Background: Asthma is the most frequent chronic disease in childhood. Severe asthma accounts for only 5% of patients but more than half of the costs of the disease. It is characterized by bronchial remodeling, including an increase in bronchial smooth muscle (BSM) mass associated with an increase in bronchial smooth muscle cell proliferation. This increased cell proliferation has been related with an abnormal calcium homeostasis and an enhanced mitochondrial biogenesis. Moreover, the release of growth factors by inflammatory cells and bronchial epithelial cells reinforces BSM cell proliferation. BSM remodeling is correlated with the severity and poor control of the disease. In children, bronchial remodeling could be assessed by performing endobronchial biopsies during bronchoscopy. However, this method is invasive and cannot be repeated to follow the remodeling. Non-invasive quantification of proximal bronchial thickness by computed tomography (CT) has been demonstrated. However, CT was unable to differentiate bronchial inflammation from remodeling on the one hand, and, on the other hand, CT is a radiating examination, and the repetition of X-ray exposures can be a source of cancer. Recently, the team evaluated at the university hospital of Bordeaux new MRI sequences to obtain 3D high-resolution images (3D-UTE), close to that of CT. This 3D-UTE has the advantage of being performed without any injection or inhalation of contrast medium and non-irradiating. Moreover, MRI provides additional data on signal intensity or bronchial inflammation that is not accessible on CT imaging. However, to the best of our knowledge, the use of 3D -UTE high resolution MRI has not been demonstrated in a pediatric population with severe asthma.

Purpose: To compare bronchial parameters measured in 3D-UTE MRI according to the presence or not of bronchial remodeling measured by immunohistochemistry on bronchial biopsies obtained from severe asthmatic children.

Methods: The investigator's team therefore wishes to carry out pulmonary MRI without anesthesia, nor injection nor inhalation of contrast product in severe asthmatic children. Two sequences in axial cuts will be performed for a total duration of 12 minutes (6 minutes by sequence). Bronchial quantitative data obtained by MRI will be compared to those obtained from immunohistochemical analysis of bronchial biopsies performed during bronchoscopy. Severe asthmatic children requiring bronchoscopy will be identified during consultation or hospitalization relative of the disease. The inclusion visit (V1) will be performed during a consultation for asthma of the usual care of the child. Then, the second visit (V2) will correspond to the anesthesia consultation of the usual care of the child to check the absence of contraindications to anesthesia and bronchoscopy. The third visit (V3) will correspond to the pulmonary MRI and flexible bronchoscopy. According to the classic procedure, bronchoscopy will be performed during a hospitalization. The pulmonary MRI, as well as blood tests and lung function testing, will be performed at the beginning of the hospitalization, the day before the bronchoscopy. Children will be monitored 24 hours in hospitalization after the bronchoscopy.