Brain imaging is not ready for use as a diagnostic tool in ADHD, Shaw said."It is still too early to use neuroanatomical scans for diagnosis," he said. "We had to scan hundreds of children to identify subtle differences. They [the differences] are very real, but they are subtle. So the scan of any one child will not tell you a great deal about whether [he or she has] ADHD or not. Currently, the diagnosis of ADHD remains clinical."
http://www.kennedykrieger.org/overview/news/brain-imaging-st...
Really a search for "adhd brain development" yields plenty of results. :)