ar-
row) at the top of the mass.
Paragangliomas (glomus tumors) that invade the
CPA arise from minute bodies present in the
jugular foramen along the vagus nerve (glomus
jugulare tumor) or along the Jacobson nerve on
the promontory of the middle ear (glomus tym-
panicum tumor) (38). These benign but locally
aggressive tumors can destroy the petrous bone ing soft-tissue masses associated with moth-eaten
and enlarge enough to invade the CPA. At CT, erosion of the bony margins of the site of tumor
paragangliomas appear as well-deﬁned, enhanc- origin (ie, jugular foramen or promontory). At
MR imaging, paragangliomas appear as soft-tis-
sue masses with hypervascularity; they demon-
strate punctate and serpentine signal voids pro-
RG f Volume 21 ● Number 2 Bonneville et al 429
Figure 12. Paraganglioma in a 54-year-old
woman with right facial nerve palsy, vertigo, and
tinnitus. (a) Axial T2-weighted MR image shows a
huge paraganglioma destroying the petrous bone
and invading the right CPA. Massive ﬂow voids (ar-
rowheads) reﬂect the hypervascularity of the lesion.
Note the thin layer of trapped CSF (arrow) between
the mass and the brainstem, which indicates an ex-
traaxial origin. (b) Axial T1-weighted MR image
shows the suggestive salt-and-pepper appearance of
the paraganglioma. (c) Contrast-enhanced axial
T1-weighted MR image shows intense enhance-
ment of the lesion along with unusual dural tail en-
hancement of the meninges (arrows).
the skull base. They often arise from the petro-
occipital or spheno-occipital synchondrosis and
destroy the adjacent bones (Fig 13) (40). Chon-
dromatous tumors can be hypoattenuating at CT,
possibly with a marginal high-attenuation area
due to a dense matrix of hyaline cartilage or mas-
duced by high-ﬂow blood vessels. In addition, sive calciﬁcation. Lytic bone erosion may be seen.
focal intratumoral hemorrhages with methemo- At MR imaging, the tumor is hypointense on T1-
globin appear as high signal intensity on T1- weighted images and heterogeneously hyperin-
weighted images, producing a characteristic salt- tense on T2-weighted images; it enhances poorly
and-pepper appearance (Fig 12) (39). The tumor due to its hypovascularity (41).
enhances intensively after contrast material ad-
Chondroma and Chondrosarcoma
Chondromatous tumors develop from embryonic
cartilaginous remnants enclosed in the bones of
430 March-April 2001 RG f Volume 21 ● Number 2
Figure 13. Chondrosarcoma in a 25-year-old woman with intracranial hypertension. (a) Axial
T2-weighted MR image shows a well-deﬁned chondrosarcoma with marked mass effect on the
pons and fourth ventricle. (b) Axial T1-weighted MR image shows a skull base pedicle. (c) Con-
trast-enhanced coronal T1-weighted MR image shows punctate enhancement, which could sug-
gest a chondromatous lesion. (d) Axial CT scan shows an eroded petro-occipital synchondrosis
( ), which reﬂects the cartilaginous origin of the chondrosarcoma, although no calciﬁcations are
Chordoma (Fig 14). The overall appearance can be quite
Chordomas develop from remnants of the noto- similar to that of chondroma.
chord and are located near the dorsum sellae,
from which they can expand into the CPA (42). Endolymphatic Sac Tumor
At CT, chordomas appear hypoattenuating with Endolymphatic sac tumors are papillary adeno-
possible slight calciﬁcation at the periphery of the matous tumors that originate from the endolym-
tumor in association with irregular bone erosion. phatic sac, which is located in the distal portion of
At MR imaging, especially on T2-weighted im- the vestibular aqueduct of the petrous bone (44).
ages, chordomas usually appear as lobulated, These tumors occur sporadically but are frequent
large, hyperintense masses with septa of low sig- in von Hippel–Lindau disease (45). At CT, the
nal intensity (43). Slight enhancement is present tumor destroys the retrolabyrinthine petrous bone
with geographic or moth-eaten margins, and in-
tratumoral spiculated or reticulated bone can be
seen (46). At MR imaging, endolymphatic sac
tumors appear heterogeneous on both T1- and
RG f Volume 21 ● Number 2 Bonneville et al 431
Figure 15. Endolymphatic sac tumor in a 26-year-old woman with von Hippel–Lindau disease and vertigo.
