1. ABSTRACT ID: IRIA - 1044

2.  51 year old male came with complaints of chronic headache.  He had 3 episodes of generalized seizures.  There is weakness of all four limbs.  There is no history of fever, trauma.  Blood parameters were normal.  Patient was referred for MRI brain.

3. MR imaging of the brain revealed an extra-axial, lobulated ill defined mass occupying the fourth ventricle with extension into CP angle which is hypo intense on T1 & hyper intense on T2 insinuating the vessels around CP angle with no evidence 0f blooming on gradient imaging with no enhancement in contrast.

4. The Fourth ventricle mass which is iso intense to CSF in T1 & T2 is appearing of mixed intensity in FLAIR sequences (not suppressed completely). The mass is causing mild dilatation of lateral ventricles.

5.  The lesion had homogeneous CSF intensity on both T1- weighted and T2-weighted images and remained hypo intense on FLAIR sequences.  There is no evidence of any altered signal intensity within the mass lesion.

6.  Epidermoid cyst, or pearly tumour, is congenital in origin and accounts for about 1% of intracranial tumours.  It is a benign extra cerebral intradural lesion and in about 40% of cases is located in the cerebellopontine angle.  They are believed to form between the 3rd and 5th week of embryonic development as a result of displaced epithelial remnants that remain after the neural tube closes.

7.  Although acquired epidermoid tumours may develop as a result of trauma, this is uncommon in the brain.  Grossly, epidermoid tumors are typically well defined lesions with an irregular nodular outer surface and a shiny “mother of pearl” appearance.  The cyst content is derived from desquamated epithelial cells composed mainly of keratin in concentric layers and cholesterol in a solid crystalline state.

8.  On MR imaging, epidermoid tumours typically have low signal intensity on T1-weighted images, high signal intensity on T2- weighted images, and no enhancement on gadolinium-enhanced images.  Epidermoid tumors showing unusual signal intensity changes have been reported.

9.  The tumour can be a so-called white epidermoid with short T1 values and fatty attenuation on CT, characterized surgically as being cystic and having a high lipid content comprising mixed triglycerides containing unsaturated fatty acid residues, and no cholesterol.  It can show hyper intensity on T1- and T2-weighted MR images, caused by a semi liquid cystic content with high protein concentration.

10.  The intra cystic haemorrhage can cause a high signal intensity on both T1- and T2-weighted images because of the paramagnetic effect of heme iron (Fe3) in methemoglobin and other haemoglobin breakdown products.

11.  CSF collections, e.g. Arachnoid cyst or mega cisterna magna  Less lobulated follows CSF on all sequences, including FLAIR and DWI  Dermoid cyst  Often fat density due to sebum  Inflammatory cyst, e.g. Neurocysticercosis  Smaller, but may be multiple  May enhance peripherally  May have associated oedema  Usually no restricted disffusion  Cystic tumour, e.g. Acoustic schwannoma or craniopharyngioma  Solid enhancing component is usually identifiable  Neurenteric cyst

12.  Epidermoid cysts typically show undulating margins and model their shape to conform to the cerebropontine angle.  The cyst has a tendency to insinuate itself around the nerves and blood vessels in the cerebropontine angle.  They usually do not enhance with gadolinium and do not bleed.  From a practical point of view, recognition of case as a hemorrhagic epidermoid cyst is important.

13.  Mohanty A, Venkatrama SK, Rao BR, et al. Experience with cerebellopontine angle epidermoids. Neurosurgery 1997;40:24–30 .Tampieri D, Melanson D, Ethier R. MR imaging of epidermoid cysts. AJNR Am J Neuroradiol 1989;10:351–56  Gao PY, Osborn AG, Smirniotopoulos JG, et al. Radiologic-pathologic correlation: epidermoid tumor of the cerebellopontine angle.AJNRAmJ Neuroradiol 1992;13:865–72  Salazar J, Vaquero J, Saucedo G, et al. Posterior fossa epidermoid cysts. Acta Neurichir 1987;85:34–39  de Souza CE, deSouza R, Costa SD, et al. Cerebellopontine angle epidermoid cysts: a report on 30 cases. J Neurol Neurosurg Psychiatry 1989;52:986–90  Ochi M, Hayashi K, Hayashi T, et al. Unusual CT and MR appearance of an epidermoid tumor of the cerebellopontine angle. AJNR Am J Neuroradiol 1998;19:1113–15