Study of Pulmonary MRI for the Diagnosis of Bronchiolitis Obliterans Syndrome After Allogeneic Stem Cell Transplantation

lung MRI (1.5T Siemens Aera) using the following sequences: 3D Fast gradient-echo pulse sequences with ultra-short echo time (UTE), acquisitions at end-inspiration breath hold, end-expiration breath hold, and free-breathing using an echonavigator positioned on the diaphragm, acquisitions using routine pulse sequences (SSFP, T2FSE) and the administration of oxygen during the MRI: O2 will be administered at 15L/min during 6 minutes.