the investigators make the following assumptions: 1) neuroinflammation in MDD can be measured by the [18 F ] DPA- 714 ; 2) it is accompanied by anatomical and functional changes in the frontal subcortical loops, strongly involved in MDD ; 3) neuroinflammation in patients might be a biomarker related to resistance to treatment in patients with MDD. If this assumptions are validated, then this study will enable a better understanding of the neuroinflammatory processes. This breakthrough could have a long term therapeutic impact, helping to target more specifically antidepressant drugs with anti-inflammatory action and / or drugs targeting neuroinflammation.