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

Abstract

The assessment of fiber architecture is of major interest in the progression of myocardial disease. Recent techniques such as Magnetic Resonance (MR) Diffusion Tensor Imaging or Ultrasound Elastic Tensor Imaging (ETI) can derive the fiber directions by measuring the anisotropy of water diffusion or tissue elasticity, but these techniques present severe limitations in clinical setting. In this study, we propose a new technique, the Backscatter Tensor Imaging (BTI) which enables determining the fibers directions in skeletal muscles and myocardial tissues, by measuring the spatial coherence of ultrasonic speckle. We compare the results to ultrasound ETI. Acquisitions were performed using a linear transducer array connected to an ultrasonic scanner mounted on a motorized rotation device with angles from 0° to 355° by 5° increments to image ex vivo bovine skeletal muscle and porcine left ventricular myocardial samples. At each angle, multiple plane waves were transmitted and the backscattered echoes recorded. The coherence factor was measured as the ratio of coherent intensity over incoherent intensity of backscattered echoes. 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. In ETI, the shear wave speed variation with the probe angle was found to follow the coherence variation. Spatial coherence can thus reveal the anisotropy of the ultrasonic speckle in skeletal muscle and myocardium. BTI could be used on any type of ultrasonic scanner with rotative phased-array probes or 2-D matrix probes for non-invasive evaluation of myocardial fibers.

Angular coherence in ultrasound imaging: Theory and applications

“…Recently, this technique has been proposed in a number of applications. For example, it has been used in "ultrafast" imaging 4 applications and transient elastography, 1 in color-encoded speckle imaging, 5 to characterize blood flow, 4 to analyze speckle coherence in anisotropic media using backscatter tensor imaging, 6 to obtain functional brain imaging on rodents, 7,8 and more recently in speed-of-sound estimation. 9 Through these applications, PWT-SA has been popularized as a method to achieve high frame rate synthetic transmit aperture imaging 1,10 to improve image resolution everywhere, and to improve sensitivity in Doppler imaging.…”

Computational Sonography

“…While this method utilizes information from different directions for a combined reconstruction, it still reconstructs only scalar values per voxel, and thus directional information is not available for further processing. In [9], the spatial coherence of ultrasound information observed for different orientations and focal regions has been used for tensor reconstructions. While the method is promising, it requires both a specialized hardware setup and a fixed scanning protocol.…”

Development and Application of Optical Coherence Tomography (OCT)

“…Providing the same sensitivity and speed advantages as spectrometer-based Fourier domain OCT, SS-OCT was soon expanded by polarization sensitivity [47,71,75,[93][94][95][96][97]. In particular the advent of commercial laser technology providing longer imaging ranges led to the development of PS-OCT at ultrahigh imaging speeds of 100,000 axial scans per second [18,54,73,74,98].…”

“…Another parameter used to quantify the strain anisotropy is the fractional anisotropy FA. In a similar manner to classical diffusion tensor imaging (DTI) in MRI [93,94] or ultrasound [95], we define the fractional anisotropy as:…”