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° and 1.5° for the rotational B-mode (n=7) and 2.7° and 1.5° 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° 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°, and 95% limits of agreement was ±10°. 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.