(a) Contrast-enhanced axial T1-weighted MR image shows an endolymphatic sac tumor that destroys the right
temporal bone and invades the CPA with hyperintense blood and a protein-ﬁlled cyst. (b) Axial T2-weighted
MR image shows the cystic component of the tumor more clearly ( ).
suggest the diagnosis. Heterogeneous enhance-
ment is seen after contrast material administra-
Rarely, aggressive and invasive pituitary macroad-
enomas can destroy the dorsum sellae, extend
behind the clivus (47), and invade the CPA later-
ally. Invasive pituitary adenomas appear as en-
hancing soft-tissue masses centered around the
pituitary fossa, which erode adjacent bony struc-
tures and invade surrounding normal structures.
Rare metastases from pituitary adenoma can also
be encountered in the CPA (48). In both situa-
tions, a history of pituitary adenoma may be
present and is helpful in making the diagnosis.
Figure 14. Chordoma in a 61-year-old man with left
trigeminal neuralgia and headaches. Contrast-en- Apex Petrositis
hanced axial T1-weighted MR image shows a chor- Apex petrositis (petrous apicitis) involves an in-
doma invading the left CPA with unusual sparing of ﬂammatory lesion of the petrous apex secondary
the clivus. There are suggestive enhanced septa (arrow-
to otitis media (49). In addition to ear pain, pa-
tients experience ﬁfth and sixth cranial nerve
palsy; this clinical complex constitutes Gradenigo
T2-weighted images with focal high signal inten- syndrome (50). CT scans show a destructive le-
sity due to subacute hemorrhages (Fig 15) and sion of the petrous apex, often in association with
low signal intensity due to calciﬁcation or hemo-
siderin. Blood-ﬁlled cysts and protein-ﬁlled cysts,
both of which appear hyperintense on T1- and
T2-weighted images, may be present (46) and
432 March-April 2001 RG f Volume 21 ● Number 2
Figure 16. Apex petrositis in a 50-year-old woman with Gradenigo syndrome at clinical evalua-
tion. (a) Axial T1-weighted MR image shows an irregular lesion at the tip of the petrous apex (ar-
row). (b) Contrast-enhanced axial T1-weighted MR image shows right-sided apex petrositis as an
enhancing lesion along the courses of cranial nerves V and VI (arrow).
ﬂuid in the middle ear. MR imaging can show an
enhancing mass at the petrous tip (Fig 16) and
extension of this mass, as well as its relationship
to cranial nerves V and VI (51).
with CPA Impingement
Sometimes, intraaxial or intraventricular tumors
can be pedunculated or large enough to invade
the CPA or to manifest as a CPA mass. In these
cases, the intraaxial origin may be almost impos-
sible to conﬁrm. Diagnosis is difﬁcult, but subtle
signs like narrowing of the cisterns, irregularity of
the tumor-brain interface, and edema are helpful,
along with the site of origin and the age and clini-
cal history of the patient.
Pedunculated Brainstem Glioma
In young adults, brainstem gliomas can manifest Figure 17. Brainstem glioma in a 23-year-old man
as asymmetric expansion of the brainstem with a with vertigo and hypoacusia. Contrast-enhanced axial
possible pedicle into the CPA and even then can T1-weighted MR image shows an unusual round grade
mimic an acoustic neuroma (Fig 17) (52,53). III glioma located in front of the porus. The tumor
Gliomas appear as hypoattenuating masses at CT demonstrates central enhancement.
with variable enhancement depending on the
glioma grade. At MR imaging, T1-weighted im- plexus and recapitulate the structure of normal
ages show hypointense masses, whereas T2- choroid plexus when benign (54). Therefore,
weighted images show hyperintense tumors papillomas can be encountered wherever choroid
(Table 3) as well as hyperintense adjacent edema. plexus is. In adults, choroid plexus papillomas
In addition, the exophytic component in the cer- often arise in the fourth ventricle and extend to
ebellopontine cistern is much better demon- the CPA through the foramen of Luschka, but
strated than at CT. they can also primarily develop in the CPA (55).
