It's somewhere in this talk (broken down as a playlist). It's been a while since I've watched it but I do recall him saying that they can see physically differences in the brain of adhd folks

https://www.youtube.com/watch?v=BzhbAK1pdPM&list=PLzBixSjmbc...

Seeing physical differences in brain volume, connectivity or activity between groups of many adhd folks and non-adhd folks is one thing and that can be done. However, performing a clinical diagnosis of adhd in an individual, solely based on a brain scan, is a completely different thing.