Giant perivascular cystic space
Perivascular spaces are pial lined interstitial fluid filled spaces that accompany penetrating arteries. They do not have direct connection with the subarachnoid space.
They are variable sized and well-circumscribed, and are most commonly seen in the basal ganglia.
They are usually less than 5mm, but can become quite large and cause mass effect. These giant perivascular spaces are often seen in the midbrain.