Reliability of Rectus Femoris Ultrasound Measurements and Relationship With Truncal Muscle Mass in Healthy Individuals Using Concurrent CT Measurements as the Reference Standard

In chronic diseases, accelerated muscle mass loss is associated with poor clinical outcomes. Computed tomography (CT) is considered a reference standard for assessing muscle mass, but it is limited for longitudinal assessment. Ultrasound (US) is more suitable for longitudinal measurements, but limited reliability data or reference values exist to inform clinical adoption. This pilot study evaluated the reliability of rectus femoris (RF) muscle US measurements cross-sectional area (CSA) and shear-wave elastography (SWE) stiffness and investigated their relationship with CT-derived truncal muscle mass. Forty healthy living liver donors undergoing abdominal CT were included. CT-derived skeletal muscle area and skeletal muscle index at T12 and L3 were quantified using deep learning. US B-mode and SWE RF images obtained with manual and automated measurements. Reliability was assessed using intraclass correlation (ICC). Agreement between manual and automated methods was evaluated using the Dice coefficient. US and CT measurements associations were evaluated using Pearson correlation and multiple linear regression. Inter-reader agreement for manual US CSA was excellent (ICC=0.95, 95% CI: 0.88-0.97). Test-retest reliability of SWE was good (ICC=0.78, 95% CI: 0.67-0.87). Automated and manual methods showed strong agreement (Dice coefficient 0.94) and good reliability (ICC=0.85, 95% CI: 0.75-0.91). RF CSA demonstrated weak but significant correlations with CT-derived skeletal muscle area at both T12 and L3 levels ( r =0.37 to 0.40, P <0.05). US parameters showed moderate predictive value for CT-derived skeletal muscle index at L3 (adjusted R ²=0.70). In conclusion, RF US measurements are reliable, and automated measurements are feasible but show a modest correlation with CT-derived muscle mass measurements.