A physical grid prevents scattered photons from reaching the image receptor. A virtual grid removes the contrast-reducing low frequency effect of the scatter, but the scattered photons (which contain no imaging information) still reach the receptor carrying with them additional photon noise. Conventionally the noise variance is proportional to the number of photons per unit area recorded. The overall effect of the virtual grid could, then, be to increase the relative quantum noise in the image. Most manufacturer seem to dodge this by combining their virtual grid product with noise reduction i.e. pixel averaging in some form. This latter processing, if it is beneficial, could of course have been added to the conventional gridded image, so the two elements (virtual grid and noise averaging) are separate interventions.