An X-ray system (1, 10) for imaging and a gain calibration method are disclosed. An assembly (4, 14) of the system is moveable to different scan coordinates to acquire X-ray projection images at a plurality of said scan coordinates in a dynamic mode while the assembly is moving. The assembly is also moveable to a predetermined reference coordinate to acquire an X-ray projection image at said reference coordinate in a static mode while the assembly is resting. At least one processing unit (5), comprised by the X-ray system, is configured for receiving a plurality of dynamic gain calibration parameters as first inputs, for obtaining a static gain calibration parameter as a second input and for determining a plurality of adjusted dynamic gain calibration parameters. Each dynamic gain calibration parameter is obtained from an X-ray projection image acquired in the dynamic mode at one of the scan coordinates. The static gain calibration parameter is obtained from an X-ray projection image acquired in the static mode at the predetermined reference coordinate.
