Intensive care unit acquired muscle weakness (ICUAW) is a common disease. After 7 days of mechanical ventilation, a quarter of patients develop an ICUAW responsible of a 15-20% of muscle loss. This is a serious pathology associated with high morbidity and mortality.

Clinical diagnosis of ICUAW is difficult and need a patient awoken. CT scan and Magnetic Resonance Imaging (MRI) are considered as "gold standards" to quantify and monitor changes in muscle mass. But these tools are not adapted to ICU patients.

Ultrasound (US) is an easy access tool at the bedside to assess muscle mass and does not expose the patient to additional radiation.

The objective of this study is to evaluate the correlation between US and CT scan to measure quadriceps muscle thickness of ICU patients.

As part of the monitoring of the neurological disease, brain CT scans are frequently performed. At the same time, an acquisition sequence centered on the quadriceps will follow the brain sequences. Up to three brain CT scans coupled with a quadriceps imaging will be performed between the day of admission and on the tenth day of hospitalization. An ultrasound of the quadriceps will be performed concomitantly. The thickness of the quadriceps is measured by an independent operator on CT and ultrasound images.

Inclusion Criteria:

• Patient over 18 years admitted to intensive care for a severe brain injury (Glasgow <8) and with a predictive duration of mechanical of at least 48hours

Exclusion Criteria:

• Pregnancy,
• Bilateral trauma of thighs,
• Coma induced by poisoning or by a metabolic disorder,
• Morbid obesity (BMI > 35).