Another article on the MRI studies

Researchers at the University of Illinois at Chicago (UIC) are the first to use brain imaging to examine the effects of emotion on working memory function in children with pediatric bipolar disorder or attention deficit hyperactivity disorder. Using functional magnetic resonance imaging (fMRI), researchers at UIC examined how working memory circuits of children are affected as they performed a working memory task while viewing faces with different emotions (e.g. angry, happy or neutral affects).
Article:

excerpt from same article:
(one of the excellent reasons and uses of this study is shown succinctly here)
Now that researchers are starting to differentiate between the two disorders at a brain network level, rather than just at a behavioral level, the long-term goal is to develop diagnostic tests based on neurological and behavioral markers of illness that can be used in a clinical setting. Currently, patients are diagnosed using clinical measures, questionnaires, behavior scales and interviews with parents.