Diffusion Tensor Imaging of Healthy Skeletal Muscles: A Comparison Between 7 T and 3 T.

Objective: The aim of this study was to assess and compare the overall performance, reliability, variability, as well as the accuracy of diffusion tensor imaging (DTI) and fiber tracking metrics, for the muscles of the calf at 3 T and 7 T.
Materials and Methods: Ten volunteers (5 males; mean age, 29.1 ± 4.7 years), with no history of muscle disease, were examined twice at 3 T and 7 T, using a stimulated-echo acquisition mode DTI sequence. Signal-to-noise ratio (SNR) and DTI metrics (track numbers [trn], length [trl], volume [trv], fractional anisotropy [FA], mean [MD], axial [AD], and radial diffusivity [RD]) of the whole-calf muscles, the tibialis anterior, the gastrocnemius medialis, the gastrocnemius lateralis, and the soleus were collected. The Student t test was used to compare SNR and DTI metrics obtained at 3 T and 7 T. The coefficients of variation and the intraclass correlation coefficients were derived to assess the variability and the reliability of the DTI measurements at 3 T and 7 T. To further assess the accuracy of the measurements, the absolute difference was computed for each DTI metric at 3 T and 7 T and then compared (Student t test). The applied level of significance for all the statistical analyses was P < 0.05.
Results: As expected, the SNR was higher at 7 T than at 3 T (+111%; P < 0.001). At 7 T, the tracked fibers of the whole calf muscles, the gastrocnemii, and the soleus were more numerous (trn +5.5%, +3.1%, +8.5%, and +15.1%, respectively), longer (trl +13.1%, +18.8%, +19.3%, and +33.3%, respectively), and showed a greater volume (trv +12.1%, +12.2%, +14.7%, and +15.7%, respectively) than at 3 T (P < 0.05 each). The soleus demonstrated higher FA (+14.3%), lower MD (-1.7%), AD (-1.9%), and RD (-2%) at 7 T than at 3 T (P < 0.05 each), whereas the other muscles showed more heterogeneous results. The coefficients of variation were good (ie, <10%) for all DTI metrics at both 3 T and 7 T. The intraclass correlation coefficient was excellent (>0.750) at 7 T and 3 T for several DTI metrics, such as the trn of the gastrocnemii and the soleus, the trv of the gastrocnemii, the FA of the whole-calf muscles, gastrocnemius medialis, tibialis anterior, and soleus, and the RD of all investigated muscles (ie, whole-calf muscles, gastrocnemii, tibialis anterior, and soleus). There were no significant differences between the 2 consecutive measurements with each device, except for the trn of the whole-calf muscles and the FA of the gastrocnemius lateralis (higher mean absolute difference at 3 T and 7 T, respectively; P < 0.05 each).
Conclusions: Despite the numerous challenges associated with DTI of the muscles, both 3 T and 7 T demonstrated reliable and precise results.