The quality of ultrasound images is limited due to granular speckle noise. The presented despeckle algorithm compensates the depth-dependent shape of granular speckles in sector scans by an initial coordinate transform. This yields a horizontally oriented speckle pattern of constant resolution and hence allows the use of constant filter templates. The signal-dependent nature of multiplicative speckle noise is considered by a ratio pyramid containing noise-normalized, subsampled scales corrupted by signal-independent noise. Since speckles can be identified as positive and negative impulses on the subsampled scales, they are removed by selfdual nonlinear multistage filters (NMF). The templates are adapted to the granular appearance of the speckles and the degree of filtering is individually controlled by the local noise power in each scale. We propose a new selfdual morphological pyramid with the common erosion/dilation as analysis operators and the reconstructive dilation/erosion as synthesis operators. T he resulting closing-by-reconstruction and opening-by-reconstruction branches consider local intensity amplifications and attenuations, respectively. They are generated separately and combined only for scale-selective restoration by NMFs. Besides morphological decomposition, a ratio Laplacian pyramid is evaluated and its performance is compared with the proposed morphological decomposition. Both methods lead to significant noise reduction, where the morphological method introduces less signal degenerations.
|Journal||Proceedings of SPIE - The International Society for Optical Engineering|
|Number of pages||12|
|Publication status||Published - 03.07.2001|