Reliability and criterion validity of abductor hallucis muscle volume measurements using ultrasound and MRI by assessors with limited image analysis experience

Purpose The abductor hallucis (ABDH) contributes to foot function and mobility, which can influence quality of life, making accurate assessment of muscle volume important. While magnetic resonance imaging (MRI) is often considered to be the reference standard of medical imaging, ultrasound (US) provides a cost-effective, rapid and dynamic way of assessing muscle size and function. This study aimed to evaluate the criterion validity and intrarater reliability of US-derived ABDH muscle volume measurements compared with MRI when image analysis was performed by assessors with limited experience. Methods Us And MRI were employed to measure the right and left ABDH muscle volume involving 22 participants (females = 12; males = 10). Following exclusions, 37 feet were available for analysis, with 36 complete MRI-US observations. Muscle volume of the ABDH was calculated across the muscle belly every 0.5 cm both manually by utilizing a truncated cone (TC) formula and with a semi-automated tool in the OsiriX (OX) software. Intrarater reliability was assessed using intraclass correlation coefficients (ICC), standard error of measurement (SEm), and minimal detectable change (MDC). Criterion validity was evaluated using Pearson correlations, Bland-Altman analyses, and two one-sided tests (TOST) with a determined ±10% equivalence margin. Results Side-specific intrarater reliability was high across methods (ICC(3,1) = 0.932–0.997), with SEm values of 0.251–0.709 cm³ and relative MDC values of 4.48%−12.95%. Very strong associations were observed among methods (r = 0.906–0.995). Bland-Altman limits-of-agreement half-widths were 6.52%−9.84% for MRI-MRI and 17.86%−27.35% for MRI-US comparisons. All three method comparisons met the ± 10% equivalence criterion. Conclusion US-derived ABDH volume measurements demonstrated high intrarater reliability and statistical equivalence to MRI within the ± 10% margin. Although MRI-US comparisons showed greater individual-level variability than MRI-MRI comparisons, US may provide a practical alternative for assessing ABDH muscle volume when MRI is impractical.