Hoskins P R, Loupas T, McDicken W N
Department of Medical Physics and Medical Engineering, Royal Infirmary of Edinburgh, Scotland.
Ultrasound Med Biol. 1990;16(4):375-89. doi: 10.1016/0301-5629(90)90067-m.
This paper is concerned with speckle suppression in Discrete Fourier Transform based Doppler signals by means of digital image processing. The Doppler spectrum is treated as a greyscale image, and three different noise smoothing algorithms are applied to it. These are the Double Window Modified Trimmed Mean filter, which is nonlinear, and Lee's and the directional filter, which are adaptive in the sense that the smoothing performed by them at each point of the image is controlled by a local image measure. In order to evaluate the performance of the filters, they were applied to a variety of regular waveforms obtained from a physiological flow phantom. Ensemble averaging of a large number of unfiltered spectra was used as the "gold standard" in the evaluation, i.e., as the output of an ideal filter which reveals the exact nature of the underlying Doppler spectrum after speckle has been eliminated. Comparison of the "gold standard" with the ensemble averaged filtered data allowed the noise reduction, bias and distortion of the maximum frequency envelope introduced by filtering to be examined. Overall, the best performance was offered by the directional filter whose action was controlled by the combination of the local edge content and the slope of the least-squares-fit line passing through the data points along each particular direction.
本文关注通过数字图像处理抑制基于离散傅里叶变换的多普勒信号中的散斑。将多普勒频谱视为灰度图像，并对其应用三种不同的噪声平滑算法。其中一种是非线性的双窗口修正截尾均值滤波器，另外两种是李滤波器和方向滤波器，它们具有自适应特性，即在图像的每个点执行的平滑操作由局部图像度量控制。为了评估这些滤波器的性能，将它们应用于从生理血流模型获得的各种规则波形。在评估中，大量未滤波频谱的总体平均被用作“金标准”，即作为理想滤波器的输出，该理想滤波器在散斑消除后揭示潜在多普勒频谱的准确特性。将“金标准”与总体平均滤波后的数据进行比较，可以检查滤波引入的噪声降低、偏差以及最大频率包络的失真情况。总体而言，方向滤波器表现最佳，其操作由局部边缘内容与沿每个特定方向穿过数据点的最小二乘拟合线的斜率的组合控制。
