How does real offset and gain correction affect the DQE in images from x-ray flat detectors?

Offset and gain corrections are indispensable to exploit images from large image sensors, because of the pixel to pixel variation in dark current and sensitivity. However, an inappropriate correction may be detrimental to the signal to noise ratio of the raw image. This is especially critical in X-ray imaging, where the quantum noise is filtered by the detector spatial response. The noise power spectrum (NPS) in the corrected image is a combination of the initial noise spectrum in the raw image (quantum noise and electronic noise) with the noise in the offset and gain images. The dark image noise power just adds up to the noise power in the current image. The noise in the gain image alters the noise of the current image in a more intricate way. This is illustrated by simulations and experimental measurements. The conditions to safeguard the signal to noise ratio in the current image are detailed: There is an optimal number of dark and 'white' images to be averaged in order to keep their electronic and quantum noise negligible compared to that of the current image. Real conditions often force trade-offs between the desirable large number of offset/gain images to be averaged and the time effectively assigned to such acquisitions. Furthermore, the residual noise spectrum in the gain images is dependent on dose, uniformity of irradiation, temperature and detector spatial response. In the appropriate conditions, the intrinsic signal to noise ratio of an image can be preserved by offset and gain correction. Nevertheless, at high dose, the gain correction unavoidably introduces some high frequency proportional noise which degrades the DQE.