At CT, these tumors are frequently hyperattenu-
Choroid Plexus Papilloma ating on nonenhanced scans with a possible cyst
Choroid plexus papillomas occur mainly in chil- and calciﬁcation. MR imaging shows an isoin-
dren but also occur in adults. These tumors de- tense, irregular, but homogeneous mass that
rive from the neuroepithelial cells of the choroid strongly enhances after injection of gadolinium
contrast material (Fig 18) and is often associated
RG f Volume 21 ● Number 2 Bonneville et al 433
Figure 18. Choroid plexus papilloma in a 49-year-old woman with vertigo and intracranial hy-
pertension. (a) Axial T2-weighted MR image shows a right CPA papilloma extending through the
foramen of Luschka. The tumor contains massive hypointense calciﬁcation (arrowhead). (b) Con-
trast-enhanced axial T1-weighted MR image shows intense enhancement of the hypervascularized
tumor. Note the normal choroid plexus in the left foramen of Luschka.
MR Imaging Characteristics and Suggestive Features of Intraaxial or Intraventricular Lesions
Invading the CPA
Appearance by Image Type
Lesion T1-weighted T2-weighted Enhancement Suggestive Features
Glioma Hypointense Hyperintense Variable Arises from brainstem
Choroid plexus papilloma Hypointense Hyperintense Intense Extends from foramen of Luschka
Lymphoma Hypointense Hyperintense Yes Edema, immunodeﬁciency
Hemangioblastoma Hypointense Hyperintense Intense von Hippel–Lindau disease, possible
Ependymoma Hypointense Hyperintense Yes Irregular, heterogeneous
Medulloblastoma Hypointense Hyperintense Yes Arises from cerebellar hemisphere
Dysembryoplastic neuro- Hypointense Hyperintense Yes Cystic component, bone erosion
with hydrocephalus. The hydrocephalus is ex- ing the CPA have no speciﬁc imaging features;
plained in part by CSF hypersecretion by the tu- they are hypointense on T1-weighted images and
mor (56). There is usually no edema in the brain- hyperintense on T2-weighted images and en-
stem and no erosion of adjacent bony structures. hance after contrast agent injection. Nevertheless,
mass effect and edema are also present and may
Lymphoma suggest the intraaxial origin (Fig 19) and there-
Lymphomas can be observed in the posterior cra- fore the correct diagnosis, especially in an immu-
nial fossa and even partly in the CPA (57). Lym- nodeﬁcient patient.
phomas appear as isoattenuating or hyperattenu-
ating homogeneous masses on CT scans with in-
tense enhancement after contrast agent admin-
istration (58). At MR imaging, lymphomas invad-
434 March-April 2001 RG f Volume 21 ● Number 2
Figure 19. Lymphoma in a 34-year-old man with acquired immunodeﬁciency syndrome, vertigo, and head-
aches. (a) Contrast-enhanced axial T1-weighted MR image shows a round lymphoma mimicking a vestibular
schwannoma in front of the right porus. (b) Axial T2-weighted MR image shows narrowing of the cisterns and
extensive edema, which suggest an intraaxial tumor.
Figure 20. Hemangioblastoma in a 28-year-old
woman with von Hippel–Lindau disease and vertigo.
(a) Axial T2-weighted MR image shows a solid he-
mangioblastoma in the left CPA. Note the vascular
pedicle (arrowhead), which appears as a ﬂow void
with all sequences. (b) Contrast-enhanced axial
T1-weighted MR image shows homogeneous en-
hancement of the hemangioblastoma (arrowhead).
