Introduction

Although vestibular schwannomas and meningiomas represent the vast majority of mass lesions in the cerebellopontine angle (CPA), a large variety of unusual lesions can also be encountered in the CPA. In part 1, we initiated the diagnosis approach in front of a CPA lesion by reviewing the different enhancing extra-axial lesions arising directly from the CPA cistern and its content. In this paper, we continue with exophytic intra-axial lesions and skull base lesions which may invade the CPA, and also describe non-enhancing extra-axial lesions. A concise algorithm is proposed to facilitate diagnosis (Fig. 1).

Fig. 1
figure 1

Drawing of a segmental approach to diagnosis of CPA lesions based on gadolinium enhancement, site of origin and key feature

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Enhancing lesions

Extra-axial lesions originating in the CPA

Extra-axial lesions are theoretically easily recognized in the CPA. They are separated from the brain parenchyma by a cleft of cerebrospinal fluid and may enlarge the cerebellopontine cistern. They also push the cranial nerves, the brain stem or the anterior aspect of the cerebellum away. Demonstration of vessels interposed between the mass lesion and the brain parenchyma is another sign of the extra-axial origin of the lesion. Such lesions mainly include schwannomas and a wide spectrum of meningeal mass lesions described in detail in part 1.

Intra-axial and intraventricular lesions

At imaging, precise anatomic landmarks are not always reliably depicted in the posterior fossa, and the intra- or extra-axial location of a lesion is not always certain, especially when the cistern itself is no longer depicted. Extensive peritumoral oedema surrounding an enhancing lesion, centred on a significant mass effect obliterating the CPA cistern is very unlikely in a benign extra-axial tumour. In this circumstance, an intra-axial tumour such as a lymphoma, a glioma or a metastasis should be suspected. The lack of clear brain-tumour interface may also favour an intra-axial origin.

Lymphoma

Primary central nervous system lymphomas may be either intra- or extra-axial in the CPA [1–3]. However, and except for the signs related to the location, imaging features are identical for both sites of origin. In immunocompetent patients, lymphomas appear with intermediate or hyperattenuation on CT with a uniform enhancement after contrast administration. At MRI, they appear with an intermediate to low signal intensity on T1-weighted images that strongly and homogeneously enhances after gadolinium administration, and a characteristic T2 low signal intensity in about 75% of the cases. This latter low signal intensity is due to the high cellularity of this tumour, which also explains the high signal intensity of lymphomas usually observed on diffusion-weighted imaging (DWI) with low apparent diffusion coefficient (ADC) values (Fig. 2) [4]. Because of the lack of neoangiogenesis and the increased permeability of the blood-brain barrier observed in lymphomas, these tumours present with rather low relative cerebral blood volume (rCBV) compared to other malignant intra-axial tumours. Indeed, their rCBV ratios are statistically lower (around 1) than those of metastases or high grade gliomas (around 5), thus providing a valuable criterion that allows differentiation among these lesions [5].

Fig. 2a, b
figure 2

Primary brain lymphoma in a 49-year-old man. a The axial gadolinium-enhanced T1-weighted MR image reveals an homogeneously enhanced right CPA lesion with intracanalicular portion that could mimic a vestibular schwannoma. b On diffusion-weighted image, the tumour appears with a high signal intensity and has restricted diffusion with a low apparent diffusion coefficient (not shown), a feature suggestive of lymphoma. Note the right cerebellar lesion also visible on a (courtesy of F. Cotton, MD, with permission from reference [4])

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In immunocompromised individuals, lymphomas present usually as multiple peripherally enhancing lesions with marked peritumoral oedema [6]. However, due to the significant overlap in findings, lymphomas cannot always be reliably distinguished from toxoplasmosis, the main differential diagnosis in this situation, based on imaging findings on conventional sequences, as well as on DWI or perfusion MR [7].

Glioma

Glial tumours of the brain stem, and especially pilocytic astrocytomas in young adults, can manifest as asymmetric expansion of the brain stem that can rarely be pedunculated and exophytic, invading the CPA (Fig. 3) [3, 8] and even mimicking a vestibular schwannoma by enlarging the porus acusticus [9]. These tumours do not have specific imaging features in this location: they appear with T2 hyperintensity, T1 hypointensity and variable enhancement depending on the glioma grade. They are usually surrounded by adjacent oedema. Diffusion and perfusion imaging of the solid portions of glial tumours also depend on their histological grade. In general, the lower the ADC value, the higher the rCBV, the higher the grade [10].

