Thank you for the pointers.
They appear to be about the same research program. They share the same author, Philip Shaw. From the first "We measured the thickness of the cortex across thousands of points in the brain in .. (466) children - half of whom had ADHD and half of whom did not" and from the second "They recruited 234 children with ADHD and 231 typically developing children ... they were able to map the trajectories of surface area development at over 80,000 points across the brain."
Even though they appear to be different efforts (the number of children involved are different and the years are different), there might still be some spurious correlations solely because the same people, with the same expectations and analysis technique, are involved.
Unfortunately, I am unable to read the primary literature. The first links to a paywalled journal article ($31.50), the second doesn't have a journal citation. I found http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3342014/ by the same author, and published a couple of months after the article in the first link. Unfortunately, which it says there are distinguishable features, along with two reference comparisons, it doesn't give enough information to judge if those differences are truly diagnostic.
I did find http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3440402/ which comments "The ADHD diagnosis rests only on these symptoms because no biological markers (e.g. genetic tests, brain imaging) have been validated [11]", but that reference [11] is from 2006, and older than the two links you gave.
A 2007 metaanalysis (so again, older than the two references you gave) at http://www.ncbi.nlm.nih.gov/pubmed/16950217 points out that at that time "Although there are many structural neuroimaging studies of attention-deficit/hyperactivity disorder (ADHD) in children, there are inconsistencies across studies and no consensus regarding which brain regions show the most robust area or volumetric reductions relative to control subjects."
To highlight the intent behind those last two previous paragraph, they are saying that there are many studies like the ones you pointed out, which show a correlation between some physical measurement and ADHD. Unfortunately, they don't seem to be consistent. Because of the well-known publication bias problem, it's likely that false positives are over-reported. It takes additional followup work to determine if those signals are really true, and if they are distinguishable enough to be used diagnostically.
This all tells me that there is no "large structural brain difference", and especially not one which has been validated as a diagnostic tool. There might be a small structural brain difference, but again, I find no evidence that identification of this difference is available as a diagnostic tool at all, even if the price were disregarded.
Again, I might be focusing on the wrong thing, but perhaps you may be misinformed? If most of your research is through the popular literature, then my earlier reference to http://www.ncbi.nlm.nih.gov/pmc/articles/PMC3440402/ ("Why Most Biomedical Findings Echoed by Newspapers Turn Out to be False: The Case of Attention Deficit Hyperactivity Disorder") may be useful:
> Because newspapers preferentially echo initial ADHD findings appearing in prominent journals, they report on uncertain findings that are often refuted or attenuated by subsequent studies. If this media reporting bias generalizes to health sciences, it represents a major cause of distortion in health science communication.