Please inform me as to what physical tests are used to detect these abnormalities and how they show up on the given medical devices (e.g., brain imagery).
You would have to go to the source to get that information, as I did not conduct the study. As the link said, the study was published by the Mayo Clinic. From the description at the link, it appears they examined the statistical incidence of ADHD (or symptoms resembling the approximate clinical signs of ADHD) in a sample group of children, and compared it to the incidence and length of general anesthesia during infancy in the study sample; and by this means identified a measurable trend, but not an absolute causation.