(c) Contrast-enhanced sagittal MR image shows a
spotty, solid intraaxial hemangioblastoma (arrow)
beneath the ﬁrst one.
RG f Volume 21 ● Number 2 Bonneville et al 435
Figure 21. Ependymoma in a 24-year-old woman with vertigo, headaches, and left facial
nerve palsy. (a) Contrast-enhanced axial T1-weighted MR image shows a heterogeneous
ependymoma with a lobulated multicystic component in the left CPA. The tumor invades
the internal auditory canal without widening the porus (arrow). (b) Coronal T2-weighted
MR image shows marked mass effect of the ependymoma on the brainstem.
Hemangioblastoma Ependymomas appear as irregular, lobulated tu-
Hemangioblastomas are frequent in von Hippel– mors that can invade the cerebellar parenchyma.
Lindau disease, but they may also occur in the They are hypointense on T1-weighted images
absence of that disease in the posterior cranial and hyperintense on T2-weighted images and
fossa of young adults, where they rarely reach demonstrate irregular enhancement. Ependymo-
the cerebellar surface and invade the CPA. mas are markedly heterogeneous due to calciﬁca-
Hemangioblastoma usually appears as a large, tion, hemorrhage, cystic components, or necrosis
well-circumscribed, homogeneous cystic mass (62).
with a small hypervascular mural nodule, but it
can also appear as a solid mass without a cystic Medulloblastoma
component (59). The nodule appears hypoin- Adult cerebellar medulloblastomas mostly occur
tense on T1-weighted images and hyperintense during the 3rd or 4th decade of life (63). They ap-
on T2-weighted images and enhances intensely pear as round or ovoid tumors with smooth mar-
after injection of gadolinium contrast material gins, are more common in the cerebellar hemi-
(60). Because of the hypervascularity, possible sphere than in the vermis, and often extend to the
high-ﬂow vessels can be observed as ﬂow voids at brain surface with possible exophytic invasion of the
the periphery of the mass (Fig 20). When present, CPA or internal auditory canal. Irregularity of some
cysts usually have smooth margins and homoge- portions of the tumor-brain interface is a clue to
neous CSF-like signal intensity. Finally, edema is their intraaxial origin (64). At nonenhanced CT,
usually slight or absent around this intraaxial tu- medulloblastomas are frequently hyperattenuating
mor (3). and uncommonly demonstrate calciﬁcation (65). At
MR imaging, these tumors are hypointense on T1-
Ependymoma weighted images and iso- or hyperintense on T2-
Ependymomas of the posterior cranial fossa are weighted images with possible cystic or necrotic
more frequent in children but can also occur in components and enhance after injection of gadolin-
young adults. Such lesions usually arise in the ium contrast material (66).
fourth ventricle and its lateral recesses and may
extend into the CPA by means of an exophytic
component (Fig 21). However, an extraaxial ori-
gin directly in the CPA is also possible (61).
436 March-April 2001 RG f Volume 21 ● Number 2
Figure 22. Dysembryoplastic neuroepithelial tumor in a 39-year-old man with mild, long-
lasting headaches. (a) Axial T2-weighted MR image shows a large dysembryoplastic neuro-
epithelial tumor of the right CPA with a cystic component ( ) associated with a possibly sug-
gestive bone erosion (arrow). Note the lack of edema. (b) Contrast-enhanced axial T1-
weighted MR image shows enhancing hamartomatous tissue ( ) and possibly suggestive
slight mass effect with a normal left cerebellopontine cistern (arrow).
Dysembryoplastic Therefore, this characteristic should be analyzed
Neuroepithelial Tumor ﬁrst. Attenuation at CT, signal intensity at MR
Dysembryoplastic neuroepithelial tumors arise imaging, enhancement, shape and margins, ex-
from dysembryoplastic cortex, yet have rarely tent, mass effect, and adjacent bone reaction are
been reported in the posterior cranial fossa also helpful in establishing the diagnosis.
(67,68). When such tumors occur in this location,
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