Abstract
INTRODUCTION: Muscle volume is a key indicator of strength and neuromuscular health, commonly assessed via Magnetic Resonance Imaging (MRI). While accurate, MRI is expensive and time-intensive. Three-dimensional ultrasonography (3DUS) offers a more accessible alternative but requires validation due to its setup-dependent accuracy. This study investigated the validity and reliability of a custom 3DUS setup for measuring lower limb muscle volumes. METHODS: Fifteen participants (8 female; 18-40 years) underwent two 3DUS and one MRI sessions. The tibialis anterior, vastus lateralis, gastrocnemii, and biceps femoris muscles were scanned using ultrasonography integrated with a motion capture system. Phantom models were also scanned. After ten participants, the scanning protocol was adapted. 3DUS and MRI volumes were analyzed using 3D Slicer by two raters or one rater, respectively. Reliability was assessed using intra-class correlation (ICC), coefficient of variation (CV%), standard error of measurement (SEM), and minimal detectable change (MDC). RESULTS: 3DUS showed excellent test-retest and inter-rater reliability (ICC = 0.97-0.99; CV% = 2.0-4.6%). MDC values were < 5 mL for all muscles. However, 3DUS systematically underestimated volumes compared to MRI (biases: -10.0 to 33.0%), with best agreement for tibialis anterior and lowest for gastrocnemii. After adapting the protocol, mean differences were reduced by ~ 70%. Phantom scans confirmed both modalities were accurate, suggesting in vivo errors arose from probe pressure and sweep inconsistencies. CONCLUSION: 3DUS demonstrated excellent reliability but underestimated volumes relative to MRI, influenced by muscle shape and location. Despite limitations, it is a promising, cost-effective method for tracking longitudinal muscle changes. Open methodology supports broader application.