A method and apparatus for reconstructing an image and a medical image system are provided. An image reconstruction processing method may include acquiring an initial value with respect to a tomographic image of an object to be inspected, initializing an auxiliary variable, acquiring one or both of a weighted value and an error value, based on a measured value, transforming a measured image signal, updating the auxiliary variable using a transform coefficient used to transform the measured image signal, and updating the measured value using the updated auxiliary variable, and one or both of the acquired weighted value and the error value.