XLH rickets is a rare disease with muscle weakness. Fat parameters such as IMAT and intraMAT could be increased in this disease. IMAT and intraMAT will be calculated on MRI for 11 XLH children versus 20 typically developing children. The investigator will compare the percentage of IMAT in the XLH group versus control group and the difference concerning the intraMAT between the two groups.

Single-center, prospective, non-randomized, controlled study with XLH and healthy volunteer children.

This study involves performing muscle MRI in healthy child volunteers to assess and compare the structure and composition of muscles to those of muscles in children with XLH pathology (muscle MRI of XLH patients is performed as part of standard medical care).

During this study we will have two groups: XLH group and a control group with typically developing children. In the XLH group, composed of 10 patients aged between 5 ans 17 years, female or male and ongoing growth (lesser bone age than 15 years old), have XLH and the most severe radiological deformities,clinical complications (pain and muscular weakness), have already done a muscle MRI of the lower limb during their follow-up.

The control group composed of 20 healthy volunteers aged between 5 ans 17 years old, female or male and without any endocrine pathology. A muscle MRI is necessary for healthy volunteers.

Every child will be alone After signing the informed consent by the 2 parents, the patient or the healthy volunteer and his parents, the MRI assessment will be performed on the same day.

Performing the muscle MRI requires the cooperation of the child in order to remain lying down and motionless throughout the duration of the MRI, i.e. approximately 45 minutes.

Performing MRI does not require the injection of contrast products or radiopharmaceuticals.

Inclusion Criteria common criteria to the 2 groups:

• Age: between 5 and 17 years.
• Sex: male or female.
• Informed consent, signed by both parents or holder of parental autority after being informated of the study.
• Affiliation to a social security schema or having the right to.

Inclusion criteria for XLH Patients :

• Patient suffer of X-linked hypophosphatemia rickets.
• Severity of clinical and radiological damage (pain, muscle weakness and severe bone deformities).
• Growth in progress (bone age less than 15 years).
• Child under conventional treatment: patient responding to conventional treatment or
• under treatment with Burosumab (patient in treatment failure under treatment conventional).

incluion criteria for helthy volunteers :

• Healthy voluntary subjects, not suffering from XLH.
• Matching by age (+/- 6 months) and sex.

Exclusion Criteria common to the 2 groups :

• Not being able to stay still during the MRI examination (approximately 45 min).
• Growth completed.
• History of lower limb surgery.
• Contraindications to MRI
• Holders of parental authority under AME.
• Holders of parental authority under tutorship / curatorship.

Exclusion criteria for Healthy volunteers :

• Patients with endocrine, contracted or muscular pathology.
• Patient receives a long-term treatment.
• Patients with a high athletic level.
• BMI-IOTF <20 or> 27.8