Anesthesia causes respiratory changes and complications, which are the main causes of complications and anesthesia-related deaths.~Among the complications, the rapid appearance of lung atelectasis is well known, deteriorating the hematosis during anesthesia, and inducing secondary pulmonary complications.~Prevention or limitation of the atelectasis would be a way to improve the safety of patients who have a limited respiratory function.~The application of a positive expiratory pressure (PEP) is an effective prevention method for atelectasis, that has been demonstrated only in patients under mechanical ventilation.~The high flow nasal cannula is a non-invasive technique easy to perform and allowing a PEP. High flow nasal cannula is safe to use, and the interface is free from local skin complications.~Chest X-ray (bad sensibility) or CT are usually used for the diagnosis of atelectasis, but these two exams involve irradiation for patient. MRI and pulmonary ultrasonography has now been validated in adults. The investigators has a long experience of anesthesia in spontaneous ventilation in children for MRI. MRI are relatively long, and allow to investigate the atelectasis with only a short additional acquisition time.~Thus, it would be the first study on the effect of the PEP on the impact and the volume of the atelectasis during general anesthesia in spontaneous ventilation in children. By the way this is the first study on lung's anatomical effects of high flow nasal cannula.~It is a monocentric, intervention, randomized, superiority study whose main purpose is to show the reduction of the atelectasis through the use of high flow nasal cannula versus a high concentration mask for pediatric anesthesia during MRI.~The studied population is all the children between 6 months and 5 years with a pediatric indication of general anesthesia for MRI.~The primary goal is to show a reduction of volume (cm3) of the atelectasis via the application of a PEP by high flow nasal cannula.~The main assessment criteria is the ratio of atelectasis volume/total lung volume.~The prevalence of the atelectasis will be evaluated by MRI lung and measured using 3D reconstruction software. The duration of the examination and the anesthesia is slighty lengthened, 3 to 5 minutes for a 40 minutes' exam.~The secondary objective is to show a match between the atelectasis on MRI and ultrasound.
Anesthesia causes respiratory changes and complications, which are the main causes of complications and anesthesia-related deaths.~Among the complications, the rapid appearance of lung atelectasis is well known, deteriorating the hematosis during anesthesia, and inducing secondary pulmonary complications.~Prevention or limitation of the atelectasis would be a way to improve the safety of patients who have a limited respiratory function.~The application of a positive expiratory pressure (PEP) is an effective prevention method for atelectasis, that have been only demonstrate in patients under mechanical ventilation.~The high flow nasal cannula is a non-invasive technique easy to perform and allowing a PEP. High flow nasal cannula is safe to use, and the interface is free from local skin complications.~Chest X-ray (bad sensibility) or CT are usually used for the diagnosis of atelectasis, but these two exams involve irradiation for patient. MRI and pulmonary ultrasonography has now been validated in adults. MRI are relatively long, and allow to investigate the atelectasis with only a short additional acquisition time.~Thus, it would be the first study on the effect of the PEP on the impact and the volume of the atelectasis during general anesthesia in spontaneous ventilation in children. By the way this is the first study on lung's anatomical effects of high flow nasal cannula.~It is a monocentric, intervention, randomized, superiority study whose main purpose is to show the reduction of the atelectasis through the use of high flow nasal cannula versus a high concentration mask for pediatric anesthesia during MRI.~The studied population is all the children between 6 months and 5 years with a pediatric indication of general anesthesia for MRI.~The primary goal is to show a reduction of volume (cm3) of the atelectasis via the application of a PEP by high flow nasal cannula.~The main assessment criteria is the ratio of atelectasis volume/total lung volume.~The prevalence of the atelectasis will be evaluated by MRI lung and measured using 3D reconstruction software. The duration of the examination and the anesthesia is slighty lengthened, 3 to 5 minutes for a 40 minutes' exam.~The secondary objective is to show a match between the atelectasis on MRI and ultrasound.