Fig. 3a, b
figure 3

Brain stem glioma in a 33-year-old woman with progressive left facial palsy, sensorineural hearing loss and dizziness. a Axial proton density-weighted image shows a high signal intensity lesion in the left CPA. b The axial infra-millimetric heavily T2-weighted image demonstrates the intra-axial location of the tumour with a faint tumour-parenchyma interface and the absence of intracanalicular component

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Metastasis

Intracranial metastases are ubiquitous. They may be extra-axial and mimic a meningioma or a schwannoma in the CPA, as previously described in detail in the first part of this review, or be intra-axial, exactly located in front of the IAC, often surrounded by peritumoral oedema (Fig. 4). Multiple lesions or past history of a known cancer will lead to the diagnosis. Additionally, advanced MR techniques are helpful and demonstrate mean rCBV ratios lower than high grade gliomas, and higher ADC values than the enhancing portion of high grade gliomas and abscesses [11, 12]. MR spectroscopy shows a predominant peak in lipids in metastasis, another important finding considered suggestive of the diagnosis [13].

Fig. 4a, b
figure 4

Metastasis from a colon cancer in a 56-year-old man with dizziness and headache. a Axial T2-weighted image demonstrates a hyperintense cystic lesion in front of the left IAC. Peritumoral oedema and lack of CPA cistern widening are more in favour of an intra-axial lesion than a schwannoma. b Short TE MR spectroscopy reveals a predominant peak in lipids, a feature suggestive of metastasis

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Hemangioblastoma

Hemangioblastomas are benign vascular intra-axial tumours preferentially located in the cerebellar hemispheres, with a possible extent into the CPA [3, 14]. They are sporadic in the vast majority of cases but are a manifestation of von Hipple-Lindau disease in 25%. This disorder is an autosomal-dominant phacomatosis leading to multiple retinal and cerebral hemangioblastomas, renal cell carcinomas and pancreatic cysts and tumours. Hemangioblastomas usually present as well-circumscribed masses with smooth margins, either entirely solid (40%) or cystic, with a hypervascular enhancing mural nodule (60%), around which oedema is usually absent. When cystic, the liquid component usually appears hypointense on T1-weighted images and hyperintense on T2-weighted images but slightly different than the CSF on fluid-attenuated inversion-recovery (FLAIR) sequence. The nodule is isodense on CT, isointense on both T1- and T2-weighted images, and enhances strongly and homogeneously after contrast media injection. Interestingly, the solid portions give low signal on DWI with increased ADC values, a finding usually not found in other cerebellar solid tumours. This finding may be explained by the rich vascular spaces present within hemangioblastomas [15]. Indeed, prominent flow voids in and/or around the solid portion of these hypervascular tumours are observed in some cases and are suggestive of the diagnosis. Perfusion MR imaging also reveals high rCBV ratios in these lesions (around 11), values significantly higher than those of metastases (around 5) [5]. The extensive hypervascular nature is also confirmed by cerebral angiography that shows dilated feeding arteries and a prolonged tumoral blush corresponding to the solid portion. Supraselective catheterization of the feeding vessels allows preoperative embolization of the tumour and can potentially decrease the morbidity and mortality of surgical resection [16].

Medulloblastoma

Medulloblastomas are primary neuroepithelial tumours that mainly occur midline in the posterior fossa of children. Differences in the imaging characteristics of adult medulloblastomas have been reported, including involvement of lateral cerebellar hemispheres with a possible extra-axial appearance [17], or even a primary extra-axial location mimicking either a meningioma [18, 19] or a vestibular schwannoma due to an extent into the internal auditory canal [20]. Medulloblastomas are highly cellular and consequently appear homogeneously isodense on CT, isointense or hypointense on T1- and T2-weighted images and enhance moderately after contrast administration. Irregularities at the brain-tumour interface may be a valuable clue to the intra-axial origin and therefore the diagnosis. CSF seeding frequently occurs with these particularly aggressive tumours, and should be sought with MR along the entire neuraxis before beginning the treatment. Presumably due to their high cellularity and high nuclear-to-cytoplasmic ratio, medulloblastomas show high signal intensity on DWI and have low ADCs [21]. At proton MR spectroscopy, medulloblastomas characteristically seem to show taurine, detectable at short echo time, and a massive choline peak [22].

