KNOBOLOGY

In the context of US imaging, myofibers appear hypoechoic compared to adjacent connective and nervous structures, while fibroadipose tissues (perimysium and epimysium) are hyperechogenic. 

In the transverse axis, the tissue appears to be formed by a background hypoechoic compound with hyperechoic dots inside (“starry sky”). Longitudinal axis images of muscle tissue show an alternation of parallel hypo/hyperechoic bands (“feather” or “veins on a leaf”) with a variable orientation depending on pennation angle. 

Different orientation of muscle fibers with respect to the US beam might cause artifacts of anisotropy, especially in long axis evaluation.