Towards backscatter tensor imaging (BTI): Analysis of the spatial coherence of ultrasonic speckle in anisotropic soft tissues

The assessment of fiber architecture is of major interest in the progression of myocardial disease. Recent techniques such as MR Diffusion Tensor Imaging or Ultrasound Elastic Tensor Imaging (ETI) can derive the fiber directions by measuring the anisotropy of water diffusion or tissue lasticity, but these techniques present severe limitations in clinical setting. In this study, we measure the spatial coherence of ultrasonic speckle in skeletal muscles and myocardial tissues, in order to determine the fibers directions and compare it to ETI. Acquisitions were performed using a linear transducer array mounted on a rotation device to image ex vivo bovine skeletal muscle and porcine LV myocardial samples. At each angle, multiple plane waves were transmitted and the backscattered echoes recorded. The coherence factor was measured as the integral of the correlation function. In skeletal muscle, maximal/minimal coherence factor was found for the probe parallel/perpendicular to the fibers. In myocardium, the coherence was assessed across the entire myocardial thickness, and the position of maxima and minima varied transmurally due to the complex fibers distribution. The shear wave speed variation with the probe angle was found to follow the coherence variation. Spatial correlation can thus reveal the anisotropy of the ultrasonic speckle in skeletal muscle and myocardium. BTI could be used on any type of ultrasonic scanner for non invasive evaluation of myocardial fibers.