Inclusion Criteria:  critically ill children from 0 to 17 years old magnetic resonance imaging planned for any medical reason sedated child for any medical reason absence of parental/patient refusal to participate to the study  Exclusion Criteria:  congenital neuromuscular disease no possible access to the thigh (e.g. dressing, drains, gypsum) anatomical anomaly of the limb, that would compromise localizing thigh landmarks (patella, groin) inability to obtain limb rest and/or thigh muscle decontraction risk induced by mobilization of the patient for ultrasound or magnetic resonance imaging purpose absence of social insurance