Phase asymmetry guided adaptive fractional-order total variation and diffusion for feature-preserving ultrasound despeckling

It is essential for ultrasound despeckling to remove speckle noise while simultaneously preserving edge features for accurate diagnosis and analysis in many applications. To preserve real edges such as ramp edges and low contrast edges, we first detect edges using a phase-based measure called phase asymmetry (PAS), which can distinguish small differences in transition border regions and varies from $0$ to $1$, taking $0$ in ideal smooth regions and taking $1$ at ideal step edges. We further propose three strategies to properly preserve edges. First, in observing that fractional-order anisotropic diffusion (FAD) filter has good performance in smooth regions while the fractional-order TV (FTV) filter performs better at edges, we leverage the PAS metric to keep a balance between FAD filter and FTV filter for achieving the best performance of preserving ramp edges. Second, considering that the FAD filter fails to protect low contrast edges by solely integrating gradient information into the diffusion coefficient, we integrate the PAS metric into the diffusion coefficient to properly preserve low contrast edges. Finally, different from fixed fractional order diffusion filters neglecting the differences between smooth regions and transition border regions, an adaptive fractional order is implemented based on the PAS metric to enhance edges. The experimental results show that our method outperforms other state-of-the-art ultrasound despeckling filters in both speckle reduction and feature preservation.