Evaluation of central (cerebral) effects of Pulsed RadioFrequency (PRF) on the lumbar DRG (Dorsal Root Ganglion), using fMRI (functional Magnetic Resonance Imaging ). Feasibility study (after which an interim analysis is done): 5 subjects in order to detect measurable central effects inducted by PRF on DRG, follow up study 20. Total amount of subjects 25.