Quantum noise in detectors

The variance in the number of x-ray photons absorbed by a detector is derived from theoretical considerations. The distributions in the number of x-ray photons produced by the x-ray tube, transmitted through the patient, and absorbed by the detector are described. These distributions are compounded to yield a distribution in the number of x-ray photons absorbed by a detector. The standard deviation (quantum noise) in this distribution is very well approximated by the commonly used and experimentally verified quantity square root N where N is the average number of x-ray number of x-ray photons absorbed by the detector.