Papilloma

Most papillomas in adults are located in the posterior fossa. Although they commonly arise from the fourth ventricle, they occasionally extend into the CPA through the foramen of Lushka [23] or primarily occur there [3, 24]. Choroid plexus papillomas, which are benign, are structurally similar to normal choroid plexus and therefore appear as calcified, vascular, enhancing masses at CT, with possible intratumoral cyst [25]. At MRI, papillomas appear either as homogeneous or heterogeneous cauliflower-like tumours. They are mainly iso/hypointense on T1- and T2-weighted images and strongly enhance after contrast injection, unless the tumour is highly calcified. They may also contain areas of low signal intensity corresponding to calcifications, possible foci of high signal intensity due to intratumoral hemorrhage and flow voids when high flow vessels feed the tumour (Fig. 5) [26]. Cerebral digital subtraction angiograms reveal these enlarged arteries that demonstrate a prolonged vascular blush and intratumoral arteriovenous shunting, thus potentially mimicking an hemangioblastoma in this location [27]. Finally, hydrocephalus is often associated with choroid plexus papillomas; it may be explained in part by CSF hypersecretion by the tumour, but also by fourth ventricle obstruction when the tumour is located in the posterior fossa [28].

Fig. 5a, b
figure 5

Papilloma in a 28-year-old man with vertigo and intracranial hypertension syndrome. a Axial T2-weighted image reveals a large intraventricular lesion located at the bottom of the fourth ventricle extending in the right CPA through the right foramen of Lushka. The multiple flow voids observed in and around the lesion are in favour of a hypervascular lesion, as confirmed by b, a digital subtracted angiogram, with a feature suggestive of papilloma in this location

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Ependymoma

Ependymomas are ubiquitous along the neuroaxis and may be either spinal, supratentorial or infratentorial, with a predilection for the fourth ventricle in the latter location [29]. More frequently than papillomas, ependymomas extend into the CPA by means of an exophytic component coming from the fourth ventricle through the foramen of Lushka, a pattern very suggestive of the diagnosis. Also, they arise directly within the CPA [30]. Ependymomas appear as irregular and lobulated isodense masses with common calcification at CT. At MRI, they appear heterogeneous with T1 hypointensity, T2 iso/hyperintensity and heterogeneous enhancement. They may demonstrate intratumoral microcysts, necrosis or hemorrhage [29]. Peritumoral oedema is usually absent.

Dysembryoplastic neuroepithelial tumour

Dysembryoplastic neuroepithelial tumours (DNT) have been described as clinicopathological neoplasms, usually located in the temporal lobe, associated with intractable complex partial seizure in young patients. Only rare cases have been reported in the posterior fossa [31, 32], including scattered reports in the CPA [3]. Diagnosis is based on the pathological analysis of the tumour but may be suggested in young adults who present with mild non-specific symptoms in association with a large heterogeneous lesion that may enhance. The mass may also impinge the adjacent bones if there is contact with the skull base [3]. No oedema has been reported in association with infratentorial DNTs.

Skull base lesions

A few tumours rising from the skull base may extend, partially or extensively, in the CPA. Significant bony erosion associated with the mass lesion points toward the diagnosis of this type of lesion.

Paraganglioma

Most paragangliomas located in the CPA result from the extension of paragangliomas arising at the jugular foramen (glomus jugulare tumour) or in the middle ear (glomus tympanicum tumour) [3]. Indeed, only three cases of primary paragangliomas originating from the CPA itself have been reported to date [33]. These benign but locally aggressive tumours destroy the bones of the skull base with a moth-eaten erosion pattern at CT. At MRI, paragangliomas appear as highly vascular soft tissue lesions demonstrating a mix of multiple punctuate and serpentine signal voids corresponding to high-flow intratumoral vessels and foci of high-signal intensity due to intratumoral hemorrhages with methemoglobin, producing the characteristic salt-and-pepper appearance (Fig. 6) [34]. A combination of unenhanced and contrast-enhanced 3D time-of-flight MR angiography has been proposed in addition to a standard imaging protocol to increase the detection of these intratumoral vessels [35]. Perfusion MR imaging demonstrates high vascularity patterns with high rCBV. Finally conventional angiography demonstrates an intense tumoral blush with enlarged feeding arteries, which may allow haemostatic embolisation prior to surgical resection, though it is rarely used as a primary diagnostic modality [36].

