New onset double vision and headache in a teen is concerning! If the prisms helped this is true Diplopia and not pseudo Diplopia which is normally astigmatism.
Whomever gave you the stick on prism must have been concerned. Did they refer you to another specialist? Was this an OD or an MD?
The list of possible reasons is daunting and you certainly avoid conjecture and Internet diagnosis. Though this forum seems universal in its vote for imaging I would hope you have a neuro ophthalmologist or neurologist that is guilding this process. Which scan and what tissues we are interested in seeing are critical and will help avoid further studies and get at the root cause sooner.
If you live in a metropolitan area you should seek out guidance and not just have her scanned.
For instance an orbital scan will miss the motor parts of the brain. A brain scan could miss sinus structures and a CT scan may best elucidate some pathology.
I assume that someone with knowledge of your daughter has ordered this scan. If not get under the care of someone that has a clue what they are looking for!
Best of luck and it is sad to not have follow up on these opinion forums for the benefit of these for profit companies.

Richard B. Foulkes