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Clinical History: 
An incidental finding in a 46 year old man being evaluated for possible recent cerebral infarction (the brain MR was otherwise normal)

Diagnosis:
Left middle cranial fossa arachnoid cystt

MR Technique: 
The scans were acquired on a Siemens 3 T Tim Trio system. Scan times were 1:46 and 3:54 respectively (min:sec), with each scan 2D FSE in type, with a slice thickness of 4 mm. Sequence parameters were TR/TE = 4040/135 and TR/TE/TI = 9000/118/2500, with pixel dimensions of 0.7×0.7 and 0.9×0.9 mm2 respectively. The FLAIR sequence employed spectral fat saturation and was acquired using BLADE, for reduction of motion artifacts specifically from vessel and CSF pulsation.

Imaging Findings:
A small left middle cranial fossa arachnoid cyst is seen on conventional screening T2 and FLAIR axial exams at 3 T. Arachnoid cysts will demonstrate CSF signal intensity on all pulse sequences, as illustrated here, with FLAIR particularly sensitive in discriminating fluid collections that differ from pure CSF. Arachnoid cysts display mass effect, an important point in differential diagnosis, with the middle cranial fossa being the most common location for this entity.