A fundamental statistical phenomenon that is present in all images. Noise tends to reduce the visibility of structures and objects especially those that have relatively low contrast. In medical imaging the objective is not to eliminate the noise but to reduce it to a clinically acceptable level. Noise is the point-to- point variation in image brightness that does not contain useful information. The magnitude of noise is indicated by the standard deviation of the grey values within a region of interest in the image.
(ICRP Publication 129, 2015)
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