Method of quantifying 3D strain distribution in skeletal muscle using cine phase contrast MRI

Intramuscular pressure (IMP), a correlate of muscle tension, may fill an important clinical testing void. A barrier to implementing this measure clinically is its non-uniform distribution, which is not fully understood. Pressure is generated by changes in fluid mass and volume, therefore 3D volumetric strain distribution may affect IMP distribution. The purpose of this study was to develop a method for quantifying 3D volumetric strain distribution in the human tibialis anterior (TA) during passive tension using cine phase contrast (CPC) MRI and to assess its accuracy and precision. Five healthy subjects each participated in three data collections. A custom MRI-compatible apparatus repeatedly rotated a subject's ankle between 0° and 26° plantarflexion while CPC MRI data were collected. Additionally, T2-weighted images of the lower leg were collected both before and after the CPC data collection with the ankle stationary at both 0° and 26° plantarflexion for TA muscle segmentation. A 3D hexahedral mesh was generated based on the TA surface before CPC data collection with the ankle at 0° plantarflexion and the node trajectories were tracked using the CPC data. The volumetric strain of each element was quantified. Three tests were employed to assess the measure accuracy and precision. First, to quantify leg position drift, the TA segmentations were compared before and after CPC data collection. The Hawsdorff distance measure (error) was 1.5 ± 0.7 mm. Second, to assess the surface node trajectory accuracy, the deformed mesh surface was compared to the TA segmented at 26° of ankle plantarflexion. This error was 0.6 ± 0.2 mm. Third, the standard deviation of volumetric strain across the three data collections was calculated for each element and subject. The median between-day variability across subjects and mesh elements was 0.06 mm³ mm^-3 (95% confidence interval 0.01 to 0.18 mm³ mm^-3). Overall the results demonstrated excellent accuracy and precision.