Arachnoid cyst MRI

Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1] Associate Editor(s)-in-Chief: José Eduardo Riceto Loyola Junior, M.D.[2]

On brain/spine MRI, arachnoid cysts are characterized by cystic images with similar density to CSF and non-enhancing borders, mostly found in the middle cranial fossa while they only rarely occur in the spinal cord. MRIs are more adequate than CT scans for evaluating arachnoid cysts.

• MRIs are better to diagnose and evaluate the extent of the arachnoid cyst than the CT Scan;
• Most cysts (50-60%) are found in the floor of the middle cranial fossa, while 1/4 to 1/3 of occur in the posterior fossa, particularly in the retrocerebellar, cerebellopontine, and quadrigeminal plate cisterns. Rarely, they may be found in the spinal cord.^[1][2]
• Demonstrate the exact location, extent, and relationship of the cyst;
• Can differentiate arachnoid from epidermoid cysts (arachnoid cysts are identical to CSF, while epidermoid present a higher signal with FLAIR and reduced diffusion with DWI, making them appear brighter than CSF).
• CSF signal is seen within the cyst;
• Eventually, arachnoid cysts may contain proteinaceous fluid or blood, which can cause diagnostic confusion.^[3]

As differential diagnosis, the following hypothesis must be considered:

1. Enlarged CSF space;
2. Epidermoid cyst;
3. Subdural hygroma/chronic subdural hemorrhage;
4. Cystic tumors;
5. Pilocytic astrocytoma;
6. Hemangioblastoma;
7. Neurenteric cyst;
8. Neuroglial cyst;
9. Porencephalic cyst;
10. Neurocysticercosis.^[5]

(Images courtesy of RadsWiki)

Patient #1: Left middle cranial fossa arachnoid cyst

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  Cor T2
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  Sag T1
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  Axial T1 FLAIR
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  Axial T2

Patient #2: CT and MR images demonstrate a hemorrhagic arachnoid cyst

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  CT
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  Sag T1
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  Ax T1
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  Ax FLAIR
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  Ax T2
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  Ax T1 with GAD