Subthalamic nucleus and substantia nigra pars reticulata combined stimulation for freezing of gait in Parkinson's disease. A randomized double-blinded cross-over study

INTERVENTION: group A :STN‐SNr combined stimulation, assessed after traditional STN stimulation;group B (before crossover):traditional STN stimulation, assessed after STN‐SNr combined stimulation ; CONDITION: Parkinson's disease PRIMARY OUTCOME: UPDRS axial symptom score;FOGQ; SECONDARY OUTCOME: quantitative gait analysis result;Unified Parkinson disease rating scale ( UPDRS) ; INCLUSION CRITERIA: 1. Those who meet the diagnostic criteria of primary Parkinson's disease at least 5 years; 2. Patients who take evaluation and follow‐up at Clinical Center for Parkinson's Disease, Capital Medical University, plan to undergo subthalamic nucleus (STN) DBS surgery; 3. Those who can cooperate to complete T1w, T2w and QSM MR scanning; 4. There is no contraindication of neurosurgery and the informed consent is signed; 5. Those who can complete follow‐up within 1 year after surgery.