The Biomarker Research in ADHD: the Impact of Nutrition (BRAIN) study is an open-label trial during which children with ADHD followed a 5-week few-foods diet (FFD). The ADHD rating scale (ARS) was used thrice to assess the child's behaviour: at the screening (T0), before the FFD (T1) and at the end of the FFD (T2). Functional MRI scanning was done at T1 and T2. Each scan session received a random measurement code (M-code). During data processing and initial data analyses researchers were aware of which T1 and T2 samples formed a pair, but they were blinded to changes in ADHD symptoms. All data were anonymized using a subject-code (CH-code).