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epidermoid tumors ( ets ) are extra - axial lesions observed anywhere along the neuraxis and constitute up to 1.8% of all cerebral neoplasms .
skull base is the most common location of et , mainly involving the cerebellopontine angle and parasellar regions .
we describe the clinical imaging characteristics of a unique et , primarily arising from and exclusively located in the jugular foramen ( jf ) and having the typical bright appearance on diffusion - weighted magnetic resonance imaging ( dwi ) .
primary et of the jf is also known as a congenital cholesteatoma . although jf tumors may lead to lower cranial neuropathies , in some cases they present with few signs and symptoms due to compensation by the cranial nerves on the contralateral side .
a 45-year - old male patient was seen in the ear , nose and throat surgery outpatient department of our hospital with the complaints of gradually progressive tinnitus and vertigo since 6 months .
his previous clinical history was noncontributory and had no signs or symptoms indicative of lower cranial neuropathy .
contrast - computed tomography ( ct ) scan of the temporal bones was performed , which revealed a 3.5 cm  2.2 cm nonenhanc ing cystic density lesion involving the right jf resulting in expansion and thinning of jfs roof .
no intralesional calcifications or erosions of the adjacent bone were noted [ figure 1a and b ] .
further evaluation was performed with magnetic resonance imaging ( mri ) , which indicated an oval lobulated nonenhancing lesion causing expansion of the jf without any bone erosions [ figure 2a  e ] .
the lesion revealed heterogeneous signal intensity on fluid - attenuated inversion - recovery ( flair ) images .
it was hyperintense on t2-weighted and hypointense on t1-weighted images and showed very bright signal intensity on dwi [ figure 3 ] .
diagnosis of a primary et of the jf was hence made based on characteristic imaging features , including a very bright appearance on dwi .
the lesion was unchanged in size and appearance during a follow - up mri examination 6 months after diagnosis .
axial unenhanced computed tomography ( ct ) scan of the temporal bone  bone - window - ( a ) and coronal reformatted ct image of the skull base ( b ) in the same patient , showing cystic density lesion causing enlargement of the right jugular foramen ( arrows ) axial t2-weighted images ( a ) , axial fluid - attenuated inversion - recovery ( flair ) image ( b ) , coronal t2-weighted image ( c ) , coronal precontrast t1-weighted image ( d ) and coronal postcontrast t1-weighted image ( e ) showing nonenhancing cystic intensity lesion in the right jugular foramen ( arrows ) .
note heterogeneous signal intensity on flair image ( arrow ) axial diffusion - weighted image shows hyperintense signal of the right jugular foramen lesion ( arrow ) , suggesting restricted diffusion within the lesion
differential diagnosis of jf tumors includes schwannomas , meningiomas , arachnoid cysts , chordomas , chondrosarcomas , dermoids , epidermoids , and metastatic lesions . presenting symptoms of patients with jf tumors include pulsatile tinnitus , reduced hearing , aural region fullness , dysphagia , and hoarseness . in our case ,
the patient had progressive vertigo and dizziness , which can be better explained by compressive neuropathy of the vestibulocochlear nerve .
both glomus jugulare and glomus tympanicum paragangliomas originate in the temporal bone , the former originate from paraganglia associated with cranial nerves ix , x , and xi in the jf and the latter originate from paraganglia on the middle ear promontory .
schwannomas originate from the nerve sheath schwann cells and meningiomas from the jugular bulb arachnoid cells .
pseudomass includes normal vascular asymmetry , a high jugular bulb , a jugular diverticulum , or jugular bulb flow variants ( wherein slow venous flow may be mistaken for contrast - enhancement ) .
however , in these cases the bone - windowed ct images will show a normal jf bone structure , flow - sensitive mri would confirm the vascular anatomic variant , and t2-weighted mri will usually show a hyper - intense jf .
this process likely occurs between the 3 and the 5 weeks of embryonic life when the optic and the otic vesicles are developing .
this explains the common occurrence of ets in the parasellar and the cerebellopontine angle regions .
on histopathological examination , the et demonstrate a wall made of stratified squamous epithelium supported by collagen from the outside . repeated desquamation inside the lumen of the et gives rise to lamellations .
