Demonstrated and illustrated the different lesions of the cerebellopontine angle and shown a practical algorithm for their differentiation.

I. Extra-axial lesions

• Arachnoid Cyst

It is a benign, round/ovoid extraaxial cyst that follows CSF signal. They do not communicate with the ventricular system.

MR appearance

On imaging, they are characterised as well circumscribed cysts, with an imperceptible wall, displacing adjacent structures, and following CSF signal intensity on all pulse sequences (Figure 1).

• Cholesterol granulomas

Cholesterol granulomas represent the most common cystic lesion of the petrous apex.

MR appearance

Overall high signal due to cholesterol component on T1 and T2 and methaemoglobin +/- low signal rim due to haemosiderin rim. It does not present central enhancement although weak peripheral enhancement after contrast administration. No diffusion restriction (Figure 2).

• Meningioma

Benign neoplasm arising from meningothelial arachnoid cells of CPA-IAC dura.

MR appearance

CPA dural-based enhancing mass with dural "tails". 3 morphologies were distinguished: "Mushroom cap" (hemispherical) with broad base towards posterior petrous wall (75%); plaque-like (en plaque) (20%); ovoid mass (5%).  Wide range of possible signals on T2 sequence - isointense or hypointense CPA mass (compared to gray matter) is most likely meningioma. On T1 sequence isointense or minimally hyperintense to gray matter. Intense homogeneous enhancement, with “dural tail” in 50–70% of cases (reaction of the dura). Calcifications may "bloom" on T2* GRE sequence (Figure 3,4).

• Vestibular schwannoma

Vestibular schwannomas are relatively common tumours that arise from the vestibulocochlear nerve (CN VIII) and represent ~80% of cerebellopontine angle (CPA) masses. Bilateral vestibular schwannomas are strongly suggestive of neurofibromatosis type 2 (Figure 5, 6, 7).

MR appearance

They can be completely intracanalicular or intracanalicular with cisternal component resulting in an ice cream cone appearance, rarely completely intra cisternal. It typically expands the IAC and causes flaring of the porus acousticus. On T1-w sequences, they can be iso- to hypointense with intense homogeneous contrast-enhancement post gadolinium infusion. On T2-w sequences, they usually become hyperintense, whereas most meningiomas remain hypo- or isointense. However, these signal intensity differents are not pathognomonic for schwannoma. In larger tumors, intratumoral degenerative changes, like hemorrhagic and cystic areas. Calcifications and hemorrhage on T2* GRE sequence are rare.

• Glomus jugulare paraganglioma

Glomus jugulare paragangliomas are rare neuroendocrine tumours arising from paraganglia, but it is the most common of the jugular fossa tumours.

MR appearance

MRI with intravenous contrast-enhancement shows an enhancing lesion usually extending from the jugular bulb to the middle ear. Posterior fossa and cervical extension are common in untreated tumors. Vascular flow voids are described with the characteristic “salt-and-pepper” appearance on T1 and T2 weighted images. The MRI depicts the tumor vascularity, extension along neural foramina and multicentricity, and should be the first imaging study when evaluating a patient with a suspected glomus jugulare. On T1 weighted images without intravenous contrast, paragangliomas can appear hypointense with a speckled appearance; however, on intravenous gadolinium-enhanced contrast sequences, pronounced enhancement is seen secondary to the hypervascular nature of the glomus jugulare (Figure 8).

• Ependymoma

Ependymomas are glial tumours with ependymal differentiation which tend to arise within or abutting the ventricular system of the brain or central canal of the spinal cord. They can also have both intraaxial and extraaxial appendages. The majority of posterior fossa ependymomas arise from the lateral recess of the fourth ventricle or midline floor of the fourth ventricle near the obex. Posterior fossa ependymomas are apt to extend through the foramina of Luschka and Magendie.

MR appearance

Ependymomas are typically heterogeneous masses with areas of necrosis, calcification, cystic change and hemorrhage frequently seen. This results in a heterogeneous appearance on all modalities. On T1-w sequences solid portions of ependymoma typically are isointense to hypointense relative to white matter. On T2-w sequences hyperintense to white matter. Calcifications and hemorrhage on T2* GRE sequence. Asymmetrical enhancement and obviously heterogeneous because of calcification, hemorrhage, cystic constituents, or necrosis (Figure 9).

