Like @yucandu@lemmy.world said, it would need to be an fMRI, which is primarily used in research as far as I know. And while it alone could not tell you definitively "this person has ADHD," it could help rule out other conditions (like TBI, which can also present similar to ADHD). Ultimately, your doctor is right that a standard MRI cannot diagnose it.

I like to combine the checklist with interviews (like DIVA, Diagnostic Interview for ADHD in Adults) and computerized continuous performance tests, like QBTest. Of course, there is also a lot of observation and sometimes even asking humorous questions, like "Do you have The Chair^TM^ at home?"

The Chair^TM^