the internal contents are mostly soft , white and cheesy and are made up of keratinous debris with abundance of cholesterol crystals . in some cases , the internal contents may be thick , brown and viscid
characteristic ct features of et consist of a cystic ( like cerebrospinal fluid [ csf ] ) density rounded or a lobulated space occupying lesion , which may have characteristic serrated margins .
dense epidermoid is a rare variant of et , which appears homogeneously hyper - dense on ct scans .
this is thought to be due either to high protein content or saponification of debris within the cyst to soaps of calcium .
calcification within the ets may also be due to saponification of the desquamated cyst debris .
typical mri features of et include the hypointense appearance on t1-weighted images , hyperintense appearance on t2-weighted images and heterogeneous internal contents ( in almost 65% of cases ) in the flair and proton - density ( pd)-weighted images .
heterogeneous appearance on flair and pd - weighted images helps in distinguishing et from the closely mimicking arachnoid cysts .
mri may also show insinuation of the cystic lesion into the adjacent cisterns or fissures and serrated margins of the cyst , features that are not associated with arachnoid cysts .
when contrast enhancement is present ( 35% of cases ) , it is minimal and peripheral . in our case
et appears significantly more hyperintense than csf and cerebral white matter on t2-weighted images due to an amalgamation of t2 and diffusion effects .
apparent diffusion coefficient ( adc ) of et is more or less same as that of grey matter and less than that of csf .
arachnoid cysts and other cystic lesions in the cranium , on the other hand , do not show restricted diffusion , and have signal on dwi and adc maps similar to that of csf . in this case , the et showed a very bright signal on dwi , indicative of restricted diffusion .
shine - through is defined as significant dwi hyperintensity that is not because of restricted diffusion , but due to increased t2 signal which  shines - through  to the dwi image .
extended t2 decay time in few of the normal tissues leads to t2 shine - through .
this is commonly seen in intracranial sub - acute infarcts having vasogenic edema but can also be seen in other pathologies such as epidermoid cysts .
for confirmation of real restricted diffusion , comparison may be made between the dwi image and the adc . in cases of real restricted diffusion ,
the area of high signal on dwi will appear as a low signal on adc .
adc is a value that computes the effect of diffusion independent of the influence of t2 shine - through .
adc maps thus reveal restricted diffusion , such as in ischemic injury , as low signal lesions as compared to normal brain tissue .
however , the t2 shine - through appears as normal or high signal on adc .
a previous study on ets found very high dwi signal intensity and adcs that are higher ( and not lower ) than that of the cerebral tissue , suggesting that the t2 properties ( also called as t2 shine - through ) dominated the contributions to the dwi signal intensity and even overwhelmed the effect of signal attenuation resulting from the increase in adc .
they also mentioned that there were no examples of intracranial tumors with t2 shine - through phenomenon showing such intense contrast between the lesion and brain as the ets .
this indicates that dwi has significant diagnostic value in imaging of intracranial ets and correlation with adc maps may not be useful .
this is the first report of primary et of jf showing the characteristic dwi appearance .
the other differential considerations of cystic et of jf include arachnoid cyst and other cystic tumors , which have not been described to appear bright on dwi .
when dwi is not available , the mri features are not specific , and these jf lesions can be mistaken for arachnoid cysts or other cystic tumors . to conclude , we present ct and mri features of a very rare case of a primary et in an unusual location ( jf ) , appearing characteristically bright on dwi , feature that helped differentiate it from other jf mass lesions .
we present computed tomography ( ct ) and magnetic resonance imaging ( mri ) features of a very rare case of a primary epidermoid tumor of the jugular foramen ( jf ) . a 45-year - old male patient presented with gradually progressive vertigo and tinnitus . 
 ct and mri scans revealed a 3.5 cm right - sided jf tumor with characteristic bright signal ( restricted diffusion ) on diffusion - weighted mri ( dwi ) . 
 dwi may be useful in accurately differentiating the lesion from other cystic neoplasms of the jf . 
 we describe the imaging features of intracranial epidermoid and jf tumors and discuss its differential diagnosis .