<qa>  What is the PSQI?After stabilisation of the stimulation parameters at least for more than 1 month, the second session of neuroimaging will occur T1. Minimal one month later 2 months after implantation of definitive SCS system the patient will undergo the last session of neuroimaging T2. The MRI protocol will occur after stabilisation of the stimulation parameters at least for more than 1 month. During the whole period, patients will fill in a VAS diary and at time of neuroimaging T1 and T2 a Likert-scale in order to evaluate the satisfaction grade of the patient. Additionally, patients will also wear 1 month before neuroimaging an Actiwatch, measuring sleep patterns and filling in questionnaires at baseline, T1 and T2 Pittsburgh Sleep Quality Index PSQI. 