Extra axial cerebral masses and DWI

Extraaxial masses: arachnoid cyst versus epidermoid tumor. 

Conventional MR images cannot be used to reliably distinguish epidermoid tumors from arachnoid cysts; both lesions are very hypointense relative to brain parenchyma on T1-weighted MR images and very hyperintense on T2-weighted images.

Epidermoid tumors are solid masses, however, which demonstrate ADCs similar to those of gray matter and lower than those of CSF. With the combination of T2 and diffusion effects, epidermoid tumors are markedly hyperintense compared with CSF and brain tissue on diffusion MR images.

Conversely, arachnoid cysts are fluid filled, demonstrate very high ADCs, and appear similar to CSF on DW MR images. 

Furthermore, on conventional MR images obtained after resection of an epidermoid tumor, the resection cavity and residual tumor may be similarly hypointense on T1-weighted images and hyperintense on T2-weighted images. On DW MR images, the hypointense CSF-containing cavity can easily be differentiated from the residual hyperintense epidermoid tumor.

Postoperative residual epidermoid tumor. The patient underwent resection of a large left middle cranial fossa epidermoid tumor that extended into the posterior fossa. Transverse T1-weighted (left) (650/16) and fast spin echo T2-weighted (middle) (4,000/104). MR images do not allow clear differentiation of residual mass from the resection cavity. Right: Transverse DW MR image (6,000/108) clearly demonstrates a hyperintense mass (black arrow) adjacent to the left pons and a smaller amount of mass (white arrow) in the left middle cranial fossa, consistent with residual epidermoid tumor. CSF (arrowhead) in the resection cavity is markedly hypointense.

Summary:

-Both epidermoid tumor and arachnoid cyst appear hypo-intense in T1 and hyper-intense in T2.

-In case of epidermoid tumor, it appears in DWI hyper-intense if compared with cerebro-spinal fluid.

-In case of arachnoid cyst, it appears in DWI similar to cerebro-spinal fluid.