Magnetic resonance imaging, or MRI, is a scan that uses a strong magnet and radio waves to make detailed pictures of organs and tissue, including the brain.
MRI uses no radiation. You lie still inside a tube-shaped scanner for between 15 and 90 minutes, depending on what is scanned, and it is loud. Some people find the noise and the enclosed space hard, which is worth saying in advance if sensory input affects you.
In research, MRI studies have found average differences in brain structure and activity between groups of people with and without ADHD or autism. Those are group averages. Individuals overlap a great deal, so a scan cannot diagnose one person.
No MRI scan is used to diagnose ADHD or autism. NICE guidance for autistic adults advises against routine neuroimaging for diagnosis.