Although an MRI is not done to diagnose ADHD/ADD, in studies it does show that the ADHD brain has considerably less activity than does a normal brain when MRI's are compared. Stimulant drugs increase the chemical activity level, which decreases the symptoms.
If your MRI shows a lesion that would help me, I have the same symptoms as ADHD but never had it as a child, the medical people say lesions are not really a concern but I do wonder why they are there, is it the start of MS, or AD, I don't think I am getting correct answers from the doctors, they will say you are depressed, but of course you are depressed if your mind does not function like it used to, who wouldn't be depressed from this.
There is a very reliable brain scan test called a SPECT scan for Single Photon Emission Computed Tomography. It is most commonly done at the Amen Clinic for ADD/ADHD. It shows the typical pattern of ADD when there is a decrease of blood and electricity in the prefronal cortex that shows up as two "holes" in the scan.
Many of these scans can be viewed at http//www.add-care.com.