Diffusion Tensor Imaging and Fiber Tractography of Skeletal Muscle: Optimization of b Value for Imaging at 1.5 T

OBJECTIVE. The aim of our study was to determine the optimal b value for 1.5-T diffusion tensor (DT) MRI and fiber tractography of in vivo human skeletal muscle. SUBJECTS AND METHODS. Five healthy volunteers were included in this prospective study. DT MRI of the proximal calf was performed with 15 directions of diffusion sensitization and parallel imaging with an acceleration factor of 2. Each single-shot spin-echo echo-planar imaging sequence was performed in each volunteer with eight b values ranging from 125 s/mm(2) to 1,000 s/mm(2). Fiber tractography of two calf muscles (anterior tibialis and lateral gastrocnemius) was performed, and image quality was ranked 1-8. Fractional anisotropy, signal-to-noise ratio, and fiber density index were assessed for both muscles at each b value. Statistical analysis was performed with repeated measurement analysis of variance. RESULTS. Among all fiber tractographic images assessed, those obtained at a b value of 625 s/mm(2) were qualitatively ranked first (best image quality) or second (second-best image quality). Signal-to-noise ratio was highest for both muscles at a b value of 125 s/mm(2) (anterior tibialis, 66.2; lateral gastrocnemius, 60.1). The highest fiber density indexes were found for both muscles at a b value of 625 s/mm(2) (anterior tibialis index, 7.82; lateral gastrocnemius index, 5.75). In the analysis of variance no significant differences were found between fractional anisotropy and b values for either muscle (p = 0.359 and p = 0.078). CONCLUSION. The optimal b value for in vivo DT MRI and tractographic assessment of human skeletal muscle in the calf at 1.5 T MRI was found to be 625 s/mm(2).