1. An image processing apparatus for the reconstruction of time-dependent representations I(x,t) of an object, comprising: an approximation module with memory storing the N-dimensional parameter vector a(x) of a predetermined parametric model function I*(a(x),t) that approximates the function I(x,t); an input module for the reception of a set of projections p i j of the object generated at times t i j , and an estimation module that is adapted to estimate the parameter vector a(x) with the help of said projections p i j .