Biological markers of depressive states have been studied, but their usefulness to predict the therapeutic response is unknown. This issue is major in all depressive states which have not remitted after several lines of treatment. rTMS (repetitive transcranial magnetic stimulation) is a non-pharmacological alternative in the treatment of depression, but its effects on cerebral functioning are not known in episodes which have resist to conventional treatments. The investigators will include 50 depressive patients who have failed to respond to two successive antidepressant medication, and propose them a treatment with low frequency rTMS during 3 to 6 weeks. Cerebral functional imaging with 18FDG-PET (positon emission tomography) with be assessed at the beginning and at the end of rTMS acute treatment, in order to measure induced metabolic changes and their correlation with clinical states. Patients who have responded to rTMS acute treatment may continue this therapeutic for six months, and the investigators will assess if efficacy maintenance is related with cerebral metabolic variations