Brain scans aren’t typically used to diagnose, or to rule out ADHD. In one of the ADDitude webinars, Dr. Hallowell (leading ADHD expert) said that in his view, the most significant source when diagnosing ADHD is your lifelong history of achievement (i.e. if you have ADHD, you will have exhibited symptoms since childhood). Most other healthcare professionals use rating scales to identify ADHD. Brain scans or neurological imaging are not among the top 2 methods used to identify ADHD.
What’s more, diagnoses (or definitive rule outs thereof) should never be made on the basis of only one source. If there is one source pointing towards one likely conclusion (e.g. your brain scan suggested neurotypical function) then that’s helpful information, but for a definite answer you will want to look at another source like your personal history, your grades, reports from your teachers or family members, or your responses to rating scales.