The most widespread pathophysiological hypothesis in major depressive disorder (MDD), is the hypothesis of monoamine deficit. The most used antidepressants in everyday clinical practice act by inhibiting the reuptake of monoamines. However, meta-analyzes evaluating the efficacy of antidepressants suggest that they are ineffective in 30 to 40% of patients. Inflammatory mechanisms might be related to the deficiency of monoamines, compromising the effectiveness of conventional antidepressants. Newly developed specific radiotracers allow the use of positron emission tomography (PET) imaging techniques to evaluate neuroinflammation. It has recently demonstrated the relevance of the [18F] DPA- 714 as a biomarker of neuroinflammation in humans in several neurological diseases.