Multiscale nonlocal means method for ultrasound despeckling

Speckle noise is inherent in ultrasound imaging where it causes reduced contrast resolution and degrades the detectability of small structures. In this paper we propose a real-time multiscale despeckling filter based on the recently proposed Nonlocal Means (NL-means) filter. Our proposed despeckling method applies a cascade of NL-means filters from fine to coarse scale. It has been tested using an optimized OpenCL GPU implementation. We apply the filter to the US image prior to scan conversion to allow a more uniform filter response and better adaption to the point spread function of the imaging system. The filter compares favourably to other state-of-the-art methods according to three quantitative image measures on a simulated ultrasound image. On real ultrasound images the filter achieved good smoothing of speckle, while preserving small and low contrast features. The running time is between 10-20 ms on typical image sizes using a mid-range GPU.