unit 120 which uses a system matrix to describe the nuclear imaging system 100 and an iteratively improved data model to calculate an image object i on the basis of the data set d the image object i can for example be displayed on a display 140 using well known volume rendering techniques for each iteration step the evaluation unit 130 calculates a stop criterion i e a goodness of fit parameter using a chi square gamma u03c7 u03b3 2 statistic the chi square gamma statistic is used for determining the quality of an estimated data model within a reconstruction algorithm it is further used to determine a stop criterion for the iterations of a reconstruction algorithm this statistic is discovered to be especially well suited for nuclear imaging because the iterations of the algorithm are limited based on a statistic that is well behaved for the low count data associated with nuclear imaging an example of a chi square gamma statistic is described in k j mighell u201c parameter estimation in astronomy with poisson distributed data i the u03c7 u03b3 2 statistic u201d astrophys j 1999 518 380 393 and k j mighell u201c parameter estimation in astronomy with poisson distributed data ii the modified chi square gamma statistic u201d 2000 arxiv astro ph 0007328 the contents of which are herein incorporated by reference however in this application.