The problem with this study is they looked at kids who had been diagnosed and were on long term drug therapy. This is similar to studies that show that ADHD diagnosees on long term drug therapy have "brain damage", which matches the brain damage of long term stimulant users. The findings are not an indicator of ADHD, they are indicators of brain and genetic damage from long term drug use.
Much more effective than Ritalin, and safer, for treating ADHD is the prescription drug Desoxyn. It's not used as much because of the stigma of taking methamphetamine hydrochloride. I mention this because everyone agrees that methamphetamine is not an innocuous drug to take long term, the same goes for Ritalin, they both have very similar effects on brain chemistry.