Methods for correcting a count loss and PET systems are provided according to examples of the present disclosure. In one aspect, the PET system obtain scanning data of a subject to be detected for which random correction has been performed, obtain a first correction factor corresponding to the true coincidence count according to the single-photon count rates of the two Blocks corresponding to the true coincidence count, obtain a second correction factor corresponding to the true coincidence count according to the system single-photon count rate, and correct the true coincidence count according to the first correction factor and the second correction factor corresponding to the true coincidence count.
