AIIMS November 2011 – MCQ 94

Epidermoid cyst is differentiated from arachnoid cyst by?
A. Smooth margin
B. Contrast enhancement
C. Isointense to CSF in FLAIR
D. Restriction on diffusion weighted images

Correct answer : D. Restriction on diffusion weighted images

Both FLAIR (Fluid attenuated inversion recovery) and Diffusion weighted MRI can be used to differentiate between epidermoid cyst and arachnoid cyst.
Specific to Epidermoid cyst – Restriction in diffusion weighted images. (Arachnoid cysts do not restrict on diffusion weighted images)
Specific to Arachnoid cyst – Appears isointense to CSF in FLAIR. (Epidermoid cysts are not precisely identical in signal intensity to CSF)

The major differential consideration for the epidermoid cyst is an arachnoid cyst. Arachnoid cysts are isointense to CSF at all sequences, including FLAIR. They displace rather than invade structures such as the epidermoid. Finally, arachnoid cysts do not restrict on diffusion-weighted images. Ref: radiology.rsna.org/content/239/3/650.full

The most difficult lesion to distinguish from the arachnoid cyst is an epidermoid cyst. Epidermoid cysts can appear nearly identical to CSF on CT scans. On MR images, epidermoid cysts appear isointense to CSF, although close inspection often shows they are not precisely identical in signal intensity to CSF. Arachnoid cysts typically suppress completely on FLAIR images and do not restrict on diffusion-weighted images. Occasionally an arachnoid cyst can be slightly hyperintense on images obtained with a long repetition time and a short echo time. Arachnoid cysts displace adjacent arteries and cranial nerves rather than engulf them, as epidermoid cysts often do. Ref: radiology.rsna.org/content/239/3/650.full

FLAIR:
Fluid attenuated inversion recovery (FLAIR) is a pulse sequence used in magnetic resonance imaging which was invented by Dr. Graeme Bydder. FLAIR can be used with both three dimensional imaging (3D FLAIR) or two dimensional imaging (2D FLAIR). The pulse sequence is an inversion recovery technique that nulls fluids. For example, it can be used in brain imaging to suppress cerebrospinal fluid (CSF) effects on the image, so as to bring out the periventricular hyperintense lesions, such as multiple sclerosis (MS) plaques.
Source: Wikipedia