Fig. 6a–c
figure 6

Paraganglioma in a 36-year-old woman with right facial palsy, tinnitus, vertigo and sensorineural loss. a Axial T2-weighted image shows a voluminous hyperintense mass lesion widening the right CPA cistern, with multiple flow voids within and around the lesion. b Gadolinium-enhanced axial T1-weighted image with fat suppression better depicts the extent of the tumour in the skull base and the jugular foramen it comes from. c Perfusion imaging confirms the hyperperfusion of the tumour, as illustrated by the high relative cerebral blood volume observed on curve 3, compared to the normal curve 2

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Chondromatous tumours

Chondromas and chondrosarcomas develop from cartilaginous remnants enclosed in the synchondroses of the skull base. These tumours usually originate off midline, at the petro-occiptal fissure or near the jugular foramen, and they may subsequently extend into the CPA (Fig. 7) [37]. At CT, cartilaginous tumours are hypo- to isoattenuating, often contain calcifications and often destroy the adjacent bones. At MRI, the hyaline cartilaginous matrix of these tumours provides a low signal intensity on T1-weighted images and a very intense signal on T2-weighted images. The diagnosis is strongly suggested when this pattern is observed in a characteristic location. Frequently, areas of signal void consistent with calcifications are observed within the mass [38]. Chondromatous tumours usually enhance poorly due to their hypovascularity [39].

Fig. 7a, b
figure 7

Giant chondroma in a 33-year-old man with intracranial hypertension syndrome, right hemiparesia and aphasia. a Gadolinium-enhanced T1-weighted image with fat suppression demonstrates a large heterogeneous lesion of the skull base extending in the temporal fossa and the left CPA. b CT with bone algorithm better illustrates the massive intratumoral calcification, corresponding to the multiple foci of low signal intensity seen on a, and very suggestive of the chondromatous nature of the tumour

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Chordoma

Intracranial chordomas are thought to originate from embryonic remnants of the primitive notochord and are located midline, near the clivus, from which they rarely extend into the CPA [3, 40]. Chondroid chordoma is a pathological subtype of chordoma that may have a more lateral origin in the petrous bone and grow directly in the CPA [41, 42]. All kinds of chordomas have, however, very similar imaging patterns, sometimes shared with chondrosarcomas as well, except that the latter usually have a more lateral origin [43]. At CT, intracranial chordoma typically appears as a centrally located, well-circumscribed, expansile soft-tissue mass associated with extensive lytic destruction of the clivus. At MRI, on T1-weighted MR images, chordomas demonstrate intermediate to low signal intensity, with foci of T1 signal hyperintensity that may represent either residual ossified fragments of the skull base, tumour calcification, collections of proteinaceous fluid or hemorrhage [40]. On T2-weighted MR images, they demonstrate very high signal intensity and septa of low signal intensity that are considered characteristic [44]. Slight enhancement is usually observed after contrast media administration.

Endolymphatic sac tumours

Endolymphatic sac tumours are aggressive papillary adenomatous tumours that originate from the endolymphatic sac, which is located at the posterior aspect of the petrous bone. These tumours may grow large enough to extend into the CPA and eventually compress the brain stem [45]. Endolymphatic sac tumours occur sporadically or in the context of von Hipple-Lindau disease [46, 47]. Endolymphatic sac tumour is an extradural tumour that erodes and destroys the retrolabyrinthine petrous bone with geographic or moth-eaten margins at CT, and may exhibit possible calcifications [48]. At MRI, the lesion is heterogeneous on both T1- and T2-weighted images, with foci of high signal intensity due to intratumoral subacute hemorrhage. A T1- and T2-hyperintense cystic component, rich in blood and proteins, may be present and is suggestive of the diagnosis in this very specific location [3]. Notably, the cyst may be predominant and the mass itself can be almost completely cystic in appearance in some cases [46]. Finally, in masses larger than 2 cm in diameter, flow voids can be observed within and around these hypervascular tumours [48].

