There’s a doctor who goes by Russell Barkley, PhD on his popular YouTube channel and claims to be able to brainscan for ADHD with certainty. Is his claim legitimate?

He's a legit researcher though I haven't seen his work on youtube. In terms of how accurate brain scans are at diagnosing adhd you can look at recent meta analyses (gold standard of evidence based medicine pyramid).

In a 2024 meta-analysis of MRI/fMRI machine-learning studies for ADHD, the average pooled sensitivity was 74% (the ability to correctly identify people who actually have ADHD) and the average pooled specificity was 75% (the ability to correctly identify people who do not have ADHD), that also means that there is a 26% false-negative rate (miss/Type II error) and a 25% false-positive rate (accidental diagnosis/Type I error) [https://doi.org/10.1016/j.jad.2024.03.111].

Though there is no set standards but under 80% is pretty low for a clinically useful test but can still be used in some conditions such as to supplement diagnosis. To give some numbers the rapid antigen covid tests had 72% sensitivity and 98.9% specificity. (note the lower sensitivity was acceptable at the time but the CDC guideline was for symptomatic people to retest 48 hours. Viral load and other staging factors could all confound.) [https://doi.org/10.1371/journal.pmed.1004011]

Russel Barkley is a pioneer in the field of ADHD research. I have heard of such a claim, but not from Russel Barkley. Could you provide a link of Russel Barkley making the claim please?

This is the intelligence of the average American voter. No wonder we got Trump.