Epidermoid

T2 weighted axial image of the 22 year old gentle man showing well defined hyperintense mass lesion in the basal cisterns on right side causing significant mass effect on the temporal horn of ipsilateral lateral ventricle.

T1 weighted axial image showing the lesion is hypointense to gray matter.

FLAIR axial image the lesion is showing mixed signal intensity predominantly hypointense.

Coronal T2 WI showing the lesion.

Axial B1000 Diffusion weighted image showing moderate restriction.

ADC image showing hypointense mass, which differentiates epidermoid from arachnoid cyst where the lesion will be hyperintense on ADC images.

Discussion:

Epidermoid cysts (sebaceous cysts) are benign congenital lesions of ectodermal origin. They account for approximately 1% of all intracranial tumors. Although these lesions are congenital, patients are usually not symptomatic until they are aged 20-40 years.

CT scan:

• Low attenuation - similar to CSF density.
• No enhancement.
• Calcification in 15-20%.
• Rarely they can be hyperdense secondary to high density.

MRI:

• Isointense to CSF on T1 and T2 weighted images. T1 signal intensity varies with the amount of lipid content.
• May be hyperintense on FLAIR images.
• High signal on diffusion weighted images which differentiate from the low signal arachnoid cyst.
• Minimal rim enhancement on post contrast images.

Arachnoid cyst

• CSF filled/signal
• Middle cranial fossa (MC) > cerebral convexities, basal cisterns, retrocerebellar region
• Hypogenesis of adjacent brain
• No enhancement
• Low on DWI (like ventricle).