What is the black signal anterior to the pons attributable to, and what does it signify?
The black signal (or more accurately the lack of signal) is due to rapid flow / pulsation of CSF. Superiorly one can easily see how this plume of flow passes from the interpeduncular cistern into the third ventricle; the cyst has thus been fenestrated and a third ventriculostomy performed.
A third ventriculostomy was performed, and the floor of the cyst was also opened into the interpeduncular cistern. Prominent CSF pulsation can be seen continuously from the third ventricle to the prepontine cistern.
Note the reduced distortion on the optic chiasm and pituitary infundibulum.