This step is usually done to increase the contrast or reduce the noise in an image to allow for more accurate segmentation in the steps that follow. It usually precedes organ segmentation or registration because it offers the potential of redistributing and rescaling pixel values in order to obtain more successful results in the clustering and classification of pixels. Techniques reported in the literature are designed for the spatial or the frequency domain. Spatial domain methods include logarithmic transformations and power law transformations, histogram equalization, image subtraction and averaging, and image smoothing techniques using spatial filters. Frequency (usually Fourier) domain include low-pass filters and high-pass filters for image smoothing and sharpening respectively .
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