Is there a way to measure ATP usage in the brain similarly to fMRI?
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.