An image generating apparatus for image generation and dose calculation is provided. The image generating apparatus includes a movable detector for detecting nuclear radiation during a detection period and an evaluation system. The evaluation system includes an interface system for transmitting detector data to the evaluation system. The detector data include information about the detected radiation for image generation. The evaluation system further includes a data memory portion for storing the detector data. The evaluation system further includes a program memory portion with a program for repeatedly determining at least one quality value with respect to image generation during the detection period. The image generating apparatus includes an output system including at least one output unit. The at least one output unit includes one output unit for outputting an instruction to a user for further moving the detector in dependence of the detector data. The instruction relates to at least a part of the remaining detection period.