Assessing gluteus medius volume with freehand 3DUS: validating a practical imaging tool for complex muscle morphology

Accurate and accessible imaging techniques are essential for evaluating muscle morphology in both clinical and research settings. This study examined the validity and reliability of freehand three-dimensional ultrasound (3DUS) employing a multiple-sweep technique for measuring the volume of complexly shaped muscles such as the gluteus medius (GMed), with magnetic resonance imaging (MRI) as the reference standard. Twelve healthy participants (21.1 ± 1.5 years) underwent both 3DUS and MRI scans. Each GMed was scanned using three overlapping 3DUS sweeps, and two processors independently segmented all 3DUS and MRI images to calculate muscle volume. Agreement between 3DUS and MRI was evaluated using Bland-Altman plots, while intra- and inter-processor reliability for 3DUS were assessed using intraclass correlation coefficients (ICCs), coefficients of variation (CV%), typical error (TE), and minimal detectable change (MDC). The mean difference between 3DUS and MRI was minimal, with Bland-Altman plots demonstrating good agreement and no systematic bias. Inter- and intra-processor reliability were excellent (ICC = 0.972 and 0.999, respectively). A subgroup analysis (n = 10) comparing prone and side-lying positions using 3DUS also demonstrated good between-position reliability (ICC = 0.94). Freehand 3DUS with the multiple-sweep technique provides a valid, reliable, and practical alternative to MRI for measuring GMed muscle volume in both clinical and research applications.