I am unaware of an app that does what your asking. There is the Augmented Reality Measuring tool in IOS 12 <https://support.apple.com/en-us/HT208924>, but I am unsure if it would be accurate and repeatable enough for your project.

Some thoughts:
Do you need to do bolus analysis in real time? Are you proposing to point the camera of your ipad at the fluoro monitor to gather a still picture to analyse?
You will get different image area numerical results depending on how zoomed in you are, unless there is an object in the fluoro image of known dimension.

For dimensional accuracy you would want to work with a calibrated fluoroscopy recording back in the lab for image analysis.