Non-enhancing lesions

T1 low-signal-intensity lesions

Epidermoid cyst

Epidermoid cysts are congenital lesions arising from the accidental inclusion of ectodermal epithelial tissue during neural tube closure in the first weeks of embryogenesis. About half of intracranial epidermoid cysts are located in the cerebellopontine angle, where they represent 5% of overall lesions and the third most common mass behind vestibular schwannomas and meningiomas [3]. Epidermoid cysts are lesions that grow from the slow desquamation of the stratified keratinised epithelium that lines the cyst. These malleable masses insinuate into posterior fossa cisterns, encasing cranial nerves and vessels with a specific irregular lobulated cauliflower-like outer surface [49]. Because of the relative softness of epidermoid cysts and their tendency to include rather than displace adjacent structures, clinical symptoms occur only when the masses are large. If epidermoid cysts commonly present with cranial nerve deficits and specifically trigeminal neuralgia [50], for which extension into Meckel’s cave should be meticulously sought [51], brain stem stroke due to the stretching of basilar artery branches by the lesion is very unusual [52].

At CT, epidermoid cysts appear hypoattenuating with possible marginal calcifications. At MRI, they have a fluid-like low T1 signal intensity and high T2 signal intensity, but they are slightly brighter than CSF on both T1- and T2-weighted images. They may, however, be difficult to distinguish from arachnoid cysts on these sequences, based on signal intensity alone. With advances in MRI techniques, preoperative diagnosis of epidermoid cysts, and reliable distinction of these lesions from arachnoid cysts, should no longer pose a dilemma. On FLAIR sequence, epidermoid cysts can be easily differentiated from arachnoid cysts because the former show mixed iso- to hypersignal intensities, but with poor demarcation, while the signal of the latter is suppressed, like the signal of CSF (Fig. 8) [53]. MR cisternography, by means of heavily T2-weighted 3D sequence, demonstrates an epidermoid cyst signal hypointense to CSF, reveals the lobulated margins of the tumour, and clearly depicts the anatomical relation to neighbouring nerves and vessels and its precise extent for surgical planning [53, 54]. DWI offers a finding specific for extra-axial epidermoid cysts by showing a very high signal. Restricted ADC compared to CSF, almost comparable to that of the brain, and T2 shine-through effect both play an important role in the high signal intensity of epidermoid cyst at DWI [53, 55]. DWI is also crucial in the postoperative follow-up as it allows confirmation of the presence of a possible residual tumour [56]. Finally, MR spectroscopy is also helpful as it shows only elevated lactate peaks in these tumours [57], which can be of interest in case of atypical epidermoid cysts. Indeed, in very limited cases, unusual patterns of epidermoid cysts may be observed on MRI. Such circumstances include so-called white epidermoids which have a rich protein content and appear with reversed signal intensities with homogeneous high signal intensity on T1-weighted images and low signal intensity on T2-weighted images [58], intracystic hemorrhage with heterogeneous signal intensities due to blood products [59] or malignant transformation into squamous cell carcinoma which should be considered in case of frank contrast enhancement [60].

Fig. 8a–d
figure 8

Epidermoid cyst in a 24-year-old man with cranial nerves palsies. a Sagittal T1-weighted image reveals a voluminous hypointense extra-axial lesion indenting the brain stem. b On the axial FLAIR image, the tumour appears with an heterogeneous signal intensity, different from that of cerebrospinal fluid. c Diffusion-weighted image shows high signal intensity, a feature characteristic of an epidermoid tumour in this location. d MR spectroscopy demonstrates a predominant peak in lactate, another suggestive feature of epidermoid cyst

