Validity of A-mode ultrasound for muscle assessments: Comparisons with magnetic resonance imaging

Background & aims Skeletal muscle assessments are essential in clinical and research contexts. Amplitude mode ultrasound (A-mode ultrasound) offers a practical alternative, but comparisons with the gold standard (i.e., magnetic resonance imaging, MRI) remain underexplored. This study aimed to evaluate the validity of skeletal muscle assessment from A-mode ultrasound in comparison to MRI among healthy young adults. Methods Ninety-five physically active individuals (60% female, 29 ± 6 years) had biceps and quadriceps muscle thicknesses measured using the BodyMetrix® device (i.e., A-mode ultrasound). MRI scans at the third lumbar vertebrae level were used to estimate skeletal muscle area (SMA in cm2). Pearson’s correlations and linear regressions were used to assess correlations and associations, respectively. Results Biceps muscle thickness (BMT) showed a strong correlation with SMA from MRI (r = 0.82; R2 = 0.68, all p-values < 0.001), outperforming rectus femoris (r = 0.51, R2 0.26, p-value < 0.001) and vastus intermedius (r = 0.42, R2 0.18, p-value < 0.001). Conclusion Our results demonstrate the potential use of A-mode ultrasound for skeletal muscle assessments, particularly for the biceps area (i.e., measuring BMT), although with modest value for the thigh region. Our study supports the use of A-mode ultrasound due to its lower cost, portability and bed-side feasibility and potential performance against MRI.