What features should be pushing you towards this diagnosis?
Age of the patient (15 years) and morphology of the mass: it extends along the floor of the third ventricle and down along the infundibulum rather than having its epicentre at the pituitary gland. Additionally the pineal gland appears a little bulky and shares similar enhancement.
There is a T1 isointense to brain, T2 hyperintense mass filling the inferior recesses of the third ventricle and extending down along the infundibulum into the pituitary fossa. It demonstrates intermediate relatively homogeneous contrast enhancement. The pineal gland, is a little bulky and demonstrates similar signal intensity and enhancement.
The mass elevates and compresses the optic chiasm and results in increased signal in the optic tracts bilaterally.