Teenager with headaches

MRI of cavum vellum interpositum
Sagittal (left) and axial (right) T1 MRI without contrast of the brain show a cystic structure between the atria/trigones of the lateral ventricles, behind the foramen of Monro, beneath the columns of the fornices and above the tela choroidea of the 3rd ventricle.

The diagnosis was cavum vellum interpositum.