A vast majority of children admitted to paediatric intensive care (PICU) present with faltering growth during their admission. Muscle mass loss is an early, intense and frequent phenomenon in this setting, which is associated with impaired outcomes. Recent international guidelines recommend monitoring both nutritional status and muscle mass throughout hospital stay. Recent studies have used quadriceps femoris (QF) measurements as a surrogate for lean mass assessment, and monitored them with bedside ultrasound (QF thickness and QF cross sectional area). However, ultrasound cross sectional area inter-operator reproducibility has not been validated so far, and none of these ultrasound measurements has been validated against their gold standard i.e. magnetic resonance imaging measurements. This validation process should be conducted to allow interpreting ultrasound muscle measurements, prior to the implementation of ultrasound measurments into clinical practice.

We hypothesise that ultrasound measurements of QF thickness and cross sectional area are reliable compared to the magnetic resonance imaging gold standard, and that QF cross sectional area has a reliable inter-operator reproducibility.

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