Arachnoid cyst

Clinical History: 
This 23 year old female, 21 weeks into pregnancy, complained of a history of headaches over the past several years. In the past 2 weeks, however, the headaches had worsened significantly in severity and the patient had experienced associated nausea, vomiting, photophobia, right-sided temporal and retro-orbital pain. Following the MR examination presented below, the patient underwent a right parietal craniotomy with fenestration of CSF arachnoid cyst, connecting it with the right lateral ventricle.

Diagnosis:
Arachnoid cyst.

MR Technique: 
Scans were acquired on a 1.5 T Siemens MR unit. Sequences obtained included (A) sagittal pre-contrast spin echo T1-weighted (TR/TE 435/14 ms, scan time 3:43 min:sec) images, (B) axial pre-contrast turbo spin echo T2-weighted (TR/TE 5800/96 ms, scan time 3:47 min:sec) images, and (C) axial T2-weighted fluid-attenuated inversion recovery (FLAIR) (TI/TR/TE 2500/9000/136ms, scan time 3:54 min:sec) images. Slice thickness was 5 mm in each instance.

Imaging Findings:
A large CSF signal intensity lesion measuring 6.7 cm craniocaudal x 5.9 cm AP x 5.3 cm transverse was identified. This lesion extended from the mesial right middle cranial fossa superiorly and posteriorly to the right of the interhemispheric fissure. No associated soft tissue nodule or mass was identified. A thin septation was identified, separating the lesion from the right lateral ventricle. Mass effect from the lesion resulted in flattening of the posterolateral midbrain, displacing it anteriorly. The fourth ventricle was decompressed, likely due to partial obstruction of the cerebral aqueduct.