Quantification of Skeletal Muscle Fiber Orientation in 3D Ultrasound B-Modes

Skeletal muscle exhibits transverse isotropy with a symmetry axis in the muscle fiber direction. Characterization of muscle mechanical properties with ultrasound shear wave elasticity imaging requires an accurate measurement of the 3D muscle fiber orientation (MFO): rotation and tilt. Existing approaches apply to 2D B-mode images, extract only fiber tilt, and detect individual fibers. Here, we present a Fourier-domain approach for calculating 3D MFO from 3D B-mode volumes acquired using two imaging setups: 1) a cylindrical volume acquired by rotating a linear transducer, and 2) a rectangular volume acquired by a rectilinear matrix array transducer. We imaged the vastus lateralis muscle of a healthy volunteer and also manually measured the orientation of individual fibers observed in these two B-mode volumes to assess heterogeneity. For rotation and tilt respectively, the standard deviations were 6.4 degrees and 1.5 degrees for the rotational B-mode (n=7) and 2.7 degrees and 1.5 degrees for the matrix B-mode (n=43). We validated our proposed algorithm on in silico and in vivo data: errors in rotation and tilt from the mean orientation were within 1 degrees for both imaging setups and less than the in vivo MFO heterogeneity. Lastly, we performed a Bland-Altman analysis of rotation angles estimated from B-mode and from shear wave elastography data (n=35): bias was less than than 1 degrees, and 95% limits of agreement was +/- 10 degrees. Our Fourier-domain approach precisely computes the average 3D MFO from 3D ultrasound B-mode volumes of muscle, and these orientation estimates will be used in ongoing muscle characterization studies.

Automated summary

The study proposes a Fourier-domain approach for calculating 3D muscle fiber orientation from 3D B-mode volumes acquired using two imaging setups and validate the algorithm on in silico and in vivo data. Results show that the new method provides accurate orientation estimates and can be a useful tool for ongoing muscle characterization studies.