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BackgroundMuscle wasting is a common finding in critically ill patients associated with increased days of mechanical ventilation in the ICU. Muscle wasting and associated morphological changes are hallmarks of ICU-acquired weakness. Muscle wasting can be diagnosed and quantified by muscle biopsy, but biopsies can cause multiple adverse effects. MuscleSound® has developed a non-invasive, real-time novel MusculoSkeletal (MSK) ultrasound approach to measure the ratio between percentage intramuscular adipose tissue (%IMAT) with muscle cross-section area (MCSA) (termed IMAT-Index). The present study aimed to assess the IMAT-Index in ICU patients longitudinally and compare it to age-matched healthy controls.MethodsTransverse (short-axis) ultrasound images of the rectus femoris muscle were obtained upon admission and discharge in 35 ICU patients and compared to age-matched healthy controls (n = 975). The echo intensity of the image taken from the muscle is used to automatically calculate the ratio between intramuscular adipose tissue by cm2 (IMAT-Index), the cross-sectional area of the muscle (MCSA), and muscle thickness (MT).ResultsIMAT-Index was successfully measured in all subjects. The mean IMAT-index (%IMAT/cm2) upon admission was significantly higher in critically ill patients compared to healthy controls (7.4 ± 4.3 vs. 3.1 ± 0.9, p