The invention relates to an X-ray detector (1) with an array (2) of sensor elements (21), wherein each row (i) of sensor elements is connected via an addressing line to an addressing unit (5), and wherein each column j) of sensor elements is connected via a read-out line to a read-out unit (3). In the read-out unit (3), sensor signals are preprocessed, for example amplified by amplifiers (31). The detector further comprises a control unit (6) which is adapted to set the gains of said amplifiers (31) for each column (j) and each row (i) individually. The values of the gains may particularly be derived from a priori knowledge, from previous images of the object, or from the image signals of rows that have already been read out.