A radiographic imaging apparatus (10) comprises a primary radiation source (14) which projects a beam of radiation into an examination region (16). A detector (18) converts detected radiation passing through the examination region (16) into electrical detector signals representative of the detected radiation. The detector (18) has at least one temporally changing characteristic such as an offset B(t) or gain A(t). A grid pulse means (64) turns the primary radiation source (14) ON and OFF at a rate between 1000 and 5000 pulses per second, such that at least the offset B(t) is re-measured between 1000 and 5000 times per second and corrected a plurality of times during generation of the detector signals. The gain A(t) is measured by pulsing a second pulsed source (86, 100, 138) of a constant intensity (XRef) with a second pulse means (88). The gain A(t) is re-measured and corrected a plurality of times per second during generation of the detector signals.
