Is there a way to measure ATP usage in the brain (or anywhere else) similarly to how fMRI works (measuring changes in iron resonance in oxygenated/deoxygenated blood)? Is there some sort of similar signal that can be used for ATP?
I'm pretty sure this doesn't exist. If it did, that would be awesome.
The following is 100% pure speculation with nothing to back this up at all (as a disclaimer).
Phosphorus-31 (the stable isotope) is NMR active, which means that you could theoretically use an MRI machine to visualize phosphorus. A quick google search shows scientists attempting to use phosphorus MRI for visualizing bone structure.
I don't really know much about MRI, but I do know some about small molecule NMR, which is based on the same principles. This is a pretty cool paper showing phosphorus-31 NMR used to measure ATP synthesis.
Maybe in the distant future it could be possible to use phosphorus-31 MRI to visualize ATP/ADP ratios in various tissues, including the brain. But that would be some amazing sci-fi stuff.