Quantum noise, also called quantum mottle, is the main and the most significant source of noise in plain radiography. It is a random process due to fluctuations in the number of photons reaching the detector from point to point. This means that exposing the detector in the absence of an object would result in a grainy image rather than uniform greyscale.
The number of photons reaching individual pixels on the detector follows a Poisson distribution, with an average number of photons per pixel, N. The standard deviation is approximately √N 1.
The perceived noise in an image is then = √N / N
Quantum noise has an undesirable effect mainly in images done using low radiation doses. In those instances such as in fluoroscopy, the image intensifier detects only a proportion of the signal which appears as graininess in the screen/images. The random differences in signal gets superimposed on the pattern produced by the structure and leads to poor contrast resolution of the fine image details. This can be reduced by using a higher mA thus increasing the number of x-ray photons, resulting in improved signal to noise ratio 2.
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