An X-ray system (1, 10) for imaging, comprising: an assembly (4, 14) comprising a source and a detector, the detector being arranged distantly from the source to detect radiation emitted by the source after traversal of an imaging region, the assembly being moveable to different scan coordinates defined by at least one scan trajectory around the imaging region to acquire X-ray projection images at a plurality of said scan coordinates in a dynamic mode while the assembly is moving and the assembly also being 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 configured for receiving a plurality of dynamic gain calibration parameters as first inputs, obtaining a static gain calibration parameter as a second input, determining a plurality of adjusted dynamic gain calibration parameters corresponding to each of the plurality of dynamic gain calibration parameters, based on at least the received first inputs and the obtained second input, wherein each of the plurality of dynamic gain calibration parameters is obtained from an X-ray projection image acquired in the dynamic mode at one of the scan coordinates of the at least one scan trajectory during a dynamic gain calibration period, the static gain calibration parameter is obtained from an X-ray projection image acquired in the static mode at the predetermined reference coordinate during a static gain calibration period, and a start time for the static gain calibration period occurs later than a start time for the dynamic gain calibration period such that the static gain calibration period and the dynamic gain calibration period are not overlapping in time. an assembly (4, 14) comprising a source and a detector, the detector being arranged distantly from the source to detect radiation emitted by the source after traversal of an imaging region, the assembly being moveable to different scan coordinates defined by at least one scan trajectory around the imaging region to acquire X-ray projection images at a plurality of said scan coordinates in a dynamic mode while the assembly is moving and the assembly also being 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 configured for receiving a plurality of dynamic gain calibration parameters as first inputs, obtaining a static gain calibration parameter as a second input, determining a plurality of adjusted dynamic gain calibration parameters corresponding to each of the plurality of dynamic gain calibration parameters, based on at least the received first inputs and the obtained second input, receiving a plurality of dynamic gain calibration parameters as first inputs, obtaining a static gain calibration parameter as a second input, determining a plurality of adjusted dynamic gain calibration parameters corresponding to each of the plurality of dynamic gain calibration parameters, based on at least the received first inputs and the obtained second input, each of the plurality of dynamic gain calibration parameters is obtained from an X-ray projection image acquired in the dynamic mode at one of the scan coordinates of the at least one scan trajectory during a dynamic gain calibration period, the static gain calibration parameter is obtained from an X-ray projection image acquired in the static mode at the predetermined reference coordinate during a static gain calibration period, and a start time for the static gain calibration period occurs later than a start time for the dynamic gain calibration period such that the static gain calibration period and the dynamic gain calibration period are not overlapping in time.