Scattered X-ray photons can contribute very significantly to the exposure of the image. In screen-film or flat panel type radiography, scattered radiation causes a reduction of contrast of the image. The main effect of scattered radiation is to increase the noise of image. Grids are used to reduce scattered radiation, but can also produce artifacts on the image.
In the case of scan type, the fan beam by collimator reduces scattered radiation from patient and linear structure of detector also prevents incident scattered ray from patient. Therefore, there is no need for severe image processing to remove the noise from scattered radiation and image processing speed is very fast.
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