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Noise
Theoretically, the number of photons that reach the detectors can be
described as a cascade of Bernoulli distribution, describing the
probability that a photon is emitted or not, absorbed from the patient or
not, detected or not etc. This cascade can be approximated with a
Poisson distribution and hence we can write:
The noise is theoretically uncorrelated, but the cross-talk between
adjacent detectors, the logarithmic operation to obtain the sinogram,
and especially the filtering of the sinogram and backprojection
operations make such that the noise is spatially correlated in image
domain.