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Arachnoid cyst

Arachnoid cysts are congenital, benign, intra-arachnoid pouch-like lesions filled with normal CSF. Their exact origin is uncertain, but they could result from a splitting of the embryonic meninges [49]. They are usually supratentorial, with about 70% being in the temporal fossa, mostly on the left side [61], anterior to the temporal poles. Only 10% of arachnoid cysts are located in the posterior fossa, where they most commonly develop in the CPA. The large majority of arachnoid cysts are asymptomatic and found incidentally at imaging, but they can compromise cranial nerve functions in the posterior fossa by stretching them. Spontaneous or traumatic intracystic hemorrhage can also complicate arachnoid cysts, though this has only rarely been described in the posterior fossa [62]. At neuroimaging, attenuation and signal intensities of uncomplicated arachnoid cysts exactly match those of CSF on all sequences, do not enhance after contrast media administration, and therefore, may mimic epidermoid cysts on conventional T1- and T2-weighted images. However, arachnoid cysts displace adjacent arteries and cranial nerves rather than encasing them, as epidermoid cysts usually do [3]. They also demonstrate rounded edges, smoothly deforming the adjacent brain or scalloping the bony structures. Additionally, the complete suppression of signal intensity on FLAIR sequence in arachnoid cysts and the lack of diffusion restriction of these lesions on DWI should eliminate epidermoid cyst as a likely differential diagnosis of arachnoid cysts (Fig. 9) [63].

Fig. 9a, b
figure 9

Arachnoid cyst in a 16-year-old woman with headache. a The axial infra-millimetric heavily T2-weighted image reveals a well-demarcated round hyperintense lesion in the left CPA smoothly eroding the skull base. b On FLAIR sequence, the arachnoid cyst signal appears completely suppressed, even more than the free CSF that pulses in front of the brain stem

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Neurocysticercosis

Neurocysticercosis, a parasitosis due to Taenia solium, may present in its racemose form with multiple non-enhancing subarachnoid cysts, sometimes located in the CPA [64]. They appear as lobulated cysts with no mural nodule, no enhancement and have signal intensity similar to that of CSF, which makes arachnoid cyst the main differential diagnosis. The lack of bone erosion adjacent to the cyst is helpful in distinguishing these entities, and diagnosis of neurocysticercosis should then be considered in a patient from an endemic area [64, 65]. On DWI, neurocysticercosis cysts have hypointense signal with an apparent diffusion coefficient similar to that of CSF. DWI may fail to detect racemose lesions [66]. As an alternative, non-invasive MR cisternography by means of FLAIR sequence acquired after a 5-min inhalation of 100% O2, which increases the signal of normal CSF, has been proposed as a robust technique to detect subarachnoid cysticercosis lesions [67].

T1 high-signal-intensity lesions

An intrinsic T1 high-signal intensity of a non-enhancing CPA mass lesion favours a fatty or high protein content. T1-weighted sequence with fat signal suppression should then be performed in order to distinguish the exact nature of the high signal intensity: if it is suppressed, the tumour contains fat and is likely to be a lipoma or a dermoid cyst, if it is unchanged, the lesion has a high protein content and may be a neurenteric cyst or a cholesterol granuloma.

Lipoma

Lipomas are benign lesions believed to result from a maldifferentiation of the primitive meninx. The majority of intracranial lipomas are located around the corpus callosum and about 100 cases have been reported in the CPA, where they encase normal adjacent neurovascular structures with very dense adhesions [68]. Intracranial lipomas may be asymptomatic, incidentally discovered on brain imaging. They can also produce symptoms by compressing the adjacent cerebral structures, such as the cranial nerves in the CPA [69]. At imaging, lipomas appear exactly the same as subcutaneous fat: homogeneously hypoattenuating on CT (except for possible superficial calcification) and with an homogeneous very high signal intensity on T1-weighted images, which decreases on fat-suppressed images, while no enhancement is observed after contrast administration [3].

Dermoid cyst

Dermoid cysts result from the congenital inclusion of cutaneous ectoderm. Intracranial dermoid cysts are intradural extra-axial lesions, predominantly supratentorial near midline [49]. Dermoid cysts are rare in the CPA, and may be secondary to the caudal extension of a parasellar lesion, the most frequent intracranial site of their occurrence [40]. They are heterogeneous masses containing a mix of fat, hair, calcifications and the products of sebaceous glands and desquamation of a keratinized epithelium. At imaging, a dermoid cyst appears as a well-circumscribed fatty round mass with a thick peripheral capsule that may enhance. In case of rupture, the visualisation of fatty T1-hyperintense droplets in the sulci or a fat-fluid level in the ventricles is highly suggestive of the diagnosis (Fig. 10).

