What is the diagnosis?
The hyperdensity in the middle of the brain almost certainly represents a colloid cyst.
Is there a differential?
There is almost always a differential, although it would be very unusual for this lesion not to be a colloid cyst. Other entities to consider include: calcified/hyperdense meningioma, giant cell astrocytoma (although there are no stigmata of tuberous sclerosis), pilocytic astrocytoma (unusual in this age group), blood in the region of foramen of Monro.
Where is this lesion located?
In the roof of the third ventricle, just below the foramina of Monro which form a communication between the frontal horns of the lateral ventricles and the third ventricle.
A round, hyperdense lesion, located in the interventricular foramina of Monro is present. No hydrocephalus. No other abnormality.