On an axial T2-weighted MRI at the level of the basal ganglia, a fluid-filled structure with smooth walls is identified posterolateral to the thalamus. It appears continuous with the quadrigeminal cistern and lies medial to the posterior limb of the internal capsule. This structure is most likely the:

• A Third ventricle
• B Ambient cistern ✓
• C Velum interpositum
• D Cistern of the lamina terminalis

Explanation

The ambient cistern (cisterna ambiens) wraps around the midbrain on each side, posterior and lateral to the thalamus, and communicates with the quadrigeminal cistern posteriorly. On axial MRI slices through the upper midbrain/diencephalon level it appears as a CSF-intensity structure posterolateral to the thalamus, medial to the posterior limb of the internal capsule. Monitoring ambient cistern effacement is critical in herniation syndromes. The third ventricle is midline and slit-like. The velum interpositum is a CSF space within the roof of the third ventricle. The cistern of the lamina terminalis lies anteriorly at the base of the frontal lobes.

Reference: BD Chaurasia's Human Anatomy, 8th ed.

High-yield for: NEET PGINI-CETNExTFMGEUSMLEPLABMRCP

Written and medically reviewed by the StethoPrep medical team.