Fig. 10a, b
figure 10

Ruptured dermoid cyst in a 50-year-old man with sudden headache. a Sagittal T1-weighted image depicts a large heterogeneously hyperintense CPA lesion and multiple subtle subarachnoid foci of high signal intensity consistent with fat droplets. b Axial FLAIR image demonstrates multiple fatty droplets in the supratentorial sulci, as well as a fat-fluid level in the right frontal horn (arrow), a feature very suggestive of the diagnosis

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Neurenteric cyst

Neurenteric cysts are congenital cystic masses lined by a mucin-producing epithelium of endodermal origin, closely resembling gastrointestinal tract mucosa. Neurenteric cysts adjacent to the central nervous system are mostly seen within the spinal canal, ventral to the spinal cord. Intracranial neurenteric cysts are very unusual, mainly located near the midline in the posterior fossa or in the CPA [70, 71]. They present with round and smooth margins. The signal intensity of the cyst depends on its protein content. It can rarely mimic CSF when this content is low [72], but it is often isointense to hyperintense relative to brain parenchyma on T1-weighted images and hyperintense on T2-weighted images when the protein concentration is high [49, 71]. Neurenteric cysts very rarely show peripheral rim enhancement [71]. Differences exist in reports of the signal intensity on DWI and ADC values of neurenteric cysts [71, 73]. However, based on the few cases of these cysts studied by DWI, it seems that the low signal intensity of neurenteric cysts on this sequence predominates (Fig. 11). This imaging feature may enable differentiation from epidermoid cysts, which may exhibit similar intensities on conventional sequences but a characteristic high signal intensity on DWI [73].

Fig. 11a–c
figure 11

Neurenteric cyst in 46-year-old man with cranial nerve palsies. a Axial FLAIR image reveals a cystic mass in the right CPA with smooth margins and an unusual homogeneous marked hyperintense signal. b Axial contrast-enhanced T1-weighted image with fat suppression only demonstrates a subtle peripheral linear discontinued enhancement at the posterior edge of this iso-signal-intensity lesion. c On diffusion-weighted image, the lesion does not show high signal intensity as epidermoid cysts do, but appears with isointensity, certainly because of a T2 shine-through effect and a high ADC (2,000 10-3 mm2/s)

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Cholesterol granuloma

Cholesterol granulomas result from the chronic obstruction of air cells and the subsequent accumulation of their secretions. In case of petrous apex origin, they can become large enough to expand in the CPA where they can compromise cranial nerves [74]. They appear as expansile lytic lesions of the temporal bone with sharp and smooth margins, demonstrating a central region of high signal intensity and a peripheral hypointense rim on both T1- and T2-weighted images, the latter corresponding to the association of the expanded cortical bone and hemosiderin deposits (Fig. 12) [3]. When desiccated, cholesterol granulomas can demonstrate areas of low signal intensity that give this homogeneous T1-hyperintense lesion an heterogeneous T2 pattern very suggestive of the diagnosis in this location [75]. Cholesterol granuloma could be classified as a skull base lesion invading the CPA, but the usual lack of enhancement after contrast enhancement and its characteristic intrinsic T1-high signal intensity, make its classification in this category more relevant.

Fig. 12a, b
figure 12

Cholesterol granuloma in a 34-year-old man with left trigeminal neuralgia. a Axial T2-weighted image reveals a petrous apex lesion with a typical high-signal-intensity core surrounded by a hypointense rim which represents expanded cortical bone and hemosiderin peripheral deposits. b Coronal fat-suppressed T1-weighted image demonstrates the persistence of the central high signal intensity on this sequence, indicative of a high protein content rather than fat, a feature suggestive of a cholesterol granuloma in this specific location

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Conclusion

A wide variety of lesions can be encountered in the CPA. A meticulous analysis of the site of origin, shape, density, signal intensities and behaviour after contrast media injection allows a systematic approach to the preoperative diagnosis in the majority of cases. Diffusion- and perfusion-weighted imaging, as well as MR spectroscopy may also provide crucial information that helps radiologists arrive at the correct diagnosis non-invasively.