Noninvasive assessment of myocardial anisotropy in vitro and in vivo using Supersonic Shear Wave Imaging

The knowledge of the myocardial fiber architecture is deemed essential and yet to be nondestructively investigated for myocardial mechanics and its association with the progression of myocardial diseases. In this study, Supersonic Shear Wave Imaging (SSI) was performed for its first time to noninvasively assess the fiber orientation (i.e., anisotropy) in in vitro porcine (N=5) and in vivo open-chest ovine (N=l) hearts. The SSI technique combined with the coherent plane-wave compounding method was performed to achieve both high echocardiographic image quality and ultrafast frame rate (>;8000 fps). An 8 MHz linear array probe (pitch= 0.2 mm) was mounted on a customized rotation device, which allowed 360° rotation at 5° increments. The probe was initially aligned (±90° inclination) with the longitudinal axis in the local cardiac coordinates. The variation of the myocardial fiber orientation across the wall was observed in both in vitro and in vivo cases. Myocardial fibers were oriented gradually from the left diagonal (upper left to bottom right) near the epicardium (100% wall thickness) to the right diagonal (upper right to bottom left) near the endocardium with the midwall region dominated by the circumferential fibers. This finding was in good agreement with the literature and histology and has demonstrated the feasibility of SSI in mapping myocardial anisotropy.