• Medulloblastoma

Medulloblastomas are undifferentiated embryonic neuroepithelial tumors, that originate from primitive multipotent cerebellar cells and spread along germ cell migration tract. Overall, medulloblastomas account for 12-25% of all CNS tumors in children and 30-40% of posterior fossa tumors in children. They are also seen in adults but represent only 0.4–1.0% of adult brain tumors. Recently, advanced neuroimaging techniques have revealed the presence of extraaxial medulloblastoma (Figure 10). There are four genetic (molecular) groups of medulloblastoma: the wingless (WNT)-activated and the Sonic Hedgehog (SHH)-activated groups, as well as groups, numerically referred to as "group 3" and "group 4".

MR appearance

On T1-w sequences, they can be iso- to hypointense and hyperintense on T2-weighted MR images. A characteristic diffusion restriction is predominantly observed on MR images because of its high cellularity and a high nuclear-to-cytoplasmic ratio. On contrast-enhancement MR images overall 90% enhance, often heterogeneously. WNT-activated tumours tend to vividly enhance, but group 4 tumours tend to enhance less.

• Bell palsy

Bell palsy is characterized by rapid onset of facial palsy, often resolving within 6 to 8 weeks, with no established etiology. The classic presentation is a rapidly developing unilateral facial paralysis with lower motor neuron involvement that affects both the upper and lower facial muscles.

MR appearance

In Bell palsy, the long segments of the facial nerve are enhanced evenly linearly, more intensely than the contralateral healthy side. The facial nerve most commonly affected is on either side of the geniculate ganglion, from the distal internal auditory canal to the distal segment of the tympanic membrane. The mastoid and extratemporal segments are less commonly affected (Figure 11).

II. Intra-axial lesions

• Brainstem glioma

Infratentorial gliomas are relatively rare tumors compared to their supratentorial counterparts. Brainstem tumors are defined as lesions occurring in the midbrain, pons, or medulla oblongata. They are rare in adults and account for only 1-2% of intracranial gliomas. These can be diffuse infiltrative low-grade gliomas, enhancing malignant gliomas, focal tectal gliomas and exophytic gliomas (Figure 12,13,14).

MR appearance

The appearance depends on the type of tumor. On MRI, gliomas usually result in diffuse enlargement of the brainstem with hyperintense areas on T2/Flair-weighted sequences and hypointense signals on T1-weighted images. The diffuse variant distorts the brainstem with patchy post-contrast enhancement. Focal gliomas have an exophytic component that shows significant heterogeneous post-contrast enhancement.

• Primary CNS lymphoma

Primary CNS lymphomas are relatively rare tumors, accounting for 2.5% of all brain tumors. Typically, patients diagnosed with primary CNS lymphoma are older than 50 years of age with a short duration of symptoms. There has been a strong association with HIV/AIDS and other immunocompromised conditions.

MR appearance

On imaging, primary CNS lymphoma is characterized by T1 hypointense, T2 iso- or hypointense lesions, with restricted diffusion. Most lesions show moderate-to-marked contrast enhancement. Increasing lesions are usually found in the basal ganglia and / or periventricular white matter (Figure 15). Often the corpus callosum is involved. The brainstem or cerebellum are affected in 9%–13% and the spinal cord, in only 1%–2% of patients. Hemorrhage or internal calcification within the tumor is a rare finding.

• Metastasis

Brain metastases are the most common form of adult brain neoplasms and occur in 5%–40% of patients with metastatic cancer. Posterior fossa metastases represent 15%–25% of the distribution of brain metastases. Most commonly from primaries of lung, breast, renal, thyroid and melanoma.

MR appearance

Skull based metastasis show distinct bony lytic lesion. Intra-axial metastasis to the brainstem shows significant perilesional edema. Dural based metastasis usually presents as plaque like dural thickening, whereas leptomeningeal metastasis presents as diffuse, nodular leptomeningeal enhancement. Ofen presence of peritumoral edema. Contrast enhancement can be homogeneous, nodular, inhomogeneous or ring-like (Figure 16).