1. A nuclear image system for updating an original nuclear image, the nuclear image system comprising: a data memory for storing the original three-dimensional nuclear image; a nuclear radiation detector, which is movable along a freely variable path, for measuring nuclear radiation, in order to obtain nuclear radiation values; a tracking system for tracking the nuclear radiation detector while measuring the nuclear radiation, so that detector coordinates are obtained which indicate a posture of the tracked nuclear radiation detector in relation to an image coordinate system of the nuclear image; a nuclear data input configured to receive the nuclear radiation values from the nuclear radiation detector and the detector coordinates from the tracking system, and to associate the nuclear radiation values with the respective detector coordinates; and an image updating module including an updating rule for changing the original nuclear image on the basis of the nuclear radiation values and the detector coordinates, wherein the image updating module is configured to generate an updated three-dimensional nuclear image by applying the updating rule to the original nuclear image; wherein the image updating module further comprises: an image varying function for varying the nuclear image; and an image quality functional for calculating an image quality value from the nuclear radiation values and the varied nuclear image, wherein the image updating module is configured to vary iteratively the nuclear image by means of the image varying function, so that the image updating module calculates the nuclear image in a plurality of iterations such that the nuclear image gradually approximates an extremal value of the image quality functional, and to output the nuclear image resulting from the calculation as the updated nuclear image; and wherein the image quality functional includes a consistency term and a regularization term, wherein the consistency term depends on the varied nuclear image, the nuclear radiation values, and the detector coordinates, and expresses a consistency between the varied nuclear image and the nuclear radiation values; and wherein the regularization term depends at